Routing – How to advertise a classless network in RIPv2

protocol-theoryriprouting

In OSPF and EIGRP we can advertise specific networks only using wildcard masks.

For example, if a router has networks 10.0.10.0/24 and 10.0.20.0/24, we can choose to advertise only 10.0.10.0/24 by typing network 10.0.10.0 0.0.0.255. This way other routers will learn 10.0.10.0/24 but not 10.0.20.0/24.

But in RIPv2 we do not use wildcard mask. If you type network 10.0.10.0 other routers will learn also 10.0.20.0/24 and all other networks within 10.0.0.0/8.

So is it even possible to advertise classless networks in RIPv2? If not then how did RIPv2 become classless?

Best Answer

For example, if a router has networks 10.0.10.0/24 and 10.0.20.0/24, we can choose to advertise only 10.0.10.0/24 by typing network 10.0.10.0 0.0.0.255. This way other routers will learn 10.0.10.0/24 but not 10.0.20.0/24.

No, you are confused about the network statement. In OSPF, RIPv2, and EIGRP, the network statement tells the routing process which interfaces it should include in the routing process, not what networks to advertise. The routing process will get the actual networks to advertise from the interfaces included in the routing process.

For example, if there is an interface with the address of 10.0.10.1 255.255.255.192, and you have an OSPF network statement of network 10.0.10.0 0.0.0.255, OSPF will only advertise 10.0.10.0/26, not 10.0.10.0/24.

RIPv2 can advertise non-classful networks if you use the no auto-summary command under the RIP configuration. If there are other interfaces that are included in the classful range, you can filter those in other ways, such as a distribute list, and you can make those interfaces passive with the passive-interface command. Given the same scenario as explained above with OSPF, using RIPv2 with no auto-summary and network 10.0.10.0, RIPv2 will also advertise the 10.0.10.0/26 network.

For example, if 10.0.10.0/24 is on an interface, and you have other interfaces with networks in the 10.0.0.0/8 classful network, you could use a distribute list to only permit the 10.0.10.0/24 network. Something like:

access-list 1 permit 10.0.10.0 0.255.255.255 ! remember the implicit deny all at the end of an access list
!
router rip
 version 2
 no auto-summary
 distribute-list 1 out <outbound advertising interface>

BGP, on the other hand, does use the network statement to advertise the network, but only if that exact network already exists in the routing table. For example, if 10.0.10.0/26 is in the routing table, for BGP, network 10.0.10.0 mask 255.255.255.192 will advertise the network, but network 10.0.10.0 mask 255.255.255.0 will not advertise a network.

In general, the IGP network statements tell the IGP which interfaces to include in the routing process, but BGP uses the network statement to determine which networks that exist in the routing table to advertise.

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