RIP Irregularities: Authentication, Updates: Uni-cast vs Multicast vs Broadcast

RIP (IPv4 versions) is a bit obsolete, I know, and very few production environments will use it as a "routing protocol of choice", but it´s also quite simple and there is simply no excuse for a Network Engineer to not know it in smallest details, let´s say - just in case.

If you need details about the protocol itself, look it up on Wikipedia, or even better - the RFCs:
RIPv1
RIPv2

Here I will only talk about the non-standard RIP behaviors.


RIPv2 weird behavior 1: Authentication

RIP Version 2 supports clear text and MD5 Authentication. The key-chain needs to be defined, and applied to the physical interface using the command:
(config-if)#ip rip authentication mode md5
(config-if)#ip rip authentication key-chain CISQUEROS_CHAIN

IMPORTANT!!!
The passwords and the key numbers MUST be the same on all the routers.
In case the key numbers are different:
- Router with the HIGHER key number will receive ALL the routes
- Router with the LOWER key number will IGNORE (reject) the received all routes received from the other router


RIPv2 weird behavior 2: Updates

By default Version 1 uses Broadcast to send it´s updates. Version 2 uses Multicast, with the destination address 224.0.0.9.

There is a way to "force" the routing updates to only one of the neighbours. To achieve this you need to:
- Manualy define the neighbour using the "network" command
- Define the interface towards the defined neighbour as PASSIVE, to prevent the Multicast Updates that are sent by default (If the interface is not defined as passive, both UNICAST and MULTICAST Updates will be sent)

There is also a way to force Broadcast Updates (ip 255.255.255.255 instead of default multicast destination 224.0.0.9) in Version 2 of RIP, and it´s achieved using the Interface Command:
(config-if)#ip rip v2-broadcast


RIPv2 weird behavior 3: Inject a Default Route into RIP

Another RIP-specific feature is injecting the default route using the "ip default-network" command. This is done in the Global Configuration mode:
(config)#ip default-network 4.0.0.0

Dont forget to advertise the network into RIP protocol:
(config-router)#network 4.0.0.0