Ca Ex S2m04 Distance Vector Routing Protocols

CCNA – Semester 2

Chapter 4 – Distance Vector Routing Protocols CCNA Exploration version 4.0

Objectives • •

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Identify the characteristics of distance vector routing protocols. Describe the network discovery process of distance vector routing protocols using Routing Information Protocol (RIP). Describe the processes to maintain accurate routing tables used by distance vector routing protocols. Identify the conditions leading to a routing loop and explain the implications for router performance. Recognize that distance vector routing protocols are in use today

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Introduction to Distance Vector Routing Protocols

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Distance Vector Routing Protocols

• Dynamic routing protocols help the network administrator •

overcome the time-consuming and exacting process of configuring and maintaining static routes. 28 routers shown in the figure? What happens when a link goes down? How do you ensure that redundant paths are available? Dynamic routing is the most common choice for large networks like the one shown. Học viện mạng Bách Khoa - Website: www.bkacad.com

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Distance Vector Routing Protocols – Routing Information Protocol (RIP): RFC 1508 • Hop count is used as the metric • Max hop is 15 • Routing updates are broadcast or multicast every 30 seconds – Interior Gateway Routing Protocol (IGRP) • Proprietary protocol developed by Cisco. • Bandwidth, delay, load and reliability are used to create a composite metric. • Routing updates are broadcast every 90 seconds, • IGRP is the predecessor of EIGRP and is now obsolete – Enhanced Interior Gateway Routing Protocol (EIGRP) • It can perform unequal cost load balancing. • It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path. • Routing updates are sent only when there is a change in the topology. Học viện mạng Bách Khoa - Website: www.bkacad.com

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Distance Vector Technology

• The Meaning of Distance Vector: – A router using distance vector routing protocols knows 2 things:  Distance to final destination  Vector or direction traffic should be directed Học viện mạng Bách Khoa - Website: www.bkacad.com

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Distance Vector Technology •

Characteristics of Distance Vector routing protocols:  Periodic updates  Neighbors: Routing by rumor  Broadcast updates 255.255.255.255  Entire routing table is included with routing update

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Routing Protocol Algorithms • At the core of the distance vector protocol is the algorithm. The •

algorithm is used to calculate the best paths and then send that information to the neighbors. Routing Protocol Algorithm: - Defined as a procedure for accomplishing a certain task

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Routing Protocol Characteristics • Routing Protocol Characteristics • Criteria used to compare routing protocols includes     

Time to convergence Scalability Classless (use of VLSM) or Classful Resource usage Implementation & maintenance

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Routing Protocol Characteristics

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Distance Vector Routing Protocols

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Network Discovery

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Cold Start • Router initial start up  After a cold start and before the exchange of routing information, the routers initially discover their own directly connected networks and subnet masks.

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Exchange of Routing Information Initial Exchange: • If a routing protocol is configured then – Routers will exchange routing information • Routing updates received from other routers – Router checks update for new information  If there is new information: – Metric is updated – New information is stored in routing table Exchange of Routing Information • Router convergence is reached when – All routing tables in the network contain the same network information • Routers continue to exchange routing information – If no new information is found then Convergence is reached Học viện mạng Bách Khoa - Website: www.bkacad.com

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Initial Exchange of Routing Information 10.1.0.0

10.2.0.0

10.3.0.0

10.4.0.0

Routing Table

Routing Table

Routing Table

10.1.0.0  0

10.2.0.0  0

10.3.0.0  0

10.2.0.0  0

10.3.0.0  0

10.4.0.0  0

10.3.0.0  1

10.1.0.0  1

10.2.0.0  1

10.4.0.0  1 10.4.0.0  2 Học viện mạng Bách Khoa - Website: www.bkacad.com

10.1.0.0  2 15

Convergence •

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Convergence must be reached before a network is considered completely operable Speed of achieving convergence consists of 2 interdependent categories – Speed of broadcasting routing information update – Speed of calculating routes Học viện mạng Bách Khoa - Website: www.bkacad.com

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Routing Table Maintenance

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Periodic Updates : RIPv1 & IGRP • •

Periodic updates are time intervals in which a router sends out its entire routing table. Changes may occur for several reasons, including: – Failure of a link – Introduction of a new link – Failure of a router – Change of link parameters

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Periodic Updates : RIPv1 & IGRP • RIP uses 4 timers – – – –

Update timer 30s Invalid timer 180s Holddown timer 180s Flush timer 240s

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Bounded Updates: EIGRP •

EIRPG routing updates are – Partial updates – Triggered by topology changes – Bounded: meaning the propagation of partial updates are automatically bounded so that only those routers that need the information are updated. – Non periodic

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Triggered Updates • Conditions in which triggered updates are sent – Interface changes state – Route becomes unreachable – Route is placed in routing table

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Random Jitter •

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Synchronized updates – A condition where multiple routers on multi access LAN segments transmit routing updates at the same time.  Problems with synchronized updates • Bandwidth consumption • Packet collisions Solution to problems with synchronized updates  Used of random variable called RIP_JITTER, 0% to 15% of the specified update interval (25 to 30 seconds for the default 30-second interval) Học viện mạng Bách Khoa - Website: www.bkacad.com

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Routing Loop

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Definitions & Implications • Routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination.

• The IP protocol has its own mechanism to prevent the possibility of a packet traversing the network endlessly. IP has a Time-toLive (TTL) field and its value is decremented by 1 at each router. If the TTL is zero, the router drops the packet.

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Definitions & Implications • Routing loops may be caused by:

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– Incorrectly configured static routes – Incorrectly configured route redistribution – Slow convergence – Incorrectly configured discard routes Routing loops can create the following issues – Excess use of bandwidth – CPU resources may be strained – Network convergence is degraded – Routing updates may be lost or not processed in a timely manner

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Count to Infinity • This is a routing loop whereby packets bounce infinitely around a network.

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Setting a maximum • Distance Vector routing protocols set a specified metric value to indicate infinity – Once a router “counts to infinity” it marks the route as unreachable

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Preventing Routing Loops with Holddown Timers

• Network is unstable: a interface resets as up, then down,

• •

then up again in rapid succession. The route is flapping. Using triggered updates, the routers might react too quickly and unknowingly create a routing loop. A routing loop could also be created by a periodic update that is sent by the routers during the instability. Holddown timers prevent routing loops from being created by these conditions. Holddown timers also help prevent the count to infinity condition. Holddown timers allow a router to not accept any changes to a route for a specified period of time Point of using holddown timers – Allows routing updates to propagate through network with the most current information. Học viện mạng Bách Khoa - Website: www.bkacad.com

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Holddown timers Holddown timers work in the following way: 1. A router receives an update from a neighbor indicating that a network that previously was accessible is now no longer accessible.

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Holddown timers 2. The router marks the network as possibly down and starts the holddown timer.

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Holddown timers 3. If an update with a better metric for that network is received from any neighboring router during the holddown period, the network is reinstated and the holddown timer is removed.

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Holddown timers 4. If an update from any other neighbor is received during the holddown period with the same or worse metric for that network, that update is ignored. Thus, more time is allowed for the information about the change to be propagated.

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Holddown timers 5. Routers still forward packets to destination networks that are marked as possibly down. This allows the router to overcome any issues associated with intermittent connectivity. If the destination network truly is unavailable and the packets are forwarded, black hole routing is created and lasts until the holddown timer expires.

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Split Horizon Rule • Split Horizon rule: – A router should not advertise a network through the interface from which the update came.

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Split horizon with Poison Reverse or Route Poisoning

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Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers. Split horizon with poison reverse: – The rule states that once a router learns of an unreachable route through an interface, advertise it as unreachable back through the same interface Học viện mạng Bách Khoa - Website: www.bkacad.com

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IP & TTL •

Purpose of the TTL field – The TTL field is found in an IP header and is used to prevent packets from endlessly traveling on a network

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IP & TTL • How the TTL field works – TTL field contains a numeric value – The numeric value is decreased by one by every router on the route to the destination. – If numeric value reaches 0 then Packet is discarded

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Distance Vector Routing Protocols today

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RIP and EIGRP • Factors used to determine whether to use RIP or EIGRP include – Network size – Compatibility between models of routers – Administrative knowledge

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RIP • RIP:

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– Supports split horizon & split horizon with poison reverse – Capable of load balancing – Easy to configure – Works in a multi vendor router environment RIPv2 introduced the following improvements to RIPv1: – Includes the subnet mask in the routing updates, making it a classless routing protocol. – Has authentication mechanism to secure routing table updates. – Supports variable length subnet mask (VLSM). – Uses multicast addresses instead of broadcast. – Supports manual route summarization. Học viện mạng Bách Khoa - Website: www.bkacad.com

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EIGRP • Features: – Triggered updates – EIGRP hello protocol used to establish neighbor adjacencies – Supports VLSM & route summarization – Use of topology table to maintain all routes – Classless distance vector routing protocol – Cisco proprietary protocol

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Distance Vector Routing Protocols Compared

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Summary

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