BGP: Filtering BGP Updates Matching the AS_PATH PA

to filter routes matching the AS_PATH PA IOS uses AS_PATH filters.  The logic is applied with a neighbor command:  The two main steps are

  1. Configure AS_PATH filter using: ip as-path access-list number {permit|deny} regex command
  2. Enable the AS_PATH filter using the neighbor neighbor-id filter-list as-path-filter-number {in|out} command

IOS will examine the AS_PATH PA in the sent or received updates for the stated neighbor.    NLRIs whos AS_PATH match with a deny action are filtered.

AS_PATH filters use regular expressions to apply powerful matching logic to the AS_PATH



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