CISCO IOS in a Nutshell

Route Filtering

A big part of working with BGP is filtering routes; that’s how you control how your network traffic is carried and how you implement routing policies. You might want to filter routes coming from the outside into your network, or filter routes you advertise to other networks. No matter what your reason for filtering is, there are basically three ways to do it: AS path filtering, community filtering, and aggregate filtering.

AS Path Filters

A lot of what you do with BGP is based on building AS path filters. Filters let you select specific paths (routes) through the network. AS path filters work like access lists, but with a twist: they support regular expression (regex) pattern matching. Here’s an example of a simple AS path filter:

ip as-path access-list 70 deny ^100_
ip as-path access-list 70 permit .*

Like access lists, AS paths have the following rules:

• Each line is a permit or a deny

• The first match wins

• An implicit “deny all” is added to the end of the list

In this case, we want to deny any AS path that starts with AS 100 and permit everything else. We’ve assigned the filter number 70 (with the command ip as-path access-list 70), which we use when we reference the filter in other parts of the configuration. The last part of each line is the regular expression that determines whether or not a path matches the list. Note that the number assigned to the AS path filter has nothing to do with the numbers assigned to regular IP access lists; there’s no concept of regular ...