A Prefix List, a Route Map, and an AS Path Access List all walk into a bar and at the same time ask the bartender for a drink. Bartender, all confused, can’t figure out which drink request to serve first. This same confusion exists for network professionals when it comes to figuring out which BGP filtering method will be applied first to inbound BGP updates! Is it the Route Map and then the AS Path Access List/Filter List? Or maybe the Prefix List/Distribute List is evaluated before all the others?
Sit back and relax, because in this comprehensive and deep dive tutorial we take an exhaustive look at why there is so much confusion surrounding the “correct” order of precedence when dealing with Prefix Lists/Distribute Lists, Route Maps, and AS Path Access Lists/Filter Lists when they are used to filter inbound BGP updates. I also demonstrate a sure fire way to determine, for your version of IOS, the “real” order being used and it leverages the ‘debug ip bgp updates [in]’ command.