Routing Information Protocol, version 2
The IP Summary Address for RIP feature enables Cisco routers running RIPv2 to advertise a summarized local IP address pool on a network access server so that the address pool can be provided to dialup clients. 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.
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 (routes that are created for any combination of the individual IP addresses contained within a summary address), this 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.
RIPv2 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.
Scalability
Using the summary IP address feature for RIP means that there is no entry for child routes in the RIP routing table, reducing the size of the table and allowing the router to handle more routes.
Efficiency
Summary IP address functions more efficiently than multiple individually advertised IP routes, because:
•
The summarized routes in the RIP database are processed first.
•
Any associated child routes that are included in a summarized route are skipped as RIP looks through the routing database, reducing the processing time required.
The ip summary-address rip command causes the router to summarize a given set of routes learned via RIPv2 or redistributed into RIPv2. Host routes are especially applicable for summarization. To configure IP summary addressing, complete the following steps, beginning in router configuration mode:
Step
Command
Purpose
1
Router config# rip
Enables RIP routing.
2
Router (config-if)# int e1
Enters interface configuration mode.
3
Router (config-if)# ip summary-address rip ip_address
ip_network_mask
Specifies the IP address and network mask that identify the routes to be summarized.