Example A[091

Because the AS of the service provider will receive customer routes with prepended AS-paths that have a length greater than one AS number, the provider must modify its incoming filters.

The service provider needs to create a new inbound regular expression filter, using regular expression variables and parentheses for recall.

What is needed is a filter that will allow any AS path containing one or multiple copies of the same AS number. An example of such a filter is as follows:

This filter matches any AS path beginning with any AS number and continues with none or multiple repetitions of that same AS number (the variable "\1" repeats the value in the brackets). The regular expression would therefore match AS paths 99 99 99, 2 2 2, or 100, but it would not match AS path 100 99.

4-58 Configuring BGP on Cisco Routers (BGP) v3.1 Copyright © 2004, Cisco Systems, Inc.

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