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All BGP updates that contain an announcement of IP prefixes have an AS_PATH field that lists all the autonomous systems that this update has traversed. BGP operators use filtering against this AS_PATH field to allow or deny IP prefixes and also to apply BGP policy based on AS_PATH filtering. This method offers greater flexibility in applying just a single line of filtering and not listing all IP prefixes, as in the case of distribute lists or prefix lists. Commonly seen problems are mostly the result of a lack of understanding of UNIX-like BGP regular expressions. Chapter 14 sections on AS_PATH cover the most commonly used regular expression in AS_PATH filtering in Cisco. |
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