BGP abnormal behavior in redundancy

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Jul 11th, 2009
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We have a redundancy connection with our ISP from two different sites BG1 (border gateway working), and BG2 (border gateway standby), the ISP use local preference in order to select the path and the Borders gateways use RANK (in both sites) in order to send the traffic to the ISP. Our two Border gatewas are connected through MPLS backbone using bgp between PE-CE, and the routing protocol between border gateway and CE is ospf.

The problem happened when the interface between BG1 and ISP goes down, BG1 send LSA update to the CE (1500 routes) in order to inform to ospf that the link is down (expected behavior), so all of the traffic is redirected to BG2, but after 60 seconds BG1 will send to CE1 new LSAs update (400 routes) even the interface is down, so the CE1 will try to send all those routes through BG1 but due to the interface is down those networks are not reachable.

Please guys can you help us to understand which timers or parameters are causing this abnormal behavior??.

Thanks any comment is very welcome

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Mohamed Sobair Sun, 07/12/2009 - 00:45
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Could you please let us know how is CE1 currently connected? You have mentioned that BG1 and BG2 are connected using BGP? does BG1 and BG2 represents PEs here? Does CE1 have 2 redundant links to BG1 and BG2 using OSPF? or the traffic is just routed because of the link between BG1 and BG2?

pls post a simple diagram with a describtion of the routing protocol used?

Pls also post config of BG1 and CE1?

Normally, a new LSA should be sent from BG1 to the CE IF those Networks are not reachable and another LSA should be sent when the Networks are reachable.



Giuseppe Larosa Sun, 07/12/2009 - 04:17
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Hello Mohamed,

you can find the same thread on this under WAN R&S forum

Hope to help


Mohamed Sobair Sun, 07/12/2009 - 07:05
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Hi Paola,

Flapping on the Network could result in such issue, I havent been into a problem like what you described but I think Giuseppe has provided good analysis for it.

I have seen the Power point file with the config on , I think there are too much External routes, you should keep an eye on the CPU and memory utilization though. However, CPU or memory utilization shouldnt result on LSA wrong update if you believe so.




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