I have been trying to figure a concept about the operation of BGP when configured with policy, then deleted, and then configured again:
Let say I have 1 BGP RR which learn routes via eBGP to some ISP.
those routes are filtered using route-map which sets the community for those incoming routes lets say for example - 11:11.
the RR will advertise those route to his iBGP neighbors and his other eBGP neighbors as expected from BGP.
the process steps that the router perform are as below:
- bgp adjacency is done
- neighbor advertise routes
- RR receive routes and filter them with the route-map and taging them with the community of 11:11.
- the RR advertise those routes along with that community to his other neighbors.
now my question is - how would the router operate if the route-map would be deleted and then inserted right back again?
my assumption for the process is the following:
- RR would ask the neighbor to advertise the routes again so he could make new filtering
- the router will not delete its already known routes that he learned before the route-map got deleted
- after receiving back the whole NLRI, now the router doesn’t set the community so it will advertise his neighbors the whole routes he learned in a FIFO way which means that even if we will configure right away the route-map again then this advertising process will not stop in the middle and the operation would keep advertising the whole routes to the other neighbors
after configuring the route-map again as before:
RR would ask the neighbor to send the routing table again , lets assume that it is FULL ROUTE table which already is being advertised and we configured the route-map again before that process ended - would the new configuration will stop that advertisement and send a new request for the same advertisement because we applied a new route-map for the neighborship policy?
now the RR would tag those routes with community 11:11 and advertise all of them again to his neighbors because those routes are different NLRI then the previous which doesn’t contain community?
this operation is some example of a problem we had last week in work which causes some big BGP advertisement that took the whole bandwidth for 20 long minutes and I would like to understand what exactly happend as much as possible since it is pretty tricky and advanced question which I couldn’t figure it out.
And for last - if my explenation of the pocess is not half bad , would that same process still be the same in case I wouldn’t tag those routes with any community but instead would filter some Private address network like RFC 1918?
Thanks you very much