BGP Flapping Issue due to over utilization of link

Unanswered Question
Dec 12th, 2007

Can we find a way to enhance BGP stability when we are running on reduced IMA bandwidth.

Our router has 3xT1 IMA, when all 3 T1s are running properly, we have no problems. When a Layer 1 issue occurs on one of the T1s (reduced bandwidth) then the router becomes highly unstable (BGP timeouts/flapping). BGP flaps continuously when we lose a T1, and then the service is unavailable.

Is there a way to create a policy map that will give BGP priority, so we will not experience an "outage" during a layer 1 issue? In other words, BGP will get a level of priority to insure it does not FLAP continuously.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
bvsnarayana03 Wed, 12/12/2007 - 21:47

I also thought policy-map could be of help here. But one thing to note, best practice is to create a bgp neighbor with loopback address & not physical interface address. If your neighbor is configured on physical interface address & that interface goes down, then you'll loose connection. But if you are already using loopback for neighborship, the here is the policy-map:

BGP runs by establising tcp connection with neighbor over port 179. So you may cretae a policy map & reserve BW for connections with tcp port 179.

access-list 101 permit tcp any any eq 179

clas-map neighbors

match access-list 101

policy-map bgp

class neighbors

bandwidth 100

apply this policy-map to either the physical links or multilink connecting to the neighbor devices.

hope that clarifies.

pls rate all helpful posts.

royalblues Wed, 12/12/2007 - 22:08

As BVS said, it is always advisable to use the loopback interfaces with BGP

So in case you use loopbacks just prioritise all the traffic between the 2 loopbacks using an access-list which will prioritise all the BGP traffic

access-list BGP permit ip host Loop1 hostloop2

access-list BGP permit ip host Loop2 host loop1

clas-map match-all BGP

match access-list name BGP

policy-map bgp

class BGP

bandwidth 100

Apply this policy on the interface on both ends of the routers



guruprasadr Wed, 12/12/2007 - 22:42


In addition to BVS and Narayan:

Loopback IP Address can be used in BGP for Tunnel Source and Destination. That should be highly enough.

>>If you have enough Bandwidth there is no need for reserving BW to Port 179 - the BGP TCP connection port.

Since you have overloaded Link pls go-ahead with their advices.


Best Regards,

Guru Prasad R


This Discussion