11-26-2013 12:07 PM - edited 03-04-2019 09:41 PM
We have a location where we have 2 MPLS connections coming in from 2 separate providers. These MPLS networks connect back to our Central HQ. The Primary connection is a 100MB MPLS with ISP1. The 2nd connection is a 20MB MPLS connection with ISP2. We are doing BGP with both. For bandwidth requirements, we prefer the traffic to go across ISP1, the 100MB link. What is happening is at times the connection to ISP1 will drop, or we will lose BGP adjacency, and the routing will automatically failover to the 2nd MPLS connection ISP2. The automtic faiolver is good, but that link is only 20MB and cannot handle the load for a long time. When the primary MPLS connection is re-established, and we learn routes again, and BGP adjacency is formed, the router will not fail back over to the primary. It will continue to prefer ISP 2 until we manually take down that link, forcing the traffic back across ISP1. Ideally, we want the traffic to route back across the primary when it comes back up. Is there a way to modify the BGP preferences to make this happen. Any suggestions or advice would be very much appreciated.
12-16-2013 02:02 PM
After reviewing our configuration with Cisco TAC, we were advised that along with the BGP weight modification at the remote site, we could also do AS prepending at the remote site as well to achieve this. This would make the routes getting advertised through the less preferred link, to not be preferred at HQ. This is the configuration example they gave.
ip prefix-list FILTER seq 5 permit 172.28.0.0/16
!
!
route-map FILTER-MAP permit 10
match ip address prefix-list FILTER
set as-path prepend
route-map FILTER-MAP permit 20
!
router bgp
Neighbor X.X.X.X route-map FILTER-MAP out
12-16-2013 03:42 PM
Yes, you can indeed use AS prepending so you could do that. That would mean all the config is on the remote site so it may be a better idea in terms of consolidating the config onto one router only. So the above config would simply add the remote site AS 3 times to the route advertised via the AT&T link.
You wouldn't then need to modify any config on the HQ WAN routers.
Jon
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