Saturday, February 27, 2010

Creating Prefix-Lists for BGP Routing


I spent a good bit of my morning learning and configuring BGP prefix-lists which I will wrap up tomorrow most likely. Prefix-lists provide greater flexibility over access-lists due to the fact you're allowed more granular control of where you want input your statements inside the prefix list. This differs from the standard access-list where one no command on the ACL requires you to recreate the access-list completely! I'm still not entierly sure how prefix-lists differ from ip access-list commands which allows you to enter sequence number states like prefix-lists. I do know that you can control exactly how you want a neighbor BGP autonomous sysstem (AS) to know about external routes by using the le and ge commands.

The le and ge values are used in a prefix-list statement to create a range of the prefix length to be matched more specifically compared to the network/length commands used in the prefix-list statements. Prefix lists do provide the advantage of being less performance intensive due to not requiring the amount of route lookup processing sometimes required by large access-list tables.

As you can see in the above lab I worked earlier, the prefix list tells AS_65000 to only let AS_65002 know about the 172.16.0.0 /16 external network instead of the more specific 172.16.10.0 /24 and 172.16.11.0 /24 routes.


Don't forget to check out my CCNA Lab Book available at www.configurethenetwork.com, it's only going to available for the $9.95 price for another 3 days!

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