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Ten Rules of RIPv2
Abstract
Over the years, the need for RIP has declined as IP-based networks evolve into more complex topologies. However, there is still a valid use for RIP, especially with the added features found in RIPv2. Here are ten useful rules to remember for RIPv2.
Sample
Introduction
Routing Information Protocol (RIP) was written as an RFC in 1988. The idea was to provide a routing protocol for IP-based networks that was easy to understand and implement, and conformed to the needs of networking at the time.
Over the years, IP-based networks have evolved into more complex topologies, and the need for RIP has declined. That does not mean that it has gone away. It is still used in small networks and can sometimes be found in specific areas of large networks.
There were many limitations to RIPv1 - namely the use of broadcasts to disseminate updates; that is was a classfull protocol; and that it lacked any authentication attributes to mitigate hackers from poisoning the routing table.
RIPv2 was created in 1998 to overcome the limitations of RIPv1 and to add some additional features. There are 10 simple rules to how RIPv2 works. They are detailed below with configuration, or output, examples taken from a Cisco Systems router.
Routing Information Protocol version 2 - RFC 2453
Replaces RIPv1 (RFC 1723, 1388)
Date of RFC writing - 1998
MAC address - 01:00:5e:00:00:09
IP address - 224.0.0.9
Transport protocol - UDP
Port number - 520
Configuration commands:
Router rip - starts the RIP process
Version 2 - activates RIPv2 instead of RIPv1
Network X.X.X.X - defines what interfaces you can talk about, and talk on
No auto-summary - turns off Classfull auto-summarization
Passive-interface - blocks RIP advertisements out a specific interface
Show commands:
Show ip rip database
Show ip protocol
Show ip route
Debug commands:
Debug ip rip
Debug ip rip database
Debug ip rip events
Debug ip rip trigger
The simplest of all routing protocols, RIP, uses a distance vector (Bellman-Ford) algorithm to determine the best path to take to any destination network within its reach.
Basic configuration:
R3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#router rip
R3(config-router)#version 2
R3(config-router)#network 172.22.0.0
Rule 1. No more than 15 hops to any remote network.
16 = Infinity!! (Lazy Bunny)
Distance: How far away. RIP uses a 'hop' metric to determine how far away things are. A hop is a layer 3 (router, multilayer switch) device. RIP is a lazy protocol. It can only keep track of networks that are equal to, or less than, 15 hops away. Any network that is more than 15 hops (16 or greater) is deemed infinitely far away and, thus, inaccessible.
Vector: What port to go out of. Quite simply, RIP will tell us two things. First, how far away the network is by number of hops and, secondly, what interface to depart out of to get to the network.
Example from a Cisco router receiving a RIP update:
R6# debug ip rip
Aug 19 15:52:21.153: RIP: received v2 update from 172.22.34.4 on
FastEthernet0/1
*Aug 19 15:52:21.153: 12.12.0.0/16 via 0.0.0.0 in 1 hops
*Aug 19 15:52:21.153: 172.22.0.0/24 via 0.0.0.0 in 1 hops
*Aug 19 15:52:21.153: 172.22.45.0/24 via 0.0.0.0 in 1 hops
*Aug 18 19:41:14.794: 172.22.81.0/24 via 0.0.0.0 in 16 hops
(inaccessible)
Rule 2. Updates every 30 seconds
(Chatty Cathy)
RIP is periodic in nature. That means the routers use a timed update interval to send information to each other instead of sending incremental updates (updates only when necessary). RIP likes to send updates every 30 seconds, unless there is a topology change; we will address that later. For now, assume all updates are sent every 30 seconds in a stable network. The updates contain information about the networks the router knows about. To ensure that peer routers do not send updates at exactly the same time, the 30-second internal timer uses an offset of +/- 0 to 5 seconds. This means the 30-second timer might actually trigger anywhere between 25 and 35 seconds.
Example from a Cisco router receiving a RIP update:
R6#deb ip rip events
*Aug 18 19:44:55.930: RIP: received v2 update from 172.22.36.3 on Serial0/0/3
*Aug 18 19:44:55.930: RIP: Update contains 10 routes
*Aug 18 19:45:24.630: RIP: received v2 update from 172.22.36.3 on Serial0/0/3
*Aug 18 19:45:24.630: RIP: Update contains 10 routes
Note: the time stamp difference is actually 28.560 seconds not 30.000 seconds
Rule 3. No more than 25 network entries per packet
(Small Boxes)
Within each update are a maximum of 25 network entries. However, if authentication is used, this value drops to 24 network entries to support the authentication key in one of the fields previously used for a network advertisement. On a Cisco router, you cannot change this value; however, on several competitor routers you can change the value to a maximum of 255 network entries. This ability is in direct violation of the RFC. Authentication is only supported in RIPv2 and not in RIPv1
