BGP graceful restart worthwhile ?

[douglas1942]

Hello, just trying to understand the benefit of Graceful Restart when you already have BGP Holdtime.

For example with BGP Holdtime (default 180 seconds) a BGP peer will not drop the connection until 180 seconds without an update from the other side.

Then we have Graceful restart which enables a peer to advise the other peer that it will be down for 120 seconds (default) while it upgrades or fails over.

I want to understand what the point of 'graceful restart' is, if the Holdtime will not drop the connection for a longer period anyway ?

[gfleming · Accepted solution]

BGP Holdtime and Graceful Restart are accomplishing two different things. Holdtime determines the amount of time the BGP process waits for keepalives, update messages, etc from peers before determining that they are down and flushing their learned routes.

Graceful restart allows already-installed BGP routes to be used even if the BGP process is restarted between peers. Without Graceful restart you would lose forwarding capability between peers when doing BGP process restart, etc. as the new BGP session would override the old session and routes would be flushed and re-learned.