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Route Filtering with Distribute Lists and Prefix Lists

Outbound and inbound RIP updates can be filtered at any interface, or for the entire RIP process. To filter the routes, the distribute-list command is used under router rip, referencing an IP ACL or an IP prefix list. Any subnets matched with a permit clause in the ACL make it through; any that match with a deny action are filtered. The distribution list filtering can be performed for either direction of flow (in or out) and, optionally, for a particular interface. If the interface option is omitted, all updates coming into or out of the RIP process are filtered. (Routes can also be filtered at redistribution points, a topic covered in Chapter 11.) The generic command, when creating a RIP distribution list that uses an ACL, is distribute-list {access-list-number | name} {in | out} [interface-type interface-number] Example 8-8 shows an inbound distribution list on router R2, filtering routes in the 172.31.196.0/22 range. For this example, R2 now receives several /24 and /30 routes from S2, using RIPv2. The routes are in the range of 172.31.192.0/21, and the goal is to filter the upper half of that numeric range. (Again, the network of Figure 8-2 is used for this example.)

Example 8-8 RIP Distribution List


A RIP distribute list might refer to a prefix list instead of an ACL to match routes. Prefix lists are designed to match a range of subnets, as well as a range of subnet masks associated with the subnets. The distribute list must still define the direction of the updates to be examined (in or out), and optionally an interface. Chapter 11 includes a more complete discussion of the syntax and formatting of prefix lists; this chapter focuses on how to call and use a prefix list for RIP. To reference a prefix list, use the following router rip subcommand: distribute-list {prefix list-name} {in | out } [interface-type interface-number] Example 8-9 shows the syntax, with the prefix list denying all /30 routes from the range 172.31.192.0/21. The prefix list permits all other subnets.

Example 8-9 RIP Prefix List
 

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