RIPv2

1. RIP v1 &2 has a maximum datagram size (with an eight-byte UDP header) of 512 octets. So it is up to 25 route entries (each is 20 bytes).

2. RIP v2 support authentication.

Simple password authentication: it will use the first route entry. So it leaves only 24.

MD5 authentication: it will use the first and last one. So it leaves only 23.

You need to configure the authentication under interface.

3. Unicast update.

By default RIP uses broadcast(v1) or multicast(v2) to send updates. To use unicast update, you need to enable the interface to be part of the network, then mark it as passive interface and finally configure neighbor.

Passive interface just makes the interface no sending out broadcast or multicast updates. But it will still listen to the updates.