BGP Flapping Issue due to over utilization of link

Unanswered Question
Dec 12th, 2007
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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.

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bvsnarayana03 Wed, 12/12/2007 - 21:47
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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
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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



HTH

Narayan

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


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.


DO RATE ALL HELPFUL POSTS


Best Regards,


Guru Prasad R

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