RIP V1 Notes

RIPv1: Distance Vector, Classful Routing Protocol

RIP has the following key characteristics:

RIP is a distance vector routing protocol.

RIP uses hop count as its only metric for path selection.

Advertised routes with hop counts greater than 15 are unreachable.

Messages are broadcast every 30 seconds.

The data portion of a RIP message is encapsulated into a UDP segment, with both source and destination port numbers set to 520. The IP header and data link headers add broadcast destination addresses before the message is sent out to all RIP configured interfaces

There are 2 messages used in the command field 1) the request message and 2) the response  meaasge. The start up process for a network running RIP is as follows

R1 will send out request messages on all ist interfaces that are running RIP requesting that all  neighbors send thier complete routing tables, when the response is recived only new routes and  routes with better routes are installed in the routing table.  The requesting router then sends a  triggered update to all ist neighbors informoring them of any possible new routes.

RIPv1 does not send subnet mask information in ist updates

RIP has an administrative distance of 120 RIP is the least-preferred routing protocol. IS-IS, OSPF,  IGRP, and EIGRP all have lower default AD values.

Basic RIPv1 configuration

The network command:

Enables RIP on all interfaces that belong to a specific network. Associated interfaces will now  both send and receive RIP updates.

Advertises the specified network in RIP routing updates sent to other routers every 30 seconds.

Verification and troubleshooting

For troubleshotting and verification use show ip route, show ip protocols and the debug ip rip in  that order

Here would be information about any passive interdfaces that have been configuried

Screen clipping taken: 24.06.2011 16:16

             show which routing protocols are enabled

          timers that are used and awhen the next update will be sent

         shows the interfaces the a running RIP and also the verson

     shows amout of equal -cost routes and tells us that all routes will be automatically summarized based on network class

Sending out unneeded updates on a LAN impacts the network in three ways:

1. Bandwidth is wasted transporting unnecessary updates. Because RIP updates are broadcast, switches will forward the updates out all ports.

2. All devices on the LAN must process the update up to the Transport layers, where the receiving device will discard the update.

3. Advertising updates on a broadcast network is a security risk. RIP updates can be intercepted with packet sniffing software. Routing updates can be modified and sent back to the router, corrupting the routing table with false metrics that misdirect traffic.

passive-interface command,

which prevents the transmission of routing updates through a router interface but still allows that network to be advertised to other routers

Automatic Summarization

Automatic summarization is the process of consolidating and advertising networks based on thier major class boundaries for example the networks 172.30.1.0/24, 172.30.2.0, and 172.30.3.0 would be summarized to 172.30.0.0/24.

Summarization takes place at border routers.

When a router gets a update, if the interface on which it comes in are both in the same major network the the route is place in the routing table with the subnet mask

If a router gets a update about netorks that are in a different major classful network then the network adress is placed in the router.

The advantage of route summirzation is thar smaller routing updates = less bandwith, it also useful for faster route lookups and a single subnet can be used no matter how many subnets are behid the summarizied route.

Screen clipping taken: 24.06.2011 17:59

RIP and Discontigious Networks

A discontiguous network comprises a major net that separates another major network

Because the subnet mask is not included in the update, RIPv1 and other classful routing protocols must summarize networks at major network boundaries. As you can see in the figure, RIPv1 on both the R1 and R3 routers will summarize their 172.30.0.0 subnets to the classful major network address of 172.30.0.0 when sending routing updates to R2. From the perspective of R2, both updates have an equal cost of 1 hop to reach network 172.30.0.0/16. As you will see, R2 installs both paths in the routing table.

Behavior of RIP and IGRP When Sending and Receiving Updates

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Introduction

This document explains the series of actions taken by both Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) when they send or receive the routing updates.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

The information in this document applies to these software and hardware versions:

• Cisco IOS Software Release 12.2(27)
• Cisco 2500 Series Routers

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

General Behavior

Send Updates

When RIP or IGRP send an update, they perform certain checks before they advertise the update. This list shows the sequence of events that occurs before Router 1 sends updates to Router 2. The network diagram allows you to examine the sequence of events more closely.

• Is the subnet information part of the same major net as the interface that sources the update?
• No: Router 1 summarizes at the major net boundary and advertises the network.
• Yes: Does the network have the same subnet mask as the interface that sources the update?
• Yes: Router 1 advertises the subnet.
• No: Does the network have a /32 mask ?
• Yes: If it is RIP, then the network is advertised. If it is IGRP, then Router 1 drops the network.
• No: Router 1 drops the network.

Receive Updates

When RIP or IGRP receive an update, they perform certain checks before they accept the update and apply the subnet mask. This is the sequence of events that occurs before Router 2 accepts an update from Router 1:

• Is the subnet received in the update on the same major net as the interface that received the update?
• Yes: Router 2 applies the mask of the interface that received the update. If the advertised network has a host bit set in the host portion of the update, Router 2 applies the host mask (/32). In the case of RIP, it continues to advertise the /32 route to the subsequent router, but IGRP does not.
• No: Do any subnets of this major net already exist in the routing table, known from interfaces other than the one that received the update? The network in this update should be a major net unless the link between the two routers is an unnumbered link, in which case it is possible for the update to contain subnet information.
• Yes: Router 2 ignores the update.
• No: Router 2 applies a classful mask. If the update came across an unnumbered link and contains subnet information (bits in subnet portion of network are set), then Router 2 applies a host mask. Refer to Understanding and Configuring the ip unnumbered Command for unnumbered case examples.

Specific Case

Send Updates

When Router 1 sends an update to Router 2, it performs these checks:

• Is 131.108.5.0/24 part of the same major net as 131.108.2.0/24, which sources the update?
• Yes: Does 131.108.5.0/24 have the same subnet mask as 131.108.2.0/24, which sources the update?
• Yes: Router 1 advertises the network.

• Is 137.99.88.0/24 part of the same major net as 131.108.2.0/24, which sources the update?
• No: Router 1 summarizes 137.99.88.0/24 at the major net boundary and advertises the route as 137.99.0.0.

This process results in Router 1 including 131.108.5.0 and 137.99.0.0 in its update to Router 2. You can see this in the debug ip rip command output shown on Router 1:

*Mar 25 00:22:46.177: RIP: sending v1 update to 255.255.255.255 via Serial0 (131.108.2.2) 
*Mar 25 00:22:46.178: RIP: build update entries
*Mar 25 00:22:46.182: subnet 131.108.5.0, metric 1
*Mar 25 00:22:46.185: network 137.99.0.0, metric 1

Receive Updates

When you issue the debug ip rip command, you can see the routing update received on Router 2 from Router 1:

*Mar 25 00:22:46.201: RIP: received v1 update from 131.108.2.2 on Serial0
*Mar 25 00:22:46.203:131.108.5.0 in 1 hops
*Mar 25 00:22:46.205:137.99.0.0 in 1 hops

Look at the checks Router 2 performs in order to determine what mask to apply on a received network.

• Is the received major net 137.99.0.0 the same as 131.108.2.0, which is the address assigned to the interface that received the update?
• No: Do any subnets of this major net already exist in the routing table known from other interfaces?
• No: Router 2 applies the natural mask (/16) because 137.99.0.0 is a class B address.

• Does subnet 131.108.5.0 belong to the same major net as subnet 131.108.2.0, which is the interface that received the update?
• Yes: Router 2 applies the mask /24, which is the mask of the interface that received the update.

This process results in these networks and masks in the routing table of Router 2, displayed with the show ip route command:

R    137.99.0.0/16 [120/1] via 131.108.2.2, 00:00:07, Serial0
     131.108.0.0/24 is subnetted, 3 subnets
R    131.108.5.0 [120/1] via 131.108.2.2, 00:00:08, Serial0
C    131.108.2.0 is directly connected, Serial0
C    131.108.3.0 is directly connected, Ethernet0

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note09186a0080093fd8.shtml>

This tells us the networks that rip is

Routing for

This tells us our neighbors that are

Running rip

R2:172.16.0.0/16

<<55.pdf>>

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Screen clipping taken: 25.06.2011 20:28

We configured the static route 192.168.0.0 with a /16 mask. This is fewer bits than the classful class C mask of /24. Because the mask does not match the class or a subnet of the class, RIPv1 will not include this route in its updates to other routers.

RIPv1 and other classful routing protocols cannot support CIDR routes that are summarized routes with a smaller subnet mask than the classful mask of the route. RIPv1 ignores these supernets in the routing table and does not include them in updates to other routers. This is because the receiving router would only be able to apply the larger classful mask to the update and not the shorter /16 mask.

Note: If the 192.168.0.0 static route were configured with a /24 mask or greater, this route would be included in the RIP updates. The receiving routers would apply the classful /24