Ccna2 M7 Distance Vector Routing Protocol

CCNA – Semester2

Module 7 Routing and Routing Protocols

Objectives

• Distance vector routing • RIP • IGRP

Distance Vector Routing

Distance vector routing updates • Routing table updates occur periodically or when the topology in a distance vector protocol network changes. • Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors.

Distance vector routing loop issues Routing decisions based on incorrect information results in packets taking paths that return them to already visited routers • • • • • •

Before network 1 goes down B & D goes via A to this network, C may go via B to this network When net 1 is down, A informs B & D about it, C does not yet know C sends update to D informing that it can get to net 1 via B D updates this incorrect route overwriting the previously correct update from A D sends update to A followed by A sends update to B causing both A & B to also update the incorrect route Consequence: Packet originating from C destined to net 1 will loop: CÆBÆAÆDÆC

Problem: Count to infinity •

Large number of routing loops results in forever-increasing value of routing metric, e.g. hop count, or count to infinity

Loop prevention: Maximum metric

Loop prevention: Split horizon

• The routing protocol advertises routes out an interface only if they were not learned from updates entering that interface.

Route poisoning • To overcome large routing loops • Set the hop count to one more than the maximum.

Avoiding routing loops with triggered updates • Triggered update is sent immediately in response to some change in the routing table. • Triggered updates, used in conjunction with route poisoning, ensure that all routers know of failed routes before any holddown timers can expire.

Loop prevention: Hold-down timer •

If at any time during hold-down timer 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 hold-down timer. – 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. – A different neighboring router with a poorer metric, the update is ignored.

Routing Information Protocol - RIP -

Classful Routing vs. Classless Routing • Classful routing must be used with classful network or subnets with the same subnetmask. It does not include subnetmask in its routing table (FLSM) • Classless routing allows updating routes with different subnetmasks therefore it include subnetmask in routing update (VLSM)

RIP • Distance vector protocol • Hop count is used as the metric for path selection • Maximum hop count is 15 • Routing update interval is 30s

RIPv1 and RIPv2 • RIP Version 1 is a Classful Routing Protocol, RIP Version 2 is a Classless Routing Protocol. RIP v2 enhancements include: – Ability to carry additional packet routing information. – Authentication mechanism to secure table updates. – Supports variable length subnet masking (VLSM).

Configuring RIP

ip classless command •

If a packet is destined for an unknown subnet of a network that has directly connected subnets on router, it will be discarded. The ip classless command allows it to be forward over default route.

Common RIP configuration issues Command

Description

(config-router)#timers basic update invalid holddown flush [sleeptime]

Change routing update timer parameters

(conf-router)#passive-interface type number

Stop sending routing update out this interface

(config-router)#redistribute static

propagate static routes over routing updates

#clear ip route *

Clear routing table

Configure RIPv1 and RIPv2

Show ip protocols

SHOW IP ROUTE command

Show ip route R

183.8.0.128 [120/1] via 183.8.128.130 , 00:00:17 , Serial0 via 183.8.64.130 , 00:00:17 , Serial1

• 183.8.0.128 : destination network number • [120/1] : [administrative distance/hop count] • 183.8.128.130: next hop IP address • 00:00:17 : latest update was 17 seconds ago • Serial0 : outgoing interface is Serial0 • Second line : load sharing

Troubleshooting RIP update issues

•

Other commands to troubleshoot RIP: – – – – –

show ip rip database show ip protocols show ip route debug ip rip {events} show ip interface brief

Load Balancing with RIP

Router route selection • Among multiple routes to a specific network: – Use the route with the lowest AD. – If AD are equal, use the route with the lowest metric – If AD and metric are equal, proceed load sharing

Default Route with RIP • A router running RIP can receive a default route via an update from another router running RIP. • Another option is for the router to generate the default route itself.

Interior Gateway Routing Protocol - IGRP -

IGRP • IGRP is a distance vector routing protocol developed by Cisco. • IGRP sends routing updates at 90 second intervals, advertising networks for a particular autonomous system.

IGRP features • Key design characteristics of IGRP are a follows: – The versatility to automatically handle indefinite, complex topologies – The flexibility needed to segment with different bandwidth and delay characteristics – Scalability for functioning in very large networks

IGRP Metrics • The metrics that IGRP uses are: – Bandwidth – The lowest bandwidth value in the path – Delay – The cumulative interface delay along the path – Reliability – The reliability on the link towards the destination as determined by the exchange of keepalives – Load – The load on a link towards the destination based on bits per second – MTU – The Maximum Transmission Unit value of the path.

• Default metric use only bandwidth and delay • Hop-count is used as a secondary metric to prevent loop

SHOW IP PROTOCOLS command

IGRP routes • IGRP advertises three types of routes: – Interior – System – Exterior

IGRP Routes

AS = 100

Configuring IGRP

AS = 200

Verifying IGRP configuration • Additional commands for checking IGRP configuration are as follows: – show interface interface [brief] – show running-config – show ip protocols

Troubleshooting IGRP • The following commands are useful when troubleshooting IGRP: – – – – – –

show ip protocols show ip route debug ip igrp events debug ip igrp transactions ping traceroute

Default static ip route • Create a default route on the router which is being configured: ip route 0.0.0.0 0.0.0.0 {ip_next_hop| interface}

• Static routes can be advertised to other router via dynamic routing update using: redistribute static

• IGRP has no concept of route 0.0.0.0/0

Default Network • Tell other router to choose the network as the default route: ip default-network {network_number}

• Network_number must be a known classfull network (taken from routing table) • IGRP may use this with default static ip route • For a network, default route configuration is only configured on gateway router

Default Route

Summary • • • • • • •

How routing loops occur in distance vector Preventing routing loop solution Configuring RIP IGRP features IGRP metrics IGRP routes Configuring IGRP

Lab Topology 1 - RIP F0/0

F0/0

Site1

Site2

S0/0 S0/1

Lab_E

Lab_B

S0/1

S0/0

S0/0

S0/1

S0/1

Lab_C

Lab_A

S0/0 S0/0

F0/0

F0/0

S0/1

Lab_F

Lab_D F0/0

F0/0

Port

Lab_A

Lab_B

Lab_C

Lab_D

Lab_E

Lab_F

S0/0

201.10.11.1/24

199.6.13.1/24

204.204.7.1/24

192.168.3.1/24

34.10.120.1/16

N/A

S0/1

N/A

201.10.11.2/24

199.6.13.2/24

204.204.7.2/24

192.168.3.2/24

34.10.210.5/16

F0/0

172.17.11.1/16

172.16.11.1/16

172.17.11.2/16

172.18.11.1/16

172.16.11.2/16

172.18.11.2/16

Lab Topology 2 – 2 IGRP AS with default routes Internet

172.16.2.1/16

IGRP 100

Lab_B

172.16.2.2/16

Lab_E

172.20.0.0/16

Lab_A

IGRP 200 192.168.1.0/24

Lab_C

Lab_D

Lab_F

CCNA2 – Module7