Distance Vector Routing Protocols

Distance Vector Routing

Protocols and

RIP V.1

Routing Protocols and Concepts

– Chapter 4, 5

Editted by Nugroho Agus H., M.Si.

Version 4.0

© 2007 Cisco Systems, Inc. All rights reserved.

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Routing Protocol Algorithm:


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Routing Protocol Characteristics

– Criteria used to compare routing protocols includes

• Time to convergence

• Scalability

• Resource usage

• Implementation & maintenance


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Network Discovery

 Router initial start up (Cold Starts)

– Initial network discovery

• Directly connected networks are initially placed in routing table


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 Initial Exchange of Routing Information

– 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


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 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


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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

– Speed of calculating routes


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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


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Characteristics of Distance Vector routing protocols:

– Periodic updates

– Neighbors

– Broadcast updates

– Entire routing table is included with routing update


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Examples of Distance Vector routing protocols:

– Routing Information Protocol (RIP)

– Interior Gateway Routing Protocol (IGRP)

– Enhanced Interior Gateway Routing Protocol

(EIGRP)


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Distance Vector Routing Protocols


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Routing Table Maintenance

 Periodic Updates : RIPv1 & RIPv2

– These are time intervals in which a router sends out its entire routing table

 RIP uses 4 timers

– Update timer

– Invalid timer

– Holddown timer

– Flush timer


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Routing Loops

 Routing loops are

– A condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination.


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Routing Loops

 Routing loops may be caused by :

– 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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Routing Loops

 Count to Infinity

– This is a routing loop whereby packets bounce infinitely around a network


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Routing Loops

 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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Routing Loops

 Preventing loops with holddown timers

– 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


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Routing Loops

 The Split Horizon Rule is used to prevent routing loops

 Split Horizon rule :

– A router should not advertise a network through the interface from which the update came


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Routing Loops

 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


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Routing Loops

 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

 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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Routing Protocols Today

 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

– Features of RIP:

• Supports split horizon & split horizon with poison reverse

• Capable of load balancing

• Easy to configure

• Works in a multi vendor router environment


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 EIGRP

– Features of EIGRP :

• 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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 Routing Information Protocol (RIP)

– A distance vector protocol that has 2 versions

• RIPv1 - a classful routing protocol

• RIPv2 - a classless routing protocol


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RIPv1

 RIP Characteristics

– A classful, Distance Vector (DV) routing protocol

– Metric = hop count

– Routes with a hop count > 15 are unreachable

– Updates are broadcast every 30 seconds


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RIPv1

 RIP Operation

– RIP uses 2 message types:

• Request message

– This is sent out on startup by each RIP enabled interface

– Requests all RIP enabled neighbors to send routing table

• Response message

– Message sent to requesting router containing routing table


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RIPv1

 IP addresses initially divided into classes

– Class A

– Class B

– Class C

 RIP is a classful routing protocol

– Does not send subnet masks in routing updates


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RIPv1

 Administrative Distance

– RIP’s default administrative distance is 120


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Basic RIPv1 Configuration

 A typical topology suitable for use by

RIPv1 includes:

– Three router set up

– No PCs attached to

LANs

– Use of 5 different IP subnets


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 Router RIP Command

– To enable RIP enter:

• Router rip at the global configuration prompt

• Prompt will look like R1(config-router)#


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 Specifying Networks

– Use the network command to:

• Enable RIP on all interfaces that belong to this network

• Advertise this network in RIP updates sent to other routers every

30 seconds


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Verification and Troubleshooting

 Show ip Route

 To verify and troubleshoot routing

– Use the following commands:

• show ip route

• show ip protocols

• debug ip rip


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 show ip protocols command

– Displays routing protocol configured on router


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 Debug ip rip command

– Used to display RIP routing updates as they are happening


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 Passive interface command

– Used to prevent a router from sending updates through an interface

– Example:

• Router(config-router)#passive-interface interface-type interface-number


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Passive interfaces


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Automatic Summarization

 Modified Topology

 The original scenario has been modified such that:

– Three classful networks are used:

•

172.30.0.0/16

• 192.168.4.0/24

• 192.168.5.0/24

– The 172.30.0.0/16 network is subnetted into three subnets:

•

172.30.1.0/24

•

172.30.2.0/24

• 172.30.3.0/24

– The following devices are part of the

172.30.0.0/16 classful network address:

•

All interfaces on R1

•

S0/0/0 and Fa0/0 on R2


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Configuration Details

– To remove the RIP routing process use the following command

• No router rip

– To check the configuration use the following command

• Show run


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Boundary Routers

– RIP automatically summarizes classful networks

– Boundary routers summarize RIP subnets from one major network to another


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 Advantages of automatic summarization:

– The size of routing updates is reduced

– Single routes are used to represent multiple routes which results in faster lookup in the routing table


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 Disadvantage of

Automatic

Summarization:

– Does not support discontiguous networks


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 Discontiguous

Topologies do not converge with RIPv1

 A router will only advertise major network addresses out interfaces that do not belong to the advertised route


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Default Route and RIPv1



Modified Topology: Scenario

C



Default routes

– Packets that are not defined specifically in a routing table will go to the specified interface for the default route

– Example: Customer routers use default routes to connect to an ISP router

– Command used to configure a default route is ip route 0.0.0.0 0.0.0.0 s0/0/1


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 Propagating the Default Route in RIPv1

 Default-information originate command

– This command is used to specify that the router is to originate default information, by propagating the static default route in

RIP update


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Summary

 RIP characteristics include:

– Classful, distance vector routing protocol

– Metric is Hop Count

– Does not support VLSM or discontiguous subnets

– Updates every 30 seconds

 Rip messages are encapsulated in a UDP segment with source and destination ports of 520


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Summary: Commands used by RIP

Rtr(config)#router rip

Command

Rtr(config-router)#network …. …. … …

Rtr#debug ip rip

Rtr(config-router)#passive-interface fa0/0

Rtr(config-router)#default-information originate

Rtr#show ip protocols

Command’s purpose

Enables RIP routing process

Associates a network with a RIP routing process used to view real time RIP routing updates

Prevent RIP updates from going out an interface

Used by RIP to propagate default routes

Used to display timers used by RIP


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Distance Vector Routing Protocols

Distance Vector Routing

Protocols and

RIP V.1

Modified Topology: Scenario

Default routes

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Distance Vector Routing Protocols