BGP Weight, LP, AS-Path

BGP Weight

The top four route selection methods are almost always used to make a routing decision.

1. Weight
2. Local Preference
3. Route Origin (we prefer routes we’ve originated)
4. Prefer the shorter AS Path
5. Lowest Origin Code (IGP (network command), EGP, incomplete). EGP includes redistributed routes. Incomplete (?) routes are redistributed routes via any protocol except EGP.

Influencing iBGP

Influencing iBGP Path Selection

The two most common ways to influence iBGP is through Weight and Local Preference.

BGP AS-Path

AS-Path length is the fourth tie breaker in BGP route selection after Weight,  Local Preference, and Origin. The path to the destination with the least number of autonomous systems (AS) to traverse is the route that is used.

AS Path prepending is manual manipulation of a path. It ensures proper return path selection. Prepending is effective in a scenario where a customer or site is multihomed and return traffic to the customer site should be distributed in a certain fashion over each link.

BGP MED

BGP Multi-Exit Discriminator

BGP MED attribute is used to make a routing decision only after Weight, Local Preference, AS-Path, and Origin Type attributes have been examined and the BGP algorithm is still unable to decide the route to use.