Cisco Systems 78-11741-02 manual IPC-204

Configuring Routing Information Protocol

RIP Configuration Task List

•As specifically configured, advertising a summarized local IP address pool on the specified interface (on a network access server) so that the address pool can be provided to dialup clients.

Automatic summary addressing always summarizes to the classful address boundary, while the ip summary-addressrouter configuration command summarizes addresses on a specified interface. If automatic summary addressing is enabled, automatic summarization is the default behavior for interfaces on the router not associated with dial-in clients (the “backbone”), with or without the ip summary-address rip interface command present.

For example, if a local IP address pool of 10.1.1.1 to 10.1.1.254 is configured on the network access server, you could configure the ip summary-address rip 10.1.1.0 255.255.255.0 command on the network access server port that provides addresses to dialup clients to cause the router to advertise 10.1.1.0/24 routes to dialup clients. Because a summary route is advertised, advertisement of the /32 host routes (installed when the dialup client connects) is suppressed so that the router does not advertise these routes to the network access server interface.

Automatic summary will override the configured summary address feature on a given interface except when both of the following conditions are true:

•The configured interface summary address and the IP address of the configured interface share the same major network (the classful, nonsubnetted portion of the IP address).

•Split horizon is not enabled on the interface.

Note If split horizon is enabled, neither an automatic summary address nor the interface summary address is advertised.

In the following example configuration, the major network is 10.0.0.0. The 10 in the address defines a Class A address space, allowing space for 0.x.x.x unique hosts where x defines unique bit positions in the addresses for these hosts. The summary of the major net defines the prefix as implied by the class (A, B, or C) of the address, without any network mask. The summary address 10.2.0.0 overrides the automatic summary address of 10.0.0.0, 10.2.0.0 is advertised out interface E1, and 10.0.0.0 is not advertised.

interface Ethernet1

ip address 10.1.1.1 255.255.255.0

ip summary-address rip 10.2.0.0 255.255.0.0 no ip split-horizon

router rip network 10.0.0.0

When RIP determines that a summary address is required in the RIP database, a summary entry is created in the RIP routing database. As long as there are child routes for a summary address, the address remains in the routing database. When the last child route is removed, the summary entry also is removed from the database. This method of handling database entries reduces the number of entries in the database because each child route is not listed in an entry, and the aggregate entry itself is removed when there are no longer any valid child routes for it.

RIP Version 2 route summarization requires that the lowest metric of the “best route” of an aggregated entry, or the lowest metric of all current child routes, be advertised. The best metric for aggregated summarized routes is calculated at route initialization or when there are metric modifications of specific routes at advertisement time, and not at the time the aggregated routes are advertised.

Cisco IOS IP Configuration Guide

IPC-204