Problem with multihoming

Unanswered Question
Feb 22nd, 2007

We have announced our Network /24 through Primary and Backup ISP but it is observed that intermittently we are facing an issue with an announcement of our Network it has been noticed that inspite of we configuring sec ISP as a backup link the traffic from outside world get automatically diverted to sec ISP whereas BGP peering with Primary Link is up, after shutting down sec ISP Link the incoming traffic again moves back to Primary Link.

In order to make sec ISP as a Backup we have appended AS while announcing Network through sec ISP.

For outgoing policy we have set local preference on the router and for incoming we have the AS prepend policy.

Any clues please.

-Sai.

I have this problem too.
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rajivrajan1 Thu, 02/22/2007 - 23:30

are u planning to use your sec ISP as backup or loadsharing.?

is it possible to post your configuration... want to know abt the preferences given.

ahoangphan Thu, 02/22/2007 - 23:45

AS appending isn't enough, you have to check with both ISPs to see their list of community settings to see what you should use to make one primary and one as backup.

royalblues Fri, 02/23/2007 - 01:05

Appending your AS around 6 times should be enough to chose a particular ISP as primary.

Though after all this it is possible from some part of the world that your network has a better reachability through that ISP

check the BGP table in one of the public route servers to check which ISP is chosen as primary and then manipulate accordingly.

HTH

Narayan

saimbt Fri, 02/23/2007 - 01:13

Hi Narayan,

Under ideal conditions all router servers show that traffic is flowing thru ISP1, out of the blue incoming traffic comes through ISP2 and it automatically gets normalized or we have to shut the ISP2 link.

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