Distance Vactor Routing Protocol Rip

Ch. 7 – Distance Vector Routing Protocols Part 1 of 2: Distance Vector Routing and RIP CCNA 1 version 3.0 Rick Graziani Cabrillo College

Note to instructors •

If you have downloaded this presentation from the Cisco Networking Academy Community FTP Center, this may not be my latest version of this PowerPoint. • For the latest PowerPoints for all my CCNA, CCNP, and Wireless classes, please go to my web site: http://www.cabrillo.cc.ca.us/~rgraziani/ • The username is cisco and the password is perlman for all of my materials. • If you have any questions on any of my materials or the curriculum, please feel free to email me at [email protected] (I really don’t mind helping.) Also, if you run across any typos or errors in my presentations, please let me know. • I will add “(Updated – date)” next to each presentation on my web site that has been updated since these have been uploaded to the FTP center. Thanks! Rick Rick Graziani [email protected]

2

Overview

• • • • • • • • • • •

Describe how routing loops can occur in distance vector routing Describe several methods used by distance vector routing protocols to ensure that routing information is accurate Configure RIP Use the ip classless command Troubleshoot RIP Configure RIP for load balancing Configure static routes for RIP Verify RIP Configure IGRP (Part II) Verify IGRP operation (Part II) Troubleshoot IGRP (Part II) Rick Graziani [email protected]

3

Distance Vector Routing Updates

Rick Graziani [email protected]

4

Distance Vector Routing Updates

• • • • •

No! MTU is never used as a routing metric. Some documentation is incorrect on this item.

RIP – Hop Count IGRP and EIGRP – Bandwidth, Delay, Reliability, Load Cisco’s OSPF – Bandwidth IS-IS – Cost BGP – Number of AS or policy Rick Graziani [email protected]

5

FAQs FAQs – Network Discovery Q: How often does initial network discovery happen? A: Only when the network comes first comes up. Q: Do routers share routing table information after network discovery? A: Yes, distance-vector routing protocols share their entire routing tables periodically (with or without split horizon enabled). Distance vector routing protocols on Cisco routers by default use split horizon with poison reverse (discussed in the next section). Depending upon the distance-vector routing protocol, the frequency of the updates will happen for RIP every 30 seconds, IPX RIP every 60 seconds, and IGRP every 90 seconds. Q: What happens when there is a change in the topology, link goes down, new network is added, new router, is added, etc.? A: Let’s take a look.

Rick Graziani [email protected]

6

Triggered Updates - Extra

Triggered Updates • Routers do not have to wait for the periodic update to hear about changes in the network topology. • Improvements to the distance-vector algorithm is typically made in distance-vector routing protocols, like RIP, to include triggered updates. • Even with triggered updates, large distance vector networks can suffer from long convergence times in some situations. Rick Graziani [email protected]

7

Triggered Updates

• • • •

Triggered updates are sent whenever a router sees a topology change or a change in routing information (from another router). The router does not have to wait for the period timer, but can send them immediately. Triggered updates do not need to include the entire routing table but only the modified route(s). Triggered updates must still be sent to adjacent routers, from router to router, like other routing updates. Rick Graziani [email protected]

8

Triggered Updates

• •

Most distance-vector routing protocols limit the frequency of triggered updates so that a flapping link does not put an unnecessary load on the network. (RIP: random 1 to 5 seconds) Typically, triggered updates can be “triggered” by: – Interface transition to the up or down state – A route has entered or exited an unreachable (down) state (later) – A new route is installed in the routing table Rick Graziani [email protected]

9

Routing Loop Issues

Routing Loops • Distance vector routing protocols are simple in their implementation and configuration, but this comes at a price. • Pure distance vector routing protocols suffer from possible routing loops. • Routing loops can cause major network problems, from packets getting lost (blackholed) in your network, to bringing down your entire network. • Several remedies to have been added to distance-vector algorithms to help prevent routing loops including: – Split horizon – Hold-down timers – Defining a maximum metric Rick Graziani [email protected]

10

Routing Loop Issues

What can cause routing loops? • Routing loops can occur when there are: – Incorrect or inconsistent routing updates due to slow convergence after a topology change. (Example coming up next.) – Incorrect or incomplete routing information (see presentation on Discard Routes) – Static routes incorrectly configured with an intermediate address which does not become resolved in the routing table. (see presentation on Static Routes – Additional Information) Rick Graziani [email protected]

11

Routing Loop Issues

Routing Loop Example • Assume for the remainder of this example that Router C’s preferred path to network 1 is by way of Router B. • Router C’s routing table has a distance of 3 to network 1 via Router B.

Rick Graziani [email protected]

12

Routing Loop Issues

Network 1 Fails • Router E sends an update to Router A. • Router A stops routing packets to network 1. • But Routers B, C, and D continue to do so because they have not yet been informed about the failure. • Router A sends out its update. • Routers B and D stop routing to network1, (via Router A). • However, Router C is still not updated. • To router C, network 1 is still reachable via router B. Rick Graziani [email protected]

13

Routing Loop Issues

Router C sends a periodic update to Router D • Router C sends a periodic update to Router D indicating a path to network 1 (by way) of via Router B. (4 hops). Router D’s Routing Table information for Network 1 • Current path to Network 1 = Unreachable (down) • Information from Router C: Network 1 : 4 hops by way of Router C • Normally, RouterD ignores this routing information because it usually has a better route, 2 hops, via Router A, but this route is now down. • Router D changes its routing table to reflect this (good) better, but incorrect information, Network 1 by way of Router C (4 hops) • Router D propagates the information to Router A. Rick Graziani [email protected]

14

Routing Loop Issues

Routers A changes its routing table • Router A adds new route to its routing table, get to Network 1 by way of Router D (5 hops). • Propagates the information to Routers B and E. Router B (and Router E) change their routing tables • Router B now believes it can get to Network 1 by way of Router A (6 hops).  Wow! I was about to tell Router C that Network 1 was down via Router B, but now I have new information! • Propagates the incorrect information to Router C. Rick Graziani [email protected]

15

Routing Loop Issues

Router C changes its routing table • Router C still believes it can get to Network 1 by way of Router B (7 hops).  Of course now it believes it is 7 hops instead of 3. • Propagates the newer but still incorrect information to Router D. Here we go again! • Data packets destined for Network 1 get caught in a routing loop, from Routers A to D to C to B to A to D etc. • As routing updates continue between the routers, the hop count gets greater – to infinity? (Not quite – we will see in a moment.) Rick Graziani [email protected]

16

Defining a Maximum

Problem: Count to infinity Solution: Defining a Maximum • Distance vector routing algorithms are self-correcting, but a routing loop problem can require a count to infinity. • To avoid this prolonged problem, distance vector protocols define infinity as a specific maximum number. • This number refers to a routing metric which may simply be the hop count. • When the metric value exceeds the maximum value, and as each router receives this maximum metric, the network is then considered unreachable. Rick Graziani [email protected]

17

Why only a 15 hop count limit? Question: Why does RIP use a hop count as the route metric, and why is its maximum value limited to 15? Answer: “When RIP was designed and implemented, dynamic routing protocols were not widely used. Instead, networks relied mostly on static routing. RIP, even with its hop-count-metric – which seems very poor to us today – was quite a big improvement. Counting intermediate routes is the simplest method to measure the quality of routes. Setting the infinity value for the metric is always a problem of choosing between wider networks and faster convergence when the protocol starts counting. When RIP was invented, it seemed unlikely to have a network with the maximum diameter more more than 15 routers, so 16 was chosen as the infinity value.” (Zinin, Cisco IP Routing)

Rick Graziani [email protected]

18

Split Horizon

•

“The effect of split horizon is that a router will send out different routing messages on different interfaces. In effect a router never sends out information on an interface that it learned from that interface.” (Lewis, Cisco TCP/IP Routing)

Rick Graziani [email protected]

19

Split Horizon

• •

•

This example from the curriculum is not an example of split hoizon, but using hold-down timers. “Split-horizon attempts to avoid this situation. If a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A. Split-horizon thus reduces incorrect routing information and reduces routing overhead.” Initially, this is true, but the loop is a result of Router C sending out the updates, because it has not converged. Rick Graziani [email protected]

20

Simple Split Horizon 10.1.1.0/24 .1

RTA

10.1.2.0/24 .1

.2

s0

s0

e0

RTB

10.1.3.0/24 .1 e0

Routing Table

Routing Table

Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0

Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0

Initial routing tables

Split Horizon Rule – Avoiding Routing Loops • Routers RTA and RTB have their initial routing tables and are ready to exchange routing information via a distance-vector routing protocol like RIP. Split Horizon disabled • If split horizon were disabled the routing updates would include all of the networks in their routing tables including their directly connected networks and any networks learned from any interface. Rick Graziani [email protected]

21

10.1.1.0/24 .1

RTA

10.1.2.0/24 .1

.2

s0

s0

e0

RTB

10.1.3.0/24 .1 e0

Routing Table

Routing Table

Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0

Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0

Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1 10.1.2.0/24 1 10.1.1.1

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

Routing Update Next-hop Net. Hops Address 10.1.2.0/24 1 10.1.2.2 10.1.3.0/24 1 10.1.2.2

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1

Initial routing tables 10.1.2.0/24 network is included because split horizon has been disabled

New routing tables

Split Horizon Disabled • After the initial exchange of updates everything in the routing tables look fine. • Because split horizon disabled, the 10.1.2.0/24 network is sent by both routers, but neither router includes the other’s route to 10.1.2.0/24 (1 hop) in the routing table, because it has a current route with a better metric of 0. Rick Graziani [email protected]

22

10.1.1.0/24 .1

RTA

e0

10.1.2.0/24 .1

.2

s0

s0

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1 10.1.2.0/24 1 10.1.1.1 10.1.3.0/24 2 10.1.1.1

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

RTB

10.1.3.0/24 .1 e0

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1

Routing Update Next-hop Net. Hops Address 10.1.2.0/24 1 10.1.2.2 10.1.3.0/24 1 10.1.2.2 10.1.1.0/24 2 10.1.2.2

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1

Previous routing tables Networks in red were included because split horizon has been disabled

New routing tables

Split Horizon Disabled • After the next exchange of updates everything in the routing tables look fine and the routing tables are converged. • Because split horizon disabled, the besides the 10.1.2.0/24 network, the networks learned from the other router in the previous update is also sent by both routers. • However, neither router includes the those networks, because it has a current route with a better metric of 0. Rick Graziani [email protected]

23

10.1.1.0/24 .1

RTA

e0

10.1.2.0/24 .1

.2

s0

s0

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

RTB

10.1.3.0/24 .1 e0

X

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1

Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1 10.1.2.0/24 1 10.1.1.1 10.1.3.0/24 2 10.1.1.1

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

Previous routing tables Networks in red were included because split horizon has been disabled

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 2 10.1.2.1 10.1.1.0/24 1 10.1.2.1

New routing tables

Split Horizon Disabled – 10.1.3.0/24 down • Note: Routing tables are not sent at the exactly same time. We will learn about this in Ch. 12 Routing Protocols, that this is done on purpose to avoid collisions on broadcast networks like Ethernet. • Here, the 10.1.3.0/24 network fails, and before RTB sends out its routing update, RTB receives a routing update from RTA. Rick Graziani [email protected]

24

10.1.1.0/24 .1

RTA

e0

10.1.2.0/24 .1

.2

s0

s0

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

RTB

10.1.3.0/24 .1 e0

X

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1

Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1 10.1.2.0/24 1 10.1.1.1 10.1.3.0/24 2 10.1.1.1

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

Previous routing tables Networks in red were included because split horizon has been disabled

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 2 10.1.2.1 10.1.1.0/24 1 10.1.2.1

New routing tables

Split Horizon Disabled – 10.1.3.0/24 down • RTB notices that it has a route to 10.1.3.0/24 via RTA. Even though it is 2 hops it is certainly better than its current situation of “unreachable” so it accepts this better, but incorrect information from RTA. • RTB now forwards all packets destined for 10.1.3.0/24 to RTA at 10.1.2.1. • RTA receives these packets and forwards them to RTB at 10.1.2.2. • RTB forwards them back to RTA at 10.1.2.1. [email protected] on! The packets get blackholed in this routing loop. • RickAnd Graziani 25

10.1.1.0/24 .1

RTA

e0

10.1.2.0/24 .1

.2

s0

s0

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

RTB

10.1.3.0/24 .1 e0

X

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 2 10.1.2.1 10.1.1.0/24 1 10.1.2.1

Routing Update Next-hop Net. Hops Address 10.1.2.0/24 1 10.1.2.2 10.1.3.0/24 3 10.1.2.2 10.1.1.0/24 2 10.1.2.2

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 3 10.1.2.2

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 2 10.1.2.1 10.1.1.0/24 1 10.1.2.1

Previous routing tables Networks in red were included because split horizon has been disabled

New routing tables

Split Horizon Disabled – 10.1.3.0/24 down • Meanwhile, its RTB’s turn to send its routing update. • RTB increments the hop count to 10.1.3.0/24 to 3 hops and sends it to RTA. • When RTA sends out its next routing table it will increment the hop count to 10.1.3.0/24 to 4 hops and sends it to RTB. and on, until “infinity” which in RIP is 16 hops. •RickAnd Grazianion [email protected] 26

Simple Split Horizon 10.1.1.0/24 .1

RTA

e0

10.1.2.0/24 .1

.2

s0

s0

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 16 10.1.2.2

RTB

10.1.3.0/24 .1 e0

X

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 16 10.1.2.1 10.1.1.0/24 1 10.1.2.1

Split Horizon Disabled • Once both routers have 16 hops for 10.1.3.0/24, they will both mark this network as unreachable and discontinue forwarding, drop, packets to this network. • This temporary routing loop can be easily avoided by enabling split horizon on the serial 0 interfaces. • Split horizon rule states that router never sends out information on an interface that it learned from that interface • Let’s see! Rick Graziani [email protected]

27

10.1.1.0/24 .1

Split Horizon Enabled

RTA

10.1.2.0/24 .1

.2

s0

s0

e0

RTB

10.1.3.0/24 .1 e0

Routing Table

Routing Table

Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0

Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0

Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1

Rick Graziani [email protected]

Previous routing tables

Routing Update Next-hop Net. Hops Address 10.1.3.0/24 1 10.1.2.2

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1

New routing tables

Routing Update Next-hop Net. Hops Address 10.1.3.0/24 1 10.1.2.2

28

10.1.1.0/24 .1

RTA

10.1.2.0/24 .1

.2

s0

s0

e0

RTB

10.1.3.0/24 .1 e0

Routing Table

Routing Table

Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0

Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0

Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1

Previous routing tables

Routing Update Next-hop Net. Hops Address 10.1.3.0/24 1 10.1.2.2

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1

New routing tables

Routing Update Next-hop Net. Hops Address 10.1.3.0/24 1 10.1.2.2

Split Horizon Enabled • As you can see, with split horizon enabled, RTA does not send RTB (out s0) information about 10.1.3.0/24 because it learned it from RTB (same s0), and RTB does not send RTA (out s0) information about 10.1.1.0/24 to RTA because it learned it from RTA (same s0). (This also includes the common network between them. Rick Graziani [email protected]

29

10.1.1.0/24 .1

RTA

e0

10.1.2.0/24 .1

.2

s0

s0

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

RTB

10.1.3.0/24 .1 e0

X

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 (down) e0 10.1.1.0/24 1 10.1.2.1

Previous routing tables

Routing Update Next-hop Net. Hops Address 10.1.3.0/24 16 10.1.2.2

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 (down) 10.1.2.2

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 (down) e0 10.1.1.0/24 1 10.1.2.1

New routing tables

Split Horizon Enabled – 10.1.3.0/24 down • RTB notices 10.1.3.0/24 is down and puts this route into hold-down state in its routing table. (hold-down coming next) • RTB immediately sends out a triggered update for only this route (if there were others in the routing table) with a metric of infinity, 16. • RTA receives the triggered update and puts the route for 10.1.3.0/24 into hold-down state. Rick Graziani [email protected]

30

10.1.1.0/24 .1

RTA

e0

10.1.2.0/24 .1

.2

s0

s0

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

RTB

10.1.3.0/24 .1 e0

X

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 (down) e0 10.1.1.0/24 1 10.1.2.1

Previous routing tables

Routing Update Next-hop Net. Hops Address 10.1.3.0/24 16 10.1.2.2

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 (down) 10.1.2.2

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 (down) e0 10.1.1.0/24 1 10.1.2.1

New routing tables

Split Horizon Enabled – 10.1.3.0/24 down • Notice that RTA never sends RTB a routing update for 10.1.3.0/24, because split horizon is enabled on these interfaces. Rick Graziani [email protected]

31

Split Horizon with Poison Reverse 10.1.1.0/24 .1

RTA

e0

10.1.2.0/24

.1

.2

s0

s0

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1 10.1.2.0/24 16 10.1.2.1 10.1.3.0/24 16 10.1.2.1

Split Horizon with Poison Reverse

• •

•

RTB

10.1.3.0/24

.1 e0

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1

Routing Update Next-hop Net. Hops Address 10.1.3.0/24 1 10.1.2.2 10.1.2.0/24 16 10.1.2.2 10.1.1.0/24 16 10.1.2.2

“Poisoned” routes in red. Routing tables remain the same.

Many vendor implementations of distance vector routing protocols like Cisco’s RIP and IGRP apply a special kind of split horizon, called split horizon with poison reverse. “Split horizon with poison reverse means that, instead of not advertising routes to the source, routes are advertised back to the source with a metric of 16, which will make the source router ignore the route. It is perceived that explicitly telling a router to ignore a route is better than not telling it about the route in the first place.” (Lewis, Cisco TCP/IP Routing) One drawback is that routing update packet sizes will be increased when using Poison Reverse, since they now include these routes. Rick Graziani [email protected]

32

Split Horizon with Poison Reverse 10.1.1.0/24 .1

RTA

e0

10.1.2.0/24

.1

.2

s0

s0

Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2

Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1 10.1.2.0/24 16 10.1.2.1 10.1.3.0/24 16 10.1.2.1

RTB

10.1.3.0/24

.1 e0

Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1

Routing Update Next-hop Net. Hops Address 10.1.3.0/24 1 10.1.2.2 10.1.2.0/24 16 10.1.2.2 10.1.1.0/24 16 10.1.2.2

“Poisoned” routes in red.

Split Horizon Enabled by Default Split horizon with poison reverse is enabled by default for all interfaces except: • Physical interfaces or multipoint sub-interfaces using Frame Relay or SMDS encapsulation (CCNA Semester 4 and CCNP) To disable split horizon on an interface: Router(config-if)# no ip split-horizon To enable split horizon on an interface: Router(config-if)# ip split-horizon Rick Graziani [email protected]

33

Route poisoning

•

When route poisoning is used with triggered updates it will speed up convergence time because neighboring routers do not have to wait 30 seconds before advertising the poisoned route. Rick Graziani [email protected]

34

Preventing routing loops with holddown timers

• •

•

The main function of holddown timers is to prevent the distance vector routing protocol from establishing routing loops during periods of network transition (topology changes). “The rule: Once a route is marked unreachable, it must stay in this state for a period of time assumed sufficient for all routers to receive new information about the unreachable network. In essence, we instruct the routers to let the rumors calm down and then to pick up the truth.” (Zinin, Cisco IP Routing) The amount of time a router remains in “this state” is determined by the holddown timer. Rick Graziani [email protected]

35

Preventing routing loops with holddown timers

Curriculum • A count to infinity problem can be avoided by using holddown timers. • When a router receives an update from a neighbor indicating that a previously accessible network is now inaccessible, the router marks the route as inaccessible and starts a hold-down timer.

Rick Graziani [email protected]

36

Preventing routing loops with holddown timers

Same Route from same neighbor: Network is back up (Correct News)

•

If at any time before the hold-down timer expires an update is received from the same neighbor indicating that the network is again accessible, the router marks the network as accessible and removes the holddown timer.

Rick Graziani [email protected]

37

Preventing routing loops with holddown timers

Better Route from different neighbor (Correct News)

•

If at any time before the hold-down timer expires an update arrives from a different neighboring router with a better metric than originally recorded for the network, the router marks the network as accessible and removes the hold-down timer.

Rick Graziani [email protected]

38

Preventing routing loops with holddown timers

Poorer Route from a different neighbor. (Incorrect News)

• •

If at any time before the hold-down timer expires an update arrives from a different neighboring router with a poorer metric than originally recorded for the network the update is ignored and the hold-down timer continues. Ignoring an update with a poorer metric when a hold-down is in effect allows more time for the knowledge of a disruptive change to propagate through the entire network. Rick Graziani [email protected]

39

Preventing routing loops with holddown timers

Additional Information on Holddown Timers Flapping routes • Holddown timers not only help prevent routing loops during transient periods but also help network stability by dampening unstable, flapping routes (routes which continuously go up and down). Holddown Time • As we will see with both RIP and IGRP, the amount of time the router remains in the holddown state can be modified (with caution!), even set to 0. will look at this later in the presentations on RIP and IGRP. • RickWe Graziani [email protected]

40

Preventing routing loops with holddown timers

Additional Information on Holddown Timers

Packet forwarding • Even though routing tables remain constant and routers do not accept potentially bad updates, an interesting question is whether or not routers should continue use the existing routes that are in holddown state for forwarding packets? • “In practice, routes in the holddown state are used for packet forwarding. In the worst case, packets are forwarded toward the router that was previously connected to the destination network, which drops them. In the best case, they are forwarded along a potentially suboptimal but valid path.” (Zinin, Cisco IP Routing) Rick Graziani [email protected]

41

Avoiding routing loops with triggered updates

•

Triggered update is sent immediately in response to some change in the routing table. • The router that detects a topology change immediately sends an update message to adjacent routers that, in turn, generate triggered updates notifying their adjacent neighbors of the change. • When a route fails, an update is sent immediately rather than waiting on the update timer to expire. • Triggered updates, used in conjunction with route poisoning, ensure that all routers know of failed routes before any holddown timers can expire. Rick Graziani [email protected] 42

IP’s TTL – Time To Live field IP Header 0 4-bit Version

15 16 4-bit Header Length

8-bit Type Of Service (TOS)

16-bit Total Length (in bytes) 3-bit Flags

16-bit Identification 8 bit Time To Live TTL

31

8-bit Protocol

13-bit Fragment Offset 16-bit Header Checksum

32-bit Source IP Address 32-bit Destination IP Address Options (if any) Data

Let’s look at a related item in IP, the TTL field. Taken from information added to Ch. 9 TCP/IP. Rick Graziani [email protected]

43

IP’s TTL – Time To Live field IP Header 0 4-bit Version

15 16 4-bit Header Length

8-bit Type Of Service (TOS)

16-bit Total Length (in bytes) 3-bit Flags

16-bit Identification 8 bit Time To Live TTL

31

8-bit Protocol

13-bit Fragment Offset 16-bit Header Checksum

32-bit Source IP Address 32-bit Destination IP Address Options (if any) Data

• • •

When a packet is first generated a value is entered into the TTL field. Originally, the TTL field was the number of seconds, but this was difficult to implement and rarely supported. Now, the TTL is now set to a specific value which is then decremented by each router. Rick Graziani [email protected]

44

IP’s TTL – Time To Live field IP Header 0 4-bit Version

15 16 4-bit Header Length

8-bit Type Of Service (TOS)

16-bit Total Length (in bytes) 3-bit Flags

16-bit Identification 8 bit Time To Live TTL

31

8-bit Protocol

13-bit Fragment Offset 16-bit Header Checksum

32-bit Source IP Address

Decrement by 1, if 0 drop the packet.

• •

32-bit Destination IP Address Options (if any)

If the router decrements the TTL field to 0, it will then drop the packet (unless the packet is destined specifically for the router,Data I.e. ping, telnet, etc.). Common operating system TTL values are: – UNIX: 255 – Linux: 64 or 255 depending upon vendor and version – Microsoft Windows 95: 32 – Other Microsoft Windows operating systems: 128

Rick Graziani [email protected]

45

http://www.switch.ch/docs/ttl_default.html TTL Overview - Disclaimer: The following list is a best effort overview of some widely used TCP/IP stacks. The information was provided by vendors and many helpful system administrators. We would like to thank all these contributors for their precious help ! SWITCH cannot, however, take any responsibility that the provided information is correct. Furthermore, SWITCH cannot be made liable for any damage that may arise by the use of this information. +--------------------+-------+---------+---------+ | OS Version |"safe" | tcp_ttl | udp_ttl | +--------------------+-------+---------+---------+ AIX n 60 30 DEC Pathworks V5 n 30 30 FreeBSD 2.1R y 64 64 HP/UX 9.0x n 30 30 HP/UX 10.01 y 64 64 Irix 5.3 y 60 60 Irix 6.x y 60 60 Linux y 64 64 MacOS/MacTCP 2.0.x y 60 60 OS/2 TCP/IP 3.0 y 64 64 OSF/1 V3.2A n 60 30 Solaris 2.x y 255 255 SunOS 4.1.3/4.1.4 y 60 60 Ultrix V4.1/V4.2A n 60 30 VMS/Multinet y 64 64 VMS/TCPware y 60 64 VMS/Wollongong 1.1.1.1 n 128 30 VMS/UCX (latest rel.) y 128 128 MS WfW n 32 32 MS Windows 95 n 32 32 MS Windows NT 3.51 n 32 32 MS Windows NT 4.0 y 128 128

Rick Graziani [email protected]

Assigned Numbers (RFC 1700, J. Reynolds, J. Postel, October 1994): IP TIME TO LIVE PARAMETER The current recommended default time to live (TTL) for the Internet Protocol (IP) is 64.

Safe: TCP and UDP initial TTL values should be set to a "safe" value of at least 60 today.

46

IP’s TTL – Time To Live field IP Header 0 4-bit Version

15 16 4-bit Header Length

8-bit Type Of Service (TOS)

16-bit Total Length (in bytes) 3-bit Flags

16-bit Identification 8 bit Time To Live TTL

31

8-bit Protocol

13-bit Fragment Offset 16-bit Header Checksum

32-bit Source IP Address

Decrement by 1, if 0 drop the packet.

32-bit Destination IP Address Options (if any) Data

• •

The idea behind the TTL field is that IP packets can not travel around the Internet forever, from router to router. Eventually, the packet’s TTL which reach 0 and be dropped by the router, even if there is a routing loop somewhere in the network. Rick Graziani [email protected]

47

RIP routing process

• •

Request for Comments (RFC) 1058 RIP has evolved over the years from a Classful Routing Protocol, RIP Version 1 (RIP v1), to a Classless Routing Protocol, RIP Version 2 (RIP v2). RIP v2 enhancements include: – Ability to carry additional packet routing information. – Authentication mechanism to secure table updates. – Supports variable length subnet masking (VLSM).

Rick Graziani [email protected]

48

Configuring RIP

Rick Graziani [email protected]

49

Configuring RIP

RIP and IGRP: • Classful network statements only • IOS will take subnetted networks but will translate it into the classful network for the running-config. Rick Graziani [email protected]

50

Configuring RIP

Rick’s Clarifications (This is for IGPs only and not EGPs such as BGP): • The network command does two things: 1. Determines which interfaces will participate in sending and receiving routing updates, as long as the interface IP address falls in the range of the network command. 2. Determines which networks this router will announce as being directly connected to in its routing updates to other routers. • The network numbers do not necessarily have to be based on the network class, as it depends on the routing protocol. Network numbers are based on the network class for RIP, IGRP, and usually EIGRP, but can be more specific for OSPF, EIGRP and IS-IS. Rick Graziani [email protected]

51

Triggered Extensions

No, this command refers to triggered extensions, not triggered updates! Triggered updates are already enabled in Cisco RIP. • “A router running RIP can be configured to send a triggered update when the network topology changes using the ip rip triggered command. This command is issued only on serial interfaces at the router(config-if)# prompt. After updating its routing table due to a configuration change, the router immediately begins transmitting routing updates in order to inform other network routers of the change. These updates, called triggered updates, are sent independently of the regularly scheduled updates that RIP routers forward.” Rick Graziani [email protected]

52

Triggered Extensions interface serial 0 ip rip triggered

Triggered Extensions to RIP • http://www.cisco.com/en/US/products/sw/iosswrel/ps1830/products_feature_guide0 • There were two problems using RIP to connect to a WAN: – Periodic broadcasting by RIP generally prevented WAN circuits from being closed. – Even on fixed, point-to-point links, the overhead of periodic RIP transmissions could seriously interrupt normal data transfer because of the quantity of information that hits the line every 30 seconds. • To overcome these limitations, triggered extensions to RIP cause RIP to send information on the WAN only when there has been an update to the routing database. • Periodic update packets are suppressed over the interface on which this feature is enabled. Rick Graziani [email protected]

53

Triggered Extensions interface serial 0 ip rip triggered

• •

•

RFC 2091, Triggered Extensions to RIP to Support Demand Circuits. When triggered extensions to RIP are enabled, routing updates are transmitted on the WAN only if one of the following occurs: – The router receives a specific request for a routing update. (Full database is sent.) – Information from another interface modifies the routing database. (Only latest changes are sent) – The interface comes up or goes down. (Partial database is sent.) – The router is first powered on, to ensure that at least one update is sent. (Full database is sent.) You might want to enable this feature if you are using an on-demand circuit and you are charged for usage time. Fewer routing updates will incur lower usage costs. Rick Graziani [email protected]

54

The RIPv1 Protocol Data Link Frame Header

IP Packet Header

UDP Segment Header

RIP Message

RIP Message Data Link Frame • MAC Source Address • MAC Destination Address = Broadcast IP Packet • IP Source Address • IP Destination Address = Broadcast: 255.255.255.255 • Protocol field = 17 for UDP UDP Segment • Source Port number field = 520 for RIP Message RIP Message (Data portion of IP Packet): • Routes: Network IP Address • Hops (metric) Rick Graziani [email protected]

55

Data Link Frame Header

IP Packet Header

UDP Segment Header

RIP Message

0 7 8 15 16 23 24 Command = 1 or 2 Version = 1 Must be zero Address family identifier (2 = IP) Must be zero IP Address (Network Address) Must be zero Must be zero Metric (Hops)

31

Multiple Routes, up to a maximum of 25 Address family identifier (2 = IP) Must be zero IP Address (Network Address) Must be zero Must be zero Metric (Hops)

• • • • • •

Command: 1 signifying a Request or 2 signifying a Reply Version: 1 for RIP v 1 or 2 for RIP v 2 Address Family Identifier: 2 signifying IP (only exception is for a Request for the Router’s full routing table, later Semester in RIP v 2) IP Address: The address of the destination route, which may be a network address, a subnet address of a host address. Metric: Hop count between 1 and 16. Note: With RIP the sending router increases the metric before sending out the RIP message. Note: The routing table knows the next-hop-ip-address (via) from the source IP address of the packet. Rick Graziani [email protected]

56

RIP v2 message format

• •

All the extensions to the original protocol are carried in the unused fields. The Address Family Identifier (AFI) field is set to two for IP. The only exception is a request for a full routing table of a router or host, in which case it will be set to zero. Rick Graziani [email protected]

57

RIP v2 message format

• • • •

The Route Tag field provides a way to differentiate between internal and external routes. External routes are those that have been redistributed into the RIP v2. The Next Hop field contains the IP address of the next hop listed in the IP Address field. Metric indicates how many internetwork hops, between 1 and 15 for a valid route, or 16 for an unreachable route. Rick Graziani [email protected]

58

Configuring RIP

RIP must be enabled and the networks specified. The remaining tasks are optional. Among these optional tasks are: • Applying offsets to routing metrics (Not commonly used) • Adjusting timers • Specifying a RIP version (RIPv1 or RIPv2) • Enabling RIP authentication • Configuring route summarization on an interface • Verifying IP route summarization • Disabling automatic route summarization (RIPv2) • Running IGRP and RIP concurrently (Usually, redistributing, not concurrently.) • Disabling the validation of source IP addresses • Enabling or disabling split horizon • Connecting RIP to a WAN Rick Graziani [email protected]

59

ip classless command •

IP classless only affects the operation of the forwarding processes in IOS. IP classless does not affect the way the routing table is built.

•

This command concerns classless and classful routing behavior, which is not the same as classless and classful routing protocols (later).

•

To discuss this command, we will use information which is not in the curriculum.

•

For more information: – The Routing Table: Part 1 or 2 - The Routing Table Structure (PDF) – The Routing Table: Part 2 or 2 - The Routing Table Lookup Process (PDF)

Rick Graziani [email protected]

Parent and Child Routes RouterB#show ip route

R C C C S S S*

172.16.0.0/24 is subnetted, 3 subnets 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 172.16.2.0 is directly connected, Serial0 172.16.3.0 is directly connected, FastEthernet0 192.168.1.0/24 is directly connected, Serial1 172.0.0.0/8 is directly connected, Serial1 160.0.0.0/4 is directly connected, Serial1 0.0.0.0/0 is directly connected, Serial1

Parent Route • Created automatically whenever there is a route with a mask greater than the classful mask. • For non-VLSM routes, contains the mask of the child routes. Child Routes • Routes with masks greater than the default classful mask. Rick Graziani [email protected]

Lookup what? RouterB#show ip route

R C C C S S S*

172.16.0.0/24 is subnetted, 3 subnets 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 172.16.2.0 is directly connected, Serial0 172.16.3.0 is directly connected, FastEthernet0 192.168.1.0/24 is directly connected, Serial1 172.0.0.0/8 is directly connected, Serial1 160.0.0.0/4 is directly connected, Serial1 0.0.0.0/0 is directly connected, Serial1

Routing Table process matches: • The routing table process compares the left-most bits in the packet’s destination IP address with the left-most bits in the route in the routing table, looking for a longest-bit-match. • The subnet mask of the route in the routing table specifies the minimum number of left-most bits that must match. • Before checking child routes, the classful mask of the parent route is used. • For child routes the parent route’s mask is used. • For VLSM routes, the mask is contained with the child route. Rick Graziani [email protected]

Parent and Child Routes RouterB#show ip route

R C C C S S S*

172.16.0.0/24 is subnetted, 3 subnets 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 172.16.2.0 is directly connected, Serial0 172.16.3.0 is directly connected, FastEthernet0 192.168.1.0/24 is directly connected, Serial1 172.0.0.0/8 is directly connected, Serial1 160.0.0.0/4 is directly connected, Serial1 0.0.0.0/0 is directly connected, Serial1

DA = 192.168.1.10 • 16 bits of 172.16.0.0 do not match, so child routes are not checked. • 24 bits of 192.168.1.0/24 do match, so this route is used.

Rick Graziani [email protected]

Parent and Child Routes RouterB#show ip route

R C C C S S S*

172.16.0.0/24 is subnetted, 3 subnets 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 172.16.2.0 is directly connected, Serial0 172.16.3.0 is directly connected, FastEthernet0 192.168.1.0/24 is directly connected, Serial1 172.0.0.0/8 is directly connected, Serial1 160.0.0.0/4 is directly connected, Serial1 0.0.0.0/0 is directly connected, Serial1

DA = 172.16.2.1 • 16 bits of 172.16.0.0 do match, so child routes are checked. • 24 bits of 172.16.1.0 do not match, so continue to next child route. • 24 bits of 172.16.2.0 do match, so this route is used!

Rick Graziani [email protected]

Parent and Child Routes RouterB#show ip route

R C C C S S S*

172.16.0.0/24 is subnetted, 3 subnets 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 172.16.2.0 is directly connected, Serial0 172.16.3.0 is directly connected, FastEthernet0 192.168.1.0/24 is directly connected, Serial1 172.0.0.0/8 is directly connected, Serial1 160.0.0.0/4 is directly connected, Serial1 0.0.0.0/0 is directly connected, Serial1

DA = 32.1.1.10 • 16 bits of 172.16.0.0 do not match, so child routes are not checked. • 24 bits of 192.168.1.0/24 do not match, so this route is not used. • 8 bits of 172.0.0.0/8 do not match, so this route is not used. • 4 bits of 160.0.0.0/4 do not match, so this route is not used. • 0 bits of 0.0.0.0/0 does match, so this route is used! Rick Graziani [email protected]

Parent and Child Routes RouterB#show ip route

R C C C S S S*

172.16.0.0/24 is subnetted, 3 subnets 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 172.16.2.0 is directly connected, Serial0 172.16.3.0 is directly connected, FastEthernet0 192.168.1.0/24 is directly connected, Serial1 172.0.0.0/8 is directly connected, Serial1 160.0.0.0/4 is directly connected, Serial1 0.0.0.0/0 is directly connected, Serial1

DA = 172.16.4.1 • 16 bits of 172.16.0.0 do match, so child routes are checked. • 24 bits of 172.16.1.0 do not match, so continue to next child route. • 24 bits of 172.16.2.0 do not match, so continue to next child route. • 24 bits of 172.16.3.0 do not match, no more child routes. Now what??? It depends! Rick Graziani [email protected]

Classful Routing Behavior RouterB#show ip route

R C C C S S S*

172.16.0.0/24 is subnetted, 3 subnets 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 172.16.2.0 is directly connected, Serial0 172.16.3.0 is directly connected, FastEthernet0 192.168.1.0/24 is directly connected, Serial1 172.0.0.0/8 is directly connected, Serial1 160.0.0.0/4 is directly connected, Serial1 0.0.0.0/0 is directly connected, Serial1

DA = 172.16.4.1 Router(config)# no ip classless • With classful routing behavior, if the child routes are checked but there are no matches, the routing lookup process ends and the Packet is dropped. (The packets get in, but they can’t get out!) • Supernet and default routes are not checked. • Default with IOS 11.2 and prior Rick Graziani [email protected]

Classless Routing Behavior RouterB#show ip route

R C C C S S S*

172.16.0.0/24 is subnetted, 3 subnets 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 172.16.2.0 is directly connected, Serial0 172.16.3.0 is directly connected, FastEthernet0 192.168.1.0/24 is directly connected, Serial1 172.0.0.0/8 is directly connected, Serial1 160.0.0.0/4 is directly connected, Serial1 0.0.0.0/0 is directly connected, Serial1

DA = 172.16.4.1 Router(config)# ip classless • With classless routing behavior, if the child routes are checked but there are no matches, the routing lookup process continues with other routes in the routing table, including supernet and default routes. • 8 bits of 172.0.0.0/8 do match, so this route is used! • Default with IOS 11.3 and later Rick Graziani [email protected]

Common RIP Configuration Issues Split Horizon • The following command is used to disable split horizon: GAD(config-if)#no ip split-horizon

•

The following command is used to enable (default) split horizon: GAD(config-if)#ip split-horizon

Rick Graziani [email protected]

69

Common RIP Configuration Issues Holddown Timer • The ideal setting would be to set the timer just longer that the longest possible update time for the internetwork. • To change the holddown timer: Router(config-router)#timers basic update invalid holddown flush [sleeptime]

Rick Graziani [email protected]

70

Common RIP Configuration Issues Update Timer • The default RIP update interval in Cisco IOS is 30 seconds. This can be configured for longer intervals to conserve bandwidth, or for shorter intervals to decrease convergence time. • To change the update internal: GAD(config-router)#update-timer seconds

Rick Graziani [email protected]

71

Common RIP Configuration Issues router rip passive-interface fastethernet 0/0

For RIP and IGRP, the passive interface command stops the router from sending updates to a particular neighbor, but the router continues to listen and use routing updates from that neighbor. (More later.) • Also used when there are no routers on that interface, such as stub LANs. Router(config-router)# passive-interface interface

Rick Graziani [email protected]

72

Common RIP Configuration Issues

• • •

Because RIP is a broadcast protocol, the network administrator may have to configure RIP to exchange routing information in a nonbroadcast network such as Frame Relay. In this type of network, RIP needs to be told of other neighboring RIP routers. To do this use the router rip command: Router(config-router)# neighbor ip address

Rick Graziani [email protected]

73

Common RIP Configuration Issues • •

By default, the Cisco IOS software receives RIP Version 1 and Version 2 packets, but sends only Version 1 packets. The network administrator can configure the router to only receive and send Version 1 packets or the administrator can configure the router to send only Version 2 packets.

Rick Graziani [email protected]

74

Compatibility with RIP v1 NewYork interface fastethernet0/0 ip address 192.168.50.129 255.255.255.192 ip rip send version 1 ip rip receive version 1 RIPv2

interface fastethernet0/1 ip address 172.25.150.193 255.255.255.240 ip rip send version 1 2

• • •

Interface FastEthernet0/0 is configured to send and receive RIP v1 updates. FastEthernet0/1 is configured to send both version 1 and 2 updates. FastEthernet0/2 has no special configuration and therefore sends and receives version 2 by default. Rick Graziani [email protected]

interface fastethernet0/2 ip address 172.25.150.225 225.255.255.240 router rip version 2 network 172.25.0.0 network 192.168.50.0

75

Verifying RIP configuration

Rick Graziani [email protected]

76

Verifying RIP configuration

•

Also: show running-config

Rick Graziani [email protected]

77

Troubleshooting RIP update issues

Rick Graziani [email protected]

78

Troubleshooting RIP update issues Other commands to troubleshoot RIP: • show ip rip database • show ip protocols {summary} • show ip route • debug ip rip {events} • show ip interface brief

Rick Graziani [email protected]

79

Load balancing with RIP • • •

RIP is capable of load balancing over as many as six equal-cost paths, with four paths being default. RIP performs what is referred to as “round robin” load balancing. This means that RIP takes turns forwarding packets over the parallel paths. This is only part of the story…

Rick Graziani [email protected]

80

Fast Switching and Process Switching The following information is taken from Routing TCP/IP Volume I by Jeff Doyle.

• •

•

•

Load sharing or Load balancing allows routers to take advantage of multiple paths to the same destination. Equal-cost load balancing: – Distributes packets equally among multiple paths with equal metrics – RIP, IGRP, EIGRP, OSPF, IS-IS and BGP Unequal-cost load balancing: – Distributes packets among multiple paths with different metrics, inversely proportional to the cost of the routes. – EIGRP Load sharing can be either: – Per Destination (Fast Switching) – Per Packet ( Process Switching) Rick Graziani [email protected]

81

Fast Switching – Per Destination Load Balancing Router(config-if)# ip route-cache ping 10.0.0.2

• • •

• •

ping 10.0.0.1

The default for most interfaces is Fast Switching. Load balancing is distributed according to the destination IP address. Given two paths to the same network, all packets for one destination IP address will travel over the first path, all packets for a second destination will travel over the second path, all packets for the third destination will again travel over the first path, and so on. To enable fast switching: Router(config-if)# ip route-cache To enable distributed or process switching: Router(config-if)# no ip route-cache Rick Graziani [email protected]

82

Fast Switching – Per Destination Load Balancing Router(config-if)# ip route-cache ping 10.0.0.2

ping 10.0.0.1

Fast Switching 2. Router switches first packet to a particular destination, a routing table lookup is performed and an exit interface is selected. 3. The necessary data-link information to frame the packet for the selected interface is retrieved including any ARP cache information. 4. The route and data-link information is stored in fast switching cache. 5. The router uses the cache to look up subsequent packets. 6. All other packets to the same destination are immediately switched out the same interface without the router performing another routing table lookup, including any recursive lookups. (Also no ARP cache lookup). Rick Graziani [email protected]

83

Process Switching – Per Packet Load Balancing Router(config-if)#no ip route-cache ping 10.0.0.2

ping 10.0.0.1

Process Switching • Given equal cost paths, per packet load sharing means that one packet to a destination is sent over one link, the next packet to the same destination is sent over the next link, and so on. • If the paths are unequal cost, the load balancing may be one packet over the higher-cost link for every three packets over the lower-cost link, or similar ratio. • With process switching, for every packet, the router performs a route table lookup and selects an interface, and looks up the data-link information. • To enable distributed or process switching: Router(config-if)# no ip route-cache Rick Graziani [email protected]

84

Which one? Fast Switching

ping 10.0.0.2

ping 10.0.0.1

Router(config-if)# ip route-cache

Process Switching

ping 10.0.0.2

ping 10.0.0.1

Router(config-if)#no ip route-cache

Fast Switching or Process Switching • Process switching (per packet load balancing) has a price, load balancing may be distributed more evenly but the lower switching time and processor utilization of fast switching are lost. Rick Graziani [email protected]

85

Using debug ip packet with Fast Switching and Process Switching Router# debug ip packet IP: s=192.168.3.2 (FastEthernet0), g=192.168.1.2, forward IP: s=192.168.3.2 (FastEthernet0), g=192.168.2.2, forward IP: s=192.168.3.2 (FastEthernet0), g=192.168.1.2, forward IP: s=192.168.3.2 (FastEthernet0), g=192.168.2.2, forward

• •

d=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/1), d=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/1),

debug ip packet can be used to observe packets sent and received and the interfaces that are involved. IMPORTANT: The debug ip packet command allows only process switched packets to be observed. Fast switch packets are not displayed (except for the first packet in the flow).

Rick Graziani [email protected]

86

Load balancing across multiple paths • • • • •

Note: The example used in this section of the online curriculum is really for IGRP/EIGRP and does not fit well in this section of RIP. By default, most IP routing protocols install a maximum of four parallel routes in a routing table. Static routes always install six routes. The exception is BGP, which by default allows only one path to a destination. The range of maximum paths is one to six paths. To change the maximum number of parallel paths allowed, use the following command in router configuration mode: Router(config-router)#maximum-paths [number]

Rick Graziani [email protected]

87

RIP and Administrative Distance

Rick Graziani [email protected]

88

RIP and Floating Static Routes

X

172.16.0.0/16

router rip network 192.168.14.0 ip route 172.16.0.0 255.255.0.0 bri0/1 130

• • •

Floating static routes are static routes which are used as backup routes. They are only injected into the routing table when a route with a lower administrative distance (dynamic or another static route) goes down. Should the route with the lower administrative distance come back up then the floating static route is removed from the routing table. Rick Graziani [email protected]

89

Redistribute Static 172.16.0.0/16

RIP

• • •

RouterA ip route 172.16.0.0 255.255.0.0 eth 0 Router rip redistribute static network ….

Redistributes static routes into the dynamic routing domain. 172.16.0.0/16 will be seen by other RIP routers as a dynamic route learned via RIP. The default metric is 0, so B and D will have a hop count of 1, where C will have a hop count of 2.

Rick Graziani [email protected]

90

RIPv1 Labs – 3 Scenarios

• • •

Read the following lab. In groups review the configurations and the outputs. Afterwards, we will discuss the this lab together, paying particular attention to the Reflection sections.

Rick Graziani [email protected]

91

RIPv1 Labs – 3 Scenarios Objective • In this lab, you will configure RIP routing in three different scenarios. • At the end of each scenario, all hosts and all routers should be able to reach (ping) each other. Scenario There are five separate classful networks. After configuring RIP, we want to view the RIP update messages being sent and received by each router.

• • •

Scenario 1: Running RIPv1 on classful networks Scenario 2: Running RIPv1 on subnets and between classful networks Scenario 3: Running RIPv1 on a stub network

These three scenarios can be done in sequence or separately.

Rick Graziani [email protected]

92

RIPv1 Labs – 3 Scenarios Setup • Use the 8 Steps to Success to help you configure the routers. • Be sure your cabling is correct, as this causes more troubleshooting issues than anything else. • If the routers have a startup-config already on them, erase it and reboot the routers. • Configure the routers to include hostnames and the proper interface commands including IP addresses, subnet masks, etc. • Each router should be able to ping the interface of the adjacent (neighboring) router and the host on its LAN (Ethernet) interface. • Test and troubleshoot as necessary. Basic Configurations • There is a Basic Configuration included for each scenario, but it does not include clock rate, no shutdown and some other necessary commands. • Note: Even though some of the networks are in numerical order, obviously this does not need to be the case. We only did this to make it easier to remember where the networks originated from. Rick Graziani [email protected]

93

RIPv1 Labs – 3 Scenarios Optional: Keeping outputs from interrupting our inputs Before we begin to configure RIP, lets configure the console 0 port to keep debug and other output messages from interrupting our input. Use the following command on each router to keep the debug out from interfering with you command-line input: Router(config)# line console 0 Router(config-line)# logging synchronous Optional: Changing the default timeout After 10 minutes, by default, if there is no input via the console, the user will be logged off. Although a good idea in production environment, in a lab environment this can be somewhat annoying. To turn-off the automatic timeout feature, we use the command: exec-timeout minutes [seconds], setting both the minutes and seconds to 0. Router(config)# line console 0 Router(config-line)# exec-timeout 0 0 Rick Graziani [email protected]

94

Scenario 1: Running RIPv1 on classful networks SanJose2 hostname SanJose2 interface ethernet 0 ip add 192.168.1.1 255.255.255.0 interface serial 0 ip add 192.168.2.1 255.255.255.0 SanJose1 hostname SanJose1 interface ethernet 0 ip add 192.168.3.1 255.255.255.0 interface serial 0 ip add 192.168.2.2 255.255.255.0 interface serial 1 ip add 192.168.4.2 255.255.255.0 Baypointe hostname Baypointe interface ethernet 0 ip add 192.168.5.1 255.255.255.0 interface serial 0 ip add 192.168.4.1 255.255.255.0

Rick Graziani [email protected]

95

Scenario 1: Running RIPv1 on classful networks Objective: Running RIPv1 on classful networks This scenario is the same one we used in the network discovery lab, with the same configurations and the same outputs. The concepts specific to this scenario will become more clear when we view the differences between this scenario and Scenario 2: Running RIPv1 on subnets and between classful networks. Step 1 – Configuring RIP First, lets enable RIP on each router. From global configuration you will enter the command (the default is RIPv1): Router(config)#router rip Once you are in the Router RIP configuration sub-mode, all you need to do is enter the classful network address for each directly connected network, using the network command. Router(config-router)#network directly-connected-classful-networkaddress

Rick Graziani [email protected]

96

Scenario 1: Running RIPv1 on classful networks Here are the commands for each router: SanJose2#configure terminal Enter configuration commands, one per line. SanJose2(config)#router rip SanJose2(config-router)#network 192.168.1.0 SanJose2(config-router)#network 192.168.2.0

End with CNTL/Z.

Baypointe#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Baypointe(config)#router rip Baypointe(config-router)#network 192.168.4.0 Baypointe(config-router)#network 192.168.5.0 SanJose1#configure terminal Enter configuration commands, one per line. SanJose1(config)#router rip SanJose1(config-router)#network 192.168.2.0 SanJose1(config-router)#network 192.168.3.0 SanJose1(config-router)#network 192.168.4.0 Rick Graziani [email protected]

End with CNTL/Z.

97

Step 2 – Understanding the network command SENDING RIP MESSAGES Each router will begin to send RIP update message out each interface belonging to one of the network statements. SanJose2(config)#router rip SanJose2(config-router)#network 192.168.1.0 SanJose2(config-router)#network 192.168.2.0 For example, SanJose2 to will send out RIP update messages on Ethernet 0 because that interface has an IP address that belong to the network 192.168.1.0, and on Serial 0 because that interface has an IP address that belongs to the network 192.168.2.0. Just because a router has a directly connected network does not mean it will automatically include that network in its routing updates to neighboring routers. The network command also tells the RIP to include these networks in its updates to adjacent neighbors. To view the RIP messages being sent and received use the debug ip rip command. SanJose2# debug ip rip RIP protocol debugging is on SanJose2 01:03:27: RIP: sending v1 update to 01:03:27: network 192.168.2.0, 01:03:27: RIP: sending v1 update to 01:03:27: network 192.168.1.0,

Rick Graziani [email protected]

255.255.255.255 via Ethernet0 (192.168.1.1) metric 1 255.255.255.255 via Serial0 (192.168.2.1) metric 1

98

Scenario 1: Running RIPv1 on classful networks LISTENING FOR RIP MESSAGES Routers will also listen for RIP messages on each interface belonging to one of the network statements. For example, SanJose2 to will listen for RIP update messages on Ethernet 0 because that interface has an IP address that belong to the network 192.168.1.0, and also listen for RIP update messages on Serial 0 because that interface has an IP address that belongs to the network 192.168.2.0. As RIP messages are received router, will add those networks in the messages to their routing tables: If the RIP message contains a network not currently in the routing table. If the RIP message contains a network with a better metric (fewer hops) than an entry currently in the routing table. SanJose2 01:10:56: RIP: received v1 update from 192.168.2.2 on Serial0 01:10:56: 192.168.4.0 in 1 hops 01:10:56: 192.168.3.0 in 1 hops Rick Graziani [email protected]

99

Scenario 1: Running RIPv1 on classful networks Step 3 – Viewing the debug ip rip output and the routing tables Remember that SanJose1 will learn routes to networks from SanJose2. It will then send that information to Baypointe, telling Baypointe that it is the next hop to get to those networks, and incrementing the metric (hop count) by one. After convergence, each router will continue to send its RIP update messages out the appropriate interfaces every 30 seconds. Lets look at the debug messages and the routing table for each router:

Rick Graziani [email protected]

100

SanJose2 01:30:45: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (192.168.1.1) 01:30:45: network 192.168.4.0, metric 2 01:30:45: network 192.168.5.0, metric 3 01:30:45: network 192.168.2.0, metric 1 01:30:45: network 192.168.3.0, metric 2 01:30:45: RIP: sending v1 update to 255.255.255.255 via Serial0 (192.168.2.1) 01:30:45: network 192.168.1.0, metric 1 SanJose2# 01:30:50: RIP: received v1 update from 192.168.2.2 on Serial0 01:30:50: 192.168.4.0 in 1 hops 01:30:50: 192.168.5.0 in 2 hops 01:30:50: 192.168.3.0 in 1 hops SanJose2# SanJose2#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR Gateway of last resort is not set R 192.168.4.0/24 R 192.168.5.0/24 C 192.168.1.0/24 C 192.168.2.0/24 R 192.168.3.0/24 SanJose2#

[120/1] via [120/2] via is directly is directly [120/1] via

Rick Graziani [email protected]

192.168.2.2, 00:00:10, Serial0 192.168.2.2, 00:00:10, Serial0 connected, Ethernet0 connected, Serial0 192.168.2.2, 00:00:10, Serial0

101

SanJose1 01:33:05: 01:33:05: SanJose1# 01:33:07: 01:33:07: 01:33:08: 01:33:08: 01:33:08: 01:33:08: 01:33:08: 01:33:08: 01:33:08: 01:33:08: 01:33:08: 01:33:08: 01:33:08: 01:33:08: 01:33:08:

RIP: received v1 update from 192.168.4.1 on Serial1 192.168.5.0 in 1 hops RIP: received v1 update from 192.168.2.1 on Serial0 192.168.1.0 in 1 hops RIP: sending v1 update to 255.255.255.255 via Ethernet0 (192.168.3.1) network 192.168.4.0, metric 1 network 192.168.5.0, metric 2 network 192.168.1.0, metric 2 network 192.168.2.0, metric 1 RIP: sending v1 update to 255.255.255.255 via Serial0 (192.168.2.2) network 192.168.4.0, metric 1 network 192.168.5.0, metric 2 network 192.168.3.0, metric 1 RIP: sending v1 update to 255.255.255.255 via Serial1 (192.168.4.2) network 192.168.1.0, metric 2 network 192.168.2.0, metric 1 network 192.168.3.0, metric 1

SanJose1#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP Gateway of last resort is not set C 192.168.4.0/24 is directly connected, Serial1 R 192.168.5.0/24 [120/1] via 192.168.4.1, 00:00:12, Serial1 R 192.168.1.0/24 [120/1] via 192.168.2.1, 00:00:10, Serial0 C 192.168.2.0/24 is directly connected, Serial0 C 192.168.3.0/24 is directly connected, Ethernet0 Rick Graziani [email protected]

102

Baypointe 01:34:53: RIP: 01:34:53: 01:34:53: 01:34:53: 01:34:53: 01:34:53: RIP: 01:34:53: Baypointe# 01:34:56: RIP: 01:34:56: 01:34:56: 01:34:56:

sending network network network network sending network

v1 update to 192.168.4.0, 192.168.1.0, 192.168.2.0, 192.168.3.0, v1 update to 192.168.5.0,

received v1 192.168.1.0 192.168.2.0 192.168.3.0

255.255.255.255 via Ethernet0 (192.168.5.1) metric 1 metric 3 metric 2 metric 2 255.255.255.255 via Serial0 (192.168.4.1) metric 1

update from 192.168.4.2 on Serial0 in 2 hops in 1 hops in 1 hops

Baypointe#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR Gateway of last resort is not set C C R R R

192.168.4.0/24 192.168.5.0/24 192.168.1.0/24 192.168.2.0/24 192.168.3.0/24

is directly is directly [120/2] via [120/1] via [120/1] via

Rick Graziani [email protected]

connected, Serial0 connected, Ethernet0 192.168.4.2, 00:00:23, Serial0 192.168.4.2, 00:00:23, Serial0 192.168.4.2, 00:00:23, Serial0

103

Scenario 1: Running RIPv1 on classful networks NOTE: At this point all routers should be able to ping all networks. We will discuss RIP much more in the chapter on Routing Protocols (RIP). Step 4 – Turning-off debug Don’t forget to turn-off debug when you are done collecting the output. Router# undebug all or Baypointe# undebug ip rip Step 5 – Reflections • For each router compare the RIP received messages with its routing table. Now you see how the information is entered into the routing table. • Cisco IOS uses split horizon with poison reverse, however this information is not displayed with debug ip rip command. • You will notice that the routers send RIP messages out their stub Ethernet interfaces, even though there are no routers out there to receive those messages. This does take up unnecessary bandwidth on the link; so later we will see how to keep those RIP messages from going out those interfaces. Rick Graziani [email protected]

104

Scenario 2: Running RIPv1 on subnets and between classful networks Note: This lab has some important information regarding RIP and boundary routers!

SanJose2 hostname SanJose2 interface ethernet 0 ip add 172.30.1.1 255.255.255.0 interface serial 0 ip add 172.30.2.1 255.255.255.0

SanJose1 hostname SanJose1 interface ethernet 0 ip add 172.30.3.1 255.255.255.0 interface serial 0 ip add 172.30.2.2 255.255.255.0 interface serial 1 ip add 192.168.4.9 255.255.255.252 Baypointe hostname Baypointe interface ethernet 0 ip add 192.168.5.1 255.255.255.0 interface serial 0 ip add 192.168.4.10 255.255.255.252

Rick Graziani [email protected]

105

Scenario 2: Running RIPv1 on subnets and between classful networks Objective: Running RIPv1 on subnets and between classful networks In this scenario we will see how subnetted routes are distributed with the same classful network. We will also see how RIPv1 automatically summarizes between classful network boundaries. You will notice that SanJose1 and SanJose2 have subnets belonging to the 172.30.0.0 network, but Baypointe does not. Making changes between Scenario 1 and Scenario 2 Be sure to change the IP addressing as displayed in the diagram and Basic Configuration section for Scenario 2. Sometimes when changing the IP address on a serial interface, you may need to reset that interface by doing a shutdown, wait for the LINK-5-CHANGED message, then follow it with a no shutdown command. If you have just completed Scenario 1, lets remove RIP by issuing the following command on each router: Router(config)# no router rip

Rick Graziani [email protected]

106

Scenario 2: Running RIPv1 on subnets and between classful networks Step 1 – Configuring RIP Once again, lets enable RIP on each router. Once you are in the Router RIP configuration sub-mode, all you need to do is enter the classful network address for each directly connected network, using the network command. If a router has multiple interfaces on the same classful network, you will only need to enter a single command enabling RIP on all interfaces for that network. Router(config-router)#network directly-connectedclassful-network-address

Rick Graziani [email protected]

107

Here are the commands for each router: SanJose2#configure terminal Enter configuration commands, one per line. SanJose2(config)#router rip SanJose2(config-router)#network 172.30.0.0

End with CNTL/Z.

Notice we only used a single network statement for SanJose2, which includes both interfaces, on different subnets, of the 172.30.0.0 major network. SanJose1#configure terminal Enter configuration commands, one per line. SanJose1(config)#router rip SanJose1(config-router)#network 172.30.0.0 SanJose1(config-router)#network 192.168.4.0

End with CNTL/Z.

Again, notice that we only used a single network statement for SanJose1, which includes both interfaces, on different subnets, of the 172.30.0.0 major network. Baypointe#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Baypointe(config)#router rip Baypointe(config-router)#network 192.168.4.0 Baypointe(config-router)#network 192.168.5.0 Rick Graziani [email protected]

108

Scenario 2: Running RIPv1 on subnets and between classful networks Question: What would happen if you entered a network statement that was a subnet? For example: SanJose2(config)#router rip SanJose2(config-router)#network 172.30.1.0 Answer: The IOS would automatically convert it to a classful network statement: SanJose2#show running-config router rip network 172.30.0.0

Rick Graziani [email protected]

109

Step 2 – Viewing the debug ip rip output and the routing tables SanJose2 SanJose2# debug ip rip 00:14:10: RIP: received v1 update from 172.30.2.2 on Serial0 00:14:10: 172.30.3.0 in 1 hops 00:14:10: 192.168.4.0 in 1 hops 00:14:10: 192.168.5.0 in 2 hops SanJose2# 00:14:29: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (172.30.1.1) 00:14:29: subnet 172.30.2.0, metric 1 00:14:29: subnet 172.30.3.0, metric 2 00:14:29: network 192.168.4.0, metric 2 00:14:29: network 192.168.5.0, metric 3 00:14:29: RIP: sending v1 update to 255.255.255.255 via Serial0 (172.30.2.1) 00:14:29: subnet 172.30.1.0, metric 1 SanJose2# 00:14:39: RIP: received v1 update from 172.30.2.2 on Serial0 00:14:39: 172.30.3.0 in 1 hops 00:14:39: 192.168.4.0 in 1 hops 00:14:39: 192.168.5.0 in 2 hops SanJose2# undebug all SanJose2#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP Gateway of last resort is not set 172.30.0.0/24 is subnetted, 3 subnets C 172.30.2.0 is directly connected, Serial0 R 172.30.3.0 [120/1] via 172.30.2.2, 00:00:08, Serial0 C 172.30.1.0 is directly connected, Ethernet0 R 192.168.4.0/24 [120/1] via 172.30.2.2, 00:00:08, Serial0 R 192.168.5.0/24 [120/2] via 172.30.2.2, 00:00:08, Serial0 Rick Graziani [email protected]

110

Scenario 2: Running RIPv1 on subnets and between classful networks Reflections • IMPORTANT INFORMATION: RIPv1 is a classful routing protocol. Classful routing protocols do not send the subnet mask with network in routing updates, ie. 172.30.1.0 is sent by SanJose1 to SanJose2 without any subnet mask information. • QUESTION: Notice that SanJose2 is receiving the subnet 172.30.3.0 from SanJose1, which is put in the routing table under the parent network (classful network) of 172.30.0.0 with the /24 subnet mask (172.30.0.0/24 is subnetted, 3 subnets). Also notice that the RIP message received from SanJose1 was “172.30.3.0 in 1 hops” but did not include a subnet mask for the subnet. How does SanJose2 know that this subnet has a /24 (255.255.255.0) subnet mask? • ANSWER: SanJose2 received this information on an interface belonging to the same classful network as the incoming 172.30.3.0 update. The IP address that SanJose1 received the “172.30.3.0 in 1 hops” message was on (Serial 0) with an IP address of 172.30.2.1 and a subnet mask of 255.255.255.0. SanJose2 uses its own subnet mask and applies it to this and all other 172.30.0.0 subnets it receives on this interface. The 172.30.3.0 network is placed with the other 172.30.0.0 /24 subnets in the routing table. • Routers running RIPv1 are limited to using the same subnet mask for all subnets with the same classful network. Classless routing protocols like RIPv2 allow the same major (classful) network to use different subnet masks on different subnets. This is known as VLSM (Variable Length Subnet Masks) and is discussed later (Cabrillo’s CCNA Sem 2 course and the CCNP Advanced Routing). Rick Graziani [email protected]

111

SanJose1 SanJose1#debug ip rip RIP protocol debugging is on SanJose1# 00:17:52: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (172.30.3.1) 00:17:52: subnet 172.30.2.0, metric 1 00:17:52: subnet 172.30.1.0, metric 2 00:17:52: network 192.168.4.0, metric 1 00:17:52: network 192.168.5.0, metric 2 00:17:52: RIP: sending v1 update to 255.255.255.255 via Serial0 (172.30.2.2) 00:17:52: subnet 172.30.3.0, metric 1 00:17:52: network 192.168.4.0, metric 1 00:17:52: network 192.168.5.0, metric 2 00:17:52: RIP: sending v1 update to 255.255.255.255 via Serial1 (192.168.4.9) 00:17:52: network 172.30.0.0, metric 1 SanJose1# 00:18:10: RIP: received v1 update from 172.30.2.1 on Serial0 00:18:10: 172.30.1.0 in 1 hops SanJose1# 00:18:12: RIP: received v1 update from 192.168.4.10 on Serial1 00:18:12: 192.168.5.0 in 1 hops SanJose1# undebug all SanJose1#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP Gateway of last resort is not set 172.30.0.0/24 is subnetted, 3 subnets C 172.30.2.0 is directly connected, Serial0 C 172.30.3.0 is directly connected, Ethernet0 R 172.30.1.0 [120/1] via 172.30.2.1, 00:00:14, Serial0 192.168.4.0/30 is subnetted, 1 subnets C 192.168.4.8 is directly connected, Serial1 R 192.168.5.0/24 [120/1] via 192.168.4.10, 00:00:10, Serial1 Rick Graziani [email protected]

112

Scenario 2: Running RIPv1 on subnets and between classful networks Reflections • The same subnet route information applies with routes sent from SanJose2 to SanJose1 (see Reflections for SanJose2). • SanJose1 knows that the 172.30.1.0 update has a subnet mask of /24 because it received it on an interface with a /24 subnet mask (Serial 0, 172.30.3.2 255.255.255.0).

Rick Graziani [email protected]

113

SanJose1#debug ip rip RIP protocol debugging is on SanJose1# 00:17:52: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (172.30.3.1) 00:17:52: subnet 172.30.2.0, metric 1 00:17:52: subnet 172.30.1.0, metric 2 00:17:52: network 192.168.4.0, metric 1 00:17:52: network 192.168.5.0, metric 2 00:17:52: RIP: sending v1 update to 255.255.255.255 via Serial0 (172.30.2.2) 00:17:52: subnet 172.30.3.0, metric 1 00:17:52: network 192.168.4.0, metric 1 00:17:52: network 192.168.5.0, metric 2 00:17:52: RIP: sending v1 update to 255.255.255.255 via Serial1 (192.168.4.9) 00:17:52: network 172.30.0.0, metric 1 SanJose1# 00:18:10: RIP: received v1 update from 172.30.2.1 on Serial0 00:18:10: 172.30.1.0 in 1 hops SanJose1# 00:18:12: RIP: received v1 update from 192.168.4.10 on Serial1 00:18:12: 192.168.5.0 in 1 hops SanJose1# undebug all SanJose1#show ip route Codes: Gateway of last resort is not set 172.30.0.0/24 is subnetted, 3 subnets C 172.30.2.0 is directly connected, Serial0 C 172.30.3.0 is directly connected, Ethernet0 R 172.30.1.0 [120/1] via 172.30.2.1, 00:00:14, Serial0 192.168.4.0/30 is subnetted, 1 subnets C 192.168.4.8 is directly connected, Serial1 R 192.168.5.0/24 [120/1] via 192.168.4.10, 00:00:10, Serial1 Rick Graziani [email protected]

114

Scenario 2: Running RIPv1 on subnets and between classful networks More Reflections • IMPORTANT INFORMATION: Notice the RIP update being sent out Serial 1: RIP: sending v1 update to 255.255.255.255 via Serial1 (192.168.4.9) network 172.30.0.0, metric 1

•

Compare that to the same information for the 172.30.0.0 network being sent out Serial 0 & Ethernet 0:

RIP: sending v1 update to 255.255.255.255 via Serial0 (172.30.2.2) subnet 172.30.3.0, metric 1

• •

•

Notice that the 172.30.0.0 subnets are being summarized to their classful network address of 172.30.0.0 when sent out Serial 1 to Baypointe. RIP automatically summarizes RIP updates between classful networks. Because the 172.30.0.0 update is being sent out an interface (Serial 1) on a different classful network (192.168.4.0), RIP sends out only a single update for the entire classful network instead of all of the different subnets. This is similar to what we did with summarizing several static routes into a single static route. A router like SanJose1, which has an interface in more than one classful network is sometimes called a “boundary router” in RIP. Boundary routers automatically summarize RIP subnets from one classful network to the other. Rick Graziani [email protected]

115

Scenario 2: Running RIPv1 on subnets and between classful networks More Reflections (continued) • How is this an advantage? Fewer updates sent and received, resulting in less bandwidth used for routing updates between SanJose1 and Baypointe. Just as importantly, Baypointe will now only have a single route for the 172.30.0.0/16 network, no matter how many subnets there are or how it is subnetted. This will result in faster lookup process in the routing table for Baypointe. • What do you expect to see in Baypointe’s received RIP messages and its routing table? That’s right, only a single 172.30.0.0 network via SanJose1. • Are there any disadvantages? Yes, discontinguous networks. We will see this later, but the idea here is what if Baypointe had another connection via Serial 1 to another router, SantaCruz1 on 192.168.4.12/30 subnet, which also has other 172.30.0.0/24 subnets (172.30.4.0/24, 172.30.5.0/24, etc.). Baypointe would also receive the same 172.30.0.0 network from SantaCruz1 as well. Baypointe would not know how to reach the specific subnet, and mistakenly load-balance the packets between the two routers. We will see an example of this later this semester.

Rick Graziani [email protected]

116

Baypointe Baypointe#debug ip rip RIP protocol debugging is on Baypointe# 00:20:09: RIP: received v1 update from 192.168.4.9 on Serial0 00:20:09: 172.30.0.0 in 1 hops Baypointe# 00:20:24: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (192.168.5.1) 00:20:24: network 172.30.0.0, metric 2 00:20:24: network 192.168.4.0, metric 1 00:20:24: RIP: sending v1 update to 255.255.255.255 via Serial0 (192.168.4.10) 00:20:24: network 192.168.5.0, metric 1 Baypointe# Baypointe#undebug all Baypointe#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP Gateway of last resort is not set R C C

172.30.0.0/16 [120/1] via 192.168.4.9, 00:00:11, Serial0 192.168.4.0/30 is subnetted, 1 subnets 192.168.4.8 is directly connected, Serial0 192.168.5.0/24 is directly connected, Ethernet0

Rick Graziani [email protected]

117

Scenario 2: Running RIPv1 on subnets and between classful networks Reflections • Notice that Baypointe is only receiving the classful summary of the 172.30.0.0 subnets: RIP: received v1 update from 192.168.4.9 on Serial0 172.30.0.0 in 1 hops

• •

SanJose1 automatically summarized the subnets into a single classful update. This keeps Baypointe’s routing table smaller, resulting in faster routing table lookups. This also isolates any changes in the 172.30.0.0 network on SanJose1 and SanJose2 from affecting Baypointe. In other words, SanJose1 and SanJose2 can add and delete 172.30.0.0/24 subnets without affecting Baypointe’s routing table, as Baypointe doesn’t care. Baypointe will send all packets destined for the 172.30.0.0/16 network to SanJose1. Baypointe’s routing table:

•

R

172.30.0.0/16 [120/1] via 192.168.4.9, 00:00:11, Serial0

Also, the subnet mask scheme could be changed (i.e. to /27) on the 172.30.0.0 network without affecting Baypointe’s routing table or the RIP update sent to Baypointe by SanJose1. Rick Graziani [email protected]

118

Scenario 3: Running RIPv1 on a stub network SanJose2 hostname SanJose2 interface ethernet 0 ip add 172.30.1.1 255.255.255.0 interface serial 0 ip add 172.30.2.1 255.255.255.0 SanJose1 hostname SanJose1 interface ethernet 0 ip add 172.30.3.1 255.255.255.0 interface serial 0 ip add 172.30.2.2 255.255.255.0 interface serial 1 ip add 192.168.4.9 255.255.255.252 Baypointe hostname Baypointe interface ethernet 0 ip add 192.168.5.1 255.255.255.0 interface serial 0 ip add 192.168.4.10 255.255.255.252

Rick Graziani [email protected]

119

Objective: Running RIPv1 on a stub network In this scenario we will modify Scenario 2 to only run RIP between SanJose1 and SanJose2. Scenario 3 is a very common situation for many companies. It is very common that a company will want to run a dynamic routing protocol (RIPv1 in our case) within their own network, but find in unnecessary to run a dynamic routing protocol between their company and their ISP. For Scenario 3 let us assume that Baypointe is the ISP for our Company XYZ, which consists of the SanJose1 and SanJose2 routers using the 172.30.0.0/16 major network, subnetted with a /24 mask. Company XYZ is a stub network, meaning there is only one way in and out of the 172.30.0.0/16 network, in via SanJose1 (a.k.a. the entrance router) and out via Baypointe (the ISP). It is doesn’t make sense for SanJose1 to send Baypointe the RIP update of 172.30.0.0 every 30 seconds, because Baypointe has no other way to get there. RIP update message from SanJose1 to Baypointe, if RIP were configured: RIP: received v1 update from 192.168.4.9 on Serial0 172.30.0.0 in 1 hops Instead, it makes more sense for Baypointe to have a static route configured for the 172.30.0.0/16 network via SanJose1. Well, how about traffic from Company XYZ towards the Internet? It makes no sense for Baypointe to send more than the 120,000 summarized Internet routes to SanJose1. All SanJose1 needs to know is that if it is not in the 172.30.0.0 network then send it to the ISP, Baypointe. This is the same for all other Company XYZ routers (only SanJose2 in our case), that they would send all traffic with destination IP addresses other than 172.30.0.0 to SanJose1 who would forward them on to Baypointe. Let’s see how to configure this.

Rick Graziani [email protected]

120

Making changes between Scenario 2 and Scenario 3 Be sure to change the IP addressing as displayed in the diagram and Basic Configuration section for Scenario 3. Sometimes when changing the IP address on a serial interface, you may need to reset that interface by doing a shutdown, wait for the LINK-5-CHANGED message, then follow it with a no shutdown command. If you have just completed Scenario 2, lets remove RIP by issuing the following command on each router: Router(config)# no router rip

Rick Graziani [email protected]

121

Step 1 – Configuring RIP on SanJose1 and SanJose2 Here are the commands for each router: SanJose2#configure terminal Enter configuration commands, one per line. SanJose2(config)#router rip SanJose2(config-router)#network 172.30.0.0 SanJose1#configure terminal Enter configuration commands, one per line. SanJose1(config)#router rip SanJose1(config-router)#network 172.30.0.0

End with CNTL/Z.

End with CNTL/Z.

Notice that we are only including the 172.30.0.0 interfaces, networks, for SanJose1. We will not be exchanging RIP updates with Baypointe via the 192.168.4.0/30 network.

Rick Graziani [email protected]

122

Step 2 - Configuring the default static route on SanJose1 On SanJose1, let’s configure a static default route, sending all default traffic, packets with destination IP addresses which do not match a specific route in the routing table, to Baypointe. SanJose1(config)# ip route 0.0.0.0 0.0.0.0 serial 1 Notice, since the exit interface is a point-to-point serial interface we chose to use the exitinterface instead of a intermediate-address (next-hop-ip address), saving the router from having to do a recursive lookup. However, using an intermediate-address (next-hop-ipaddress) would have worked also. Previous to IOS version 12.1, SanJose1 would propagate, send, this default route automatically via RIP with its RIP updates to all other routers (in this case SanJose2). SanJose2 and all other routers will receive this default route via RIP and forward to all other routers in the RIP routing domain. However, with IOS 12.1 and later, we need to enter the default-information originate command on Baypointe, the router with the static default route. This will tell SanJose1 to include the static default route with its RIP updates to SanJose2. SanJose1(config)#router rip SanJose1(config-router)#default-information originate

Rick Graziani [email protected]

123

Step 3 - Configuring the static route on Baypointe for the 172.30.0.0/16 network Since Baypointe and SanJose1 are not exchanging RIP updates, we need to configure a static route on Baypointe for the 172.30.0.0/16 network. This will send all 172.30.0.0/16 traffic, packets with destination IP addresses of 172.30.x.x, to SanJose1. Baypointe(config)# ip route 172.30.0.0 255.255.0.0 serial 0 Once again, notice, since the exit interface is a point-to-point serial interface we chose to use the exit-interface instead of a intermediate-address (next-hop-ip address), saving the router from having to do a recursive lookup. However, using an intermediate-address (next-hopip-address) would have worked also.

Rick Graziani [email protected]

124

SanJose1 SanJose1#debug ip rip RIP protocol debugging is on SanJose1# 02:09:10: RIP: received v1 update from 172.30.2.1 on Serial0 02:09:10: 172.30.1.0 in 1 hops SanJose1# 02:09:29: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (172.30.3.1) 02:09:29: subnet 172.30.2.0, metric 1 02:09:29: subnet 172.30.1.0, metric 2 02:09:29: default, metric 1 02:09:29: RIP: sending v1 update to 255.255.255.255 via Serial0 (172.30.2.2) 02:09:29: subnet 172.30.3.0, metric 1 02:09:29: default, metric 1 SanJose1# SanJose1#undebug all SanJose1#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP Gateway of last resort is 0.0.0.0 to network 0.0.0.0

C C R C S*

172.30.0.0/24 is subnetted, 3 subnets 172.30.2.0 is directly connected, Serial0 172.30.3.0 is directly connected, Ethernet0 172.30.1.0 [120/1] via 172.30.2.1, 00:00:13, Serial0 192.168.4.0/30 is subnetted, 1 subnets 192.168.4.8 is directly connected, Serial1 0.0.0.0/0 is directly connected, Serial1 Rick Graziani [email protected]

125

Scenario 3: Running RIPv1 on a stub network Reflections • Notice that the static default route is being propagated by SanJose1 to other routers (SanJose2) via RIP. • Notice the static route in the routing table and the “Gateway of last resort.”

Rick Graziani [email protected]

126

SanJose2 SanJose2#debug ip rip RIP protocol debugging is on SanJose2# 02:07:06: RIP: received v1 update from 172.30.2.2 on Serial0 02:07:06: 172.30.3.0 in 1 hops 02:07:07: 0.0.0.0 in 1 hops SanJose2# 02:07:23: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (172.30.1.1) 02:07:23: subnet 172.30.2.0, metric 1 02:07:23: subnet 172.30.3.0, metric 2 02:07:23: default, metric 2 02:07:23: RIP: sending v1 update to 255.255.255.255 via Serial0 (172.30.2.1) 02:07:23: subnet 172.30.1.0, metric 1 SanJose2# SanJose2#undebug all SanJose2#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR Gateway of last resort is 172.30.2.2 to network 0.0.0.0 172.30.0.0/24 is subnetted, 3 subnets C 172.30.2.0 is directly connected, Serial0 R 172.30.3.0 [120/1] via 172.30.2.2, 00:00:22, Serial0 C 172.30.1.0 is directly connected, Ethernet0 R* 0.0.0.0/0 [120/1] via 172.30.2.2, 00:00:22, Serial0 Rick Graziani [email protected]

127

Scenario 3: Running RIPv1 on a stub network Reflections • Notice that SanJose2 is receiving the default route from SanJose1. • SanJose2 forwards that default route out Ethernet 0, a RIP enabled interface, although there are no other routers on that segment. • Notice the default route in the routing table and that it was learned via RIP. • Notice the “Gateway of last resort”

Rick Graziani [email protected]

128

Baypointe No RIP messages, as we are not running RIP. Baypointe#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR Gateway of last resort is not set S C C

172.30.0.0/16 is directly connected, Serial0 192.168.4.0/30 is subnetted, 1 subnets 192.168.4.8 is directly connected, Serial0 192.168.5.0/24 is directly connected, Ethernet0

Reflections • Notice that RIP is not being used on Baypointe. The only routes that are not directly-connected is the static route. Rick Graziani [email protected]

129

Scenario 3: Running RIPv1 on a stub network show ip protocols command SanJose2 router from Scenario 3. SanJose2#show ip protocols Routing Protocol is "rip" Sending updates every 30 seconds, next due in 11 seconds Invalid after 180 seconds, hold down 180, flushed after 240 Outgoing update filter list for all interfaces is Incoming update filter list for all interfaces is Redistributing: rip Default version control: send version 1, receive any version Interface Send Recv Key-chain Ethernet0 1 1 2 Serial0 1 1 2 Routing for Networks: 172.30.0.0 Routing Information Sources: Gateway Distance Last Update 172.30.2.2 120 00:00:04 Distance: (default is 120) SanJose2#

Be sure to understand this command. We will examine it again when we take a closer look at RIPv1, RIPv2 and IGRP. Take a look at the items in bold and make sure you understand them. Rick Graziani [email protected]

130

A Few Final Notes RIP uses broadcasts • Notice that RIPv1 sends out its RIP updates via an IP broadcast. 02:07:23: RIP: sending v1 update to 255.255.255.255 via Ethernet0 (172.30.1.1) All devices on the segment will see these RIP updates. The passive-interface command • How can you keep a RIP update from being sent out an interface which does not have any other routers? (i.e The Ethernet interfaces in our network.) • Because the network statement includes all interfaces which have an IP address on that classful network, by default RIP will send out updates out each one of those interfaces. • Do keep RIP from sending updates out an interface which does not have any other routers, you can use the passive-interface command. • The passive-interface command allows the interface to receive RIP updates on the interface, but does not send RIP updates out that interface. • For example, to keep SanJose2 from sending out RIP updates out Ethernet 0, you can do the following: SanJose2(config)#router rip SanJose2(config-router)#network 172.30.0.0 SanJose2(config-router)#passive-interface Ethernet 0 Rick Graziani [email protected]

131

What is with the /30 network? • /30 or 255.255.255.252 subnet masks are quite common on serial links. • A /30 subnet mask helps maximize the hosts addresses, which is perfect for a point-topoint serial link, allowing the following for each subnet: – 1 network address – 2 host addresses – 1 broadcast address IP Class: C IP Address: 192.168.4.0 Mask Bits: 6 Subnet Mask: 255.255.255.252 Subnets: 62+1 IP Major Net: 192.168.4.0 Hosts/Subnet: 2 Major Net Bcast: 192.168.4.255 Subnets for Fixed Length Subnet Masking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . No. Subnet Hosts Hosts Broadcast Address To Address 0 192.168.4.0 192.168.4.1 192.168.4.2 192.168.4.3 1 192.168.4.4 192.168.4.5 192.168.4.6 192.168.4.7 2 192.168.4.8 192.168.4.9 192.168.4.10 192.168.4.11 3 192.168.4.12 192.168.4.13 192.168.4.14 192.168.4.15 4 192.168.4.16 192.168.4.17 192.168.4.18 192.168.4.19 5 192.168.4.20 192.168.4.21 192.168.4.22 192.168.4.23 6 192.168.4.24 192.168.4.25 192.168.4.26 192.168.4.27 7 192.168.4.28 192.168.4.29 192.168.4.30 192.168.4.31 8 192.168.4.32 192.168.4.33 192.168.4.34 192.168.4.35 9 192.168.4.36 192.168.4.37 192.168.4.38 192.168.4.39 61 192.168.4.244 192.168.4.245 192.168.4.246 192.168.4.247 62 192.168.4.248 192.168.4.249 192.168.4.250 192.168.4.251 63 192.168.4.252 192.168.4.253 192.168.4.254 192.168.4.255

Rick Graziani [email protected]

From

132

How can I remove a single network from RIP? Instead of using the following command to remove all networks from RIP: Router(config)# no router rip You can specify just the network you wish to remove by using the no network command, for example: Router(config)#router rip Router(config-router)#no network 172.30.0.0 Debug ip routing - FYI •

If you wish to see what is happening in the router’s routing table process, you can use the debug ip routing command:

SanJose2#debug ip routing IP routing debugging is on SanJose2#conf t Enter configuration commands, one per line. End with CNTL/Z. SanJose2(config)#router rip SanJose2(config-router)#network 172.30.0.0 SanJose2(config-router)# 00:15:03: RT: add 172.30.3.0/24 via 172.30.2.2, rip metric [120/1] 00:15:03: RT: add 0.0.0.0/0 via 172.30.2.2, rip metric [120/1] 00:15:03: RT: default path is now 0.0.0.0 via 172.30.2.2 00:15:03: RT: new default network 0.0.0.0 Rick Graziani [email protected]

133

End of Part I

• •

End of Part I See Part II for IGRP

Rick Graziani [email protected]

134