Hello
I am trying to reproduce the BGP OSCILLATION route problem. Is short terms, it is the problem with BGP best path selection mechanism with using IGP metric to next hop step and MED. The problem is described here:
https://www.diva-portal.org/smash/get/diva2:1007297/FULLTEXT01.pdf
I have created similar topology, and faced with one issue.
R1#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
* i 1.1.1.1/32 192.168.67.6 2 100 0 300 0 300
*>i 192.168.45.5 1 100 0 200i 0 200
As we can see, we have two paths to prefix 1.1.1.1/32 and these paths are from different ASNs.
R1#sh ip bgp 1.1.1.1/32
BGP routing table entry for 1.1.1.1/32, version 1246
Paths: (2 available, best #2, table default)
Advertised to update-groups:
9 10
Refresh Epoch 1
300, (Received from a RR-client)
192.168.67.6 (metric 15) from 192.168.17.7 (192.168.67.7)
Origin IGP, metric 2, localpref 100, valid, internal
rx pathid: 0, tx pathid: 0
Refresh Epoch 1
200, (Received from a RR-client)
192.168.45.5 (metric 20) from 192.168.14.4 (192.168.45.4)
Origin IGP, metric 1, localpref 100, valid, internal, best
rx pathid: 0, tx pathid: 0x0
Here is the problem I faced. According to BGP best path selection algorithm, MED is not used if paths come from different ASNs. Therefore, lowest IGP metric to next hop must be used. But for unknown reason, BGP prefers path with higher metric.
Any ideas why?
