Hi,
I'm having trouble with BGP's route selection criteria. I have a network with NCS 5500; one of them is route reflector and receives route 1.46.0.0/19 via iBGP through three of my routers (R1, R2 and R3) with the following metric.
R1:
AS-PATH: 2914 10089 9587 24378
MED: 30263
IGP Metric: 12609
R2:
AS-PATH: 6939 10089 9587 24378
MED: 2179
IGP Metric: 13103
R3:
AS-PATH: 6939 10089 9587 24378
MED: 3664
IGP Metric: 952
My route reflector is choosing R2's route as best.
I understand the Route Reflector is comparing the route with next hop R2 and R3 through the MED, but it is not comparing the route of R1. I understand that this behavior should not be appropriate, since R1's route is learned by AS 21914 and not 6939, so the selected route should be that of R3 due the lower IGP Metric.
The problem that is arising is that when router R3 receives from the reflector that the best route is that of R2, MED assumes that the one it learns through iBGP is better and stops announcing the route to the reflector. This has the consequence that the reflector now receives the routes of R1 and R2 and between the two it takes the route of R1 as the best, thereby announcing the route to R3 with next hop R1. This causes R3 to announce the route again since the one learned by eBGP is preferred over the one learned by R1.
This situation is constantly repeating itself, so my routers have the route in the routing table for a few seconds and are constantly changing.
I would appreciate if someone can help me understand how to solve this problem.
Kind regards
José
