Wednesday, May 28, 2008

BGP route selection with MEDs

In BGP, MED stands for Multi Exit Discriminator. It is a well-known optional attribute which allows one autonomous system to inject it's IGP route metrics into its BGP advertisements to another BGP autonomous system. This allows the second autonomous system to make intelligent routing decisions regarding which of multiple paths to take to send traffic to a particular destination in the first autonomous system.

Because different AS's use different IGP's and can calculate metrics in different ways, by default MEDs are only compared when multiple paths exist between the same two autonomous systems.

BGP MEDs are fairly late in the BGP route selection process, coming after local-preference and AS-PATH length.

In this episode we show how to inject MEDs into BGP advertisements, and how they are used to influence routing decisions.