WAN drops when LAN connected

Unanswered Question
Sep 4th, 2009

Have a very strange problem with a 2821 installation and wonder if anyone has seen this or something similar before?

The original symptom was when configuring an IP address on a LAN port sub-interface, BGP would flap on the WAN. Since then all hardware has been swapped, and now have a situation where plugging anything into the LAN port brings down the WAN BGP. Diconnecting all lan connections restores stability, but even bringing up the LAN port by turning off keepalives with no physical device attached causes the BGP to flap!

The IOS version is same as used at 100's of similar installations with no issues, and only major configuration difference at this particular site, is that it is running a local DHCP server.

I have this problem too.
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Mohamed Sobair Fri, 09/04/2009 - 10:57


Very starnge symptom.

A local DHCP server shouldnt affect your wan link anyway , but I still would check if its abug issue or hardware failure before going further.

Have you tried different routing protocol on the WAN or did you try leaving it without a routing protocol. Does the Wan interface flaps?





donlerche Fri, 09/04/2009 - 11:33

Mohamed, many thanks for your response, but just found out a colleague had already posted the same query earlier on today under "2821 - Serial/BGP Flaps when LAN connected" so could I ask if you have any further suggestions to please respond to this earlier post.

In response to your question, unfortunately we can't disable the WAN as this router is supported remotely.

donlerche Fri, 09/04/2009 - 11:20

Apologies to all (especially to Mohamed who replied) but I have just found out my colleague John has already posted a question on this problem earlier today under "2821 - Serial/BGP Flaps when LAN connected".

As you can probably tell we are both totally perplexed by this issue, however as John got in first, please post any responses to his earlier conversation.


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