Cisco Routing – RIPv2 Manual Subnet Mask Configuration

ciscoigpriprouting

As I understand Cisco IOS takes subnet mask from interface to which is network connected and put it in RIPv2 updates.

Could you advice please if it possible to somehow manualy set subnet mask of RIPv2 updates (in Cisco IOS)?

For instance assuming that I have following topology:

topology

How should be configured R1 in way to advertise only network A.B.C.D/28 but not entire network A.B.C.D/24?

It's strange that configuring of subnet mask for RIPv2 is not supported by wide range of vendors (actually I don't know the one who supports it:))

Do you know if there are any RIPv2 standart limitation for it?

Thank you.

Best Answer

RIPv2 with the no auto-summary option will advertise the specific routes, not the aggregated prefix, unless you you use the ip summary-address rip interface command.

The network statements in RIP are not specifically telling RIP what prefix(es) to advertise, they tell RIP which interfaces should participate in RIP, and RIP will get the specific prefix(es) from the interface(s) which fall into the network statement. This is true for most routing protocols on Cisco routers, and it is a source of confusion for many people. For instance:

interface FastEthernet0/0
 ip address 10.1.1.1 255.255.255.0

interface FastEthernet0/1
 ip address 10.2.2.1 255.255.255.0

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.0

router rip
 version 2
 no auto-summary
 network 10.0.0.0

The above configuration will run RIP on F0/0 and F0/1, but not F1/0. RIP will advertise two routes: 10.1.1.0/24 and 10.2.2.0/24.

There are Cisco documents, such as Configuring Routing Information Protocol which explain RIP route summarization:

Restrictions to RIP Route Summarization

Supernet advertisement (advertising any network prefix less than its classful major network) is not allowed in RIP route summarization, other than advertising a supernet learned in the routing tables. Supernets learned on any interface that is subject to configuration are still learned. For example, the following summarization is invalid:

interface E1
.
.
.
ip summary-address rip 10.0.0.0 252.0.0.0 (invalid supernet summarization)

Each route summarization on an interface must have a unique major net, even if the subnet mask is unique. For example, the following is not permitted:

interface Ethernet1
.
.
.
ip summary-address rip 10.1.0.0 255.255.0.0
ip summary-address rip 10.2.0.0 255.255.0.0 (or different mask)
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