Notes on RIP
RIP is not as common these days as it used to be but it can still have its uses in small networks. It is also still tested in the CCIE lab. Here are some notes I have written down.
- RIP is a distance vector protocol
- Uses UDP port 520 as transport
- Uses split-horizon (does not send updates back the same way it was received)
- Broadcasts updates every 30 seconds
- Uses hopcount as metric
- Maximum hopcount is 15, 16 hops is inaccessible
- Classful routing
RIP version 2 is defined in RFC 1723 by G.Malkin
Version 2 brings:
- Multicast instead of broadcast
- Route tag
The biggest improvement is sending the subnetmask in updates which means RIP is classless.
Plain text authentication is defined in the RFC but Cisco also supports MD5.
The packets are multicasted to 126.96.36.199 instead of broadcast.
Routes can be tagged with the source they originated from or to tag as a filtering mechanism.
Next-hop carried in update in case route is to non RIP speaking router.
As all distance vector protocols RIP uses split-horizon, this can pose a problem when interfaces have secondary addresses or when multipoint interfaces are used. To advertise more than one network per interface split-horizon has to be disabled. This is done with no ip slit-horizon in interface configuration mode.
RIP uses auto-summarization by default. To turn it off use no auto-summary under the router rip process. Note that advertising
supernets larger than their classful boundary is not supported as seen below:
The Cisco implementation of RIP has support for triggered updates, but only on serial point-to-point interfaces. It is enabled with ip rip triggered under the interface and it will supress periodic updates and only send updates when necessary.
To change between RIP versions you can either set the version globally with the version command under the router rip process or set it per interface with ip rip send and ip rip receive version commands. The interface specific command overrides the globally set version. If you want to send a RIP version 2 packet but broadcast it because multicast is not available this is also an option. Use ip rip v2-broadcast.
RIP version 2 has supportfor authentication either in plain text or with MD5, the MD5 extension is defined in RFC 2082. To set the mode use ip rip authentication mode text | md5. Then you need to configure a key-chain and configure RIP to use it with ip rip authentication key-chain name-of-keychain.