BGP Route Convergence

Question 1

Routing Convergence

Answer 1

Routing convergence can be broadly defined as how quickly a routing protocol can become
stable after changes occur in the network—for example, a protocol or link flap.

In terms of Border Gateway Protocol (BGP), it can be defined as converged when
all BGP neighbor sessions have been established and neighbors have been updated,
routes have been learned from all neighbors and installed into the routing table,
and all routing tables across the network are consistent after a network event
or any change in the network.

Question 2

Convergence Time

Answer 2

There are multiple paths from the source router in order to reach the
destination router, but for simplicity, consider the two paths—primary and secondary.
Primary path: R1 - R2 - R4 - R6 | Secondary path: R1 - R3 - R5 - R6

If a link on primary path fails, the best path is impacted and
leads to a traffic loss. Because of the failure event, a next-best path is computed.

The amount of time during which there was a traffic loss in the network while the alternate
path was not available to forward the traffic to the point where traffic starts flowing again
is called the convergence time.