Use BGP Prepend and Weight to change BGP path selection

BGP is a dynamic routing protocol, AS number and AS path are the mandatory attributes for the route preference. Typically, routers choose the route with the shortest AS path.

Vigor routers support AS path prepending to manipulate the length, to make Vigor the less preferred hop, such as a failover scenario.

In practice, Vigor router has a prepend level in the BGP neighbor settings, which means the AS number will be multiplied by the level, hence the AS path between R1 and Vigor router will be longer than R1 between R2. R1 selects R2 if both R2 and Vigor advertise the same route when both BGP connections are established.

In addition to AS path prepending, Vigor also supports the Weight of BGP neighbors. So that Vigor can prioritize the BGP neighbors for the route selection.

The higher neighbor is the primary hop of the same route learned by Vigor. For example, if R1 and R2 both advertise route 172.16.50.0/24, we can prioritize R1 by giving it a higher weight.

Then we will see Vigor prefers R1 for route 172.16.50.0/24 in the routing table when both BGP connections are established.