Hello, when we receive an bgp updates from neighbor that a route has been removed for example, how long will this router to accept this update to remove that route based on the updates in its own routing table?
does it have anything to do with the BGP timers hold time at all? I assume not, I assume the default 60/180 timers is for the BGP neighbor peering session timers for the peer to establish or tear down that it has nothing to do with the actual route updates, is this correct?
If so, then say I have this situation:
A (L3 switch) -- B (CE) -- C (PE) -- D (PE) -- E (CE).
A and B is eBGP.
B and C is eBGP.
C and D is MP-iBGP.
D and E is eBGP.
if I power down A, then B will see its neighbor goes down and loses all the routes learned from A.
1. Does B wait for 180 seconds before decide neighbor A is dead?
2. Does B wait for 180 seconds before it removes A's routes from its own routing table?
3. Once B removes A's routes from its own routing table, how long will B advertise this update to C? Is it immediate?
4. Once C receives this update from B, how long will C to wait before removing A's route from its own routing table?
5. Once C removes A's routes from its own routing table, is it immediate that C will advertise this update to D?
6. So same question on D and E. The bottom line is when A power down, how long will E withdraw A's routes from its own routing table going thru B, C, D to E? Does BGP timers 60/180 play any role in the route updates?
Joyce / John
If B realises that A has gone down, either using the timers or some other way then yes B can remove the routes from it's routing table and stop advertising them to downstream neighbors.
The original question was about the BGP timers ie. do they affect how long a route is kept in the routing table. And the answer is still no, not directly ie.
the BGP timers are purely used to detect a dead neighbor ie. they are not equivalent of holddown timers found in some IGPs.
So the timers only affect affect how long B keeps A's routes and only indirectly ie. once 180 seconds has passed ie. 3 missed hellos the routes are removed because B now realises A is down. On C D & E the timers are irrelevant in this scenario.
Obviously if there is another way for B to realise A is down ie. fast-external-failover then the timers are not only not relevant on C D & E but also on B.