Configuring AS and AS Prepending on BGP

You can prepend one or more autonomous system (AS) numbers at the beginning of an AS path. The AS numbers are added at the beginning of the path after the actual AS number from which the route originates has been added to the path. Prepending an AS path makes a shorter AS path look longer and therefore less preferable to BGP.

The BGP best path algorithm determines how the best path to an autonomous system (AS) is selected. The AS path length determines the best path when all of the following conditions are met:

There are multiple potential routes to an AS.

• BGP has the lowest preference value, sometimes referred to as the administrative distance, of the available    routes.
• The local preferences of the available routes are equal.

When these conditions are met, the AS path length is used as the tie breaker in the best path algorithm. When two or more routes exist to reach a particular prefix, BGP prefers the route with the shortest AS Path length.

If you have multi-homing to one or more service providers, you may prefer for incoming traffic take a particular path to reach your network. Perhaps you have two connections, but one costs less than the other. Or you might have one fast connection and another, much slower connection that you only want to use as a backup if your primary connection is down. AS path prepending is an easy method that you can use to influence inbound routing to your AS.

Netvisor One has new parameters for the vrouter-route-map-* commands:

CLI (network-admin@Leaf1) > vrouter-route-map-add

CLI (network-admin@Leaf1) > vrouter-route-map-show