RIP Auto-summary

Following my last post regarding route redistribution i found out that when i configure RIP with no auto-summary and with a specific network segment it will still keep advertise the network as major class and by that advertise other interfaces which i wasn’t interested to advertise.


Here is the topology i used:


i configured RIP on R4 as follow:

R4(config-router)#router rip
R4(config-router)#ver 2
R4(config-router)#no au
R4(config-router)#network 10.1.24.0

but when i looked on RIP database i found the following:

R4#sh ip rip database
10.0.0.0/8    auto-summary
10.1.12.0/24
    [1] via 10.1.24.2, 00:00:12, FastEthernet0/0
10.1.24.0/24    directly connected, FastEthernet0/0
10.1.34.0/24    directly connected, FastEthernet0/1
172.20.0.0/16    auto-summary
172.20.0.0/24
    [2] via 10.1.24.2, 00:00:12, FastEthernet0/0
192.168.10.0/24    auto-summary
192.168.10.1/32    redistributed
    [5] via 200.0.0.5, from 192.168.20.1,
192.168.20.0/24    auto-summary
192.168.20.1/32    redistributed
    [5] via 200.0.0.5, from 192.168.20.1,
<OUTPUT OMMITED>

note that R4 is also advertising network 10.1.34.0/24 which i didn’t configured, so i tough that i configured something wrong on RIP so i looked on the running-configuration:

R4#show running-config | s rip
redistribute rip subnets
router rip
version 2
redistribute ospf 1 metric 5
network 10.0.0.0
no auto-summary

so after looking around for an answer regarding this behavior i got a great explanation from Peter Paluch on Cisco Support Community:
The behavior you have observed is correct. Even though you configured each router with a particular network statement, the RIP configuration now contains only the network 10.0.0.0 statement - check your running-config. Cisco's RIP implementation always masks the address in the network command by the corresponding classful mask before entering it into running configuration. The roots of this behavior lie in RIPv1 and the behavior was not changed for RIPv2. This means that even if you configure each RIPv2 router using a particular subnet of the 10.0.0.0/8 range, the RIP on each router is running over all interfaces that lie in the 10.0.0.0/8 range. This is the reason why R3 advertises the 10.1.34.0/x network even though you have not explicitly added it to RIP. There is nothing you can do about it - in other words, Cisco's RIP implementation can not be configured to run only over a selected subnet of a classful network. The automatic summarization is not related to this behavior at all.

The auto-summary networks in the RIP database are always created by the RIP process. However, they are not advertised until the condition for their advertisement is met: a network from one particular classful network is going to be advertised out an interface that itself lies in a different classful network.