RIPv2 is a classless, distance vector routing protocol, that is defined in RFC 1723. Because RIPv2 is a classless routing protocol, it includes the subnet mask with the network addresses in the routing updates. As with other classless routing protocols, RIPv2 supports CIDR supernets, VLSM and discontiguous networks.
We saw that classful routing protocols like RIPv1 can not support discontiguous networks because they automatically summarize at major network boundaries. A router that receives routing updates from multiple routers advertising the same classful summary route cannot determine which subnets belong to which summary route. This inability leads to unexpected results including misrouted packets.
The default version of RIP is version 1. The command version 2 is used to modify RIP to RIPv2.
Similar to RIPv1, RIPv2 automatically summarizes at major network boundaries. However, with RIPv2 automatic summarization can be disabled with the no auto-summary command. Automatic summarization must be disabled to support discontiguous networks. RIPv2 also supports CIDR supernets and VLSM because the specific subnet mask is included with the network address in every routing update. You can use the debug ip rip command to view the RIP update sending the subnet mask with the network address as part of the route entry.
The show ip protocols command will display that RIP is now sending and receiving version 2 updates and whether or not automatic summarization is in effect.
no auto-summary command is used to disable automatic summarization
this is used when you have discontiguous subnets
Classless routing protocols have the capability of carrying the subnet mask in route advertisments
Auto summarization is enabled by default in RIPv2
