BGP Path Selection

May 9, 2007 at 2:12 pm | Posted in BGP, Routing | Leave a comment

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/bgp.htm 

BGP could possibly receive multiple advertisements for the same route from multiple sources. BGP selects only one path as the best path. When the path is selected, BGP puts the selected path in the IP routing table and propagates the path to its neighbors. BGP uses the following criteria, in the order presented, to select a path for a destination:

If the path specifies a next hop that is inaccessible, drop the update.

Prefer the path with the largest weight.

If the weights are the same, prefer the path with the largest local preference.

If the local preferences are the same, prefer the path that was originated by BGP running on this router.

If no route was originated, prefer the route that has the shortest AS_path.

If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is lower than EGP, and EGP is lower than incomplete).

If the origin codes are the same, prefer the path with the lowest MED attribute.

If the paths have the same MED, prefer the external path over the internal path.

If the paths are still the same, prefer the path through the closest IGP neighbor.

Prefer the path with the lowest IP address, as specified by the BGP router ID.

Advertisements

Leave a Comment »

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Create a free website or blog at WordPress.com.
Entries and comments feeds.

%d bloggers like this: