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BGP prepending - visibility from internet


We have a public AS with uplinks to 2 ISPs.

primary path = normal setup

backup path = As path prepending

I'm not concerned about the functionality as it does work as expected. I'm just wondering, when i have both neighbors up and everything is up and running if i'm going to a looking glass (AT&T looking glass for example) in the internet i would only see the primary path in the bgp table.

When i shut down the primary path on my side, the backup prepended path starts being learned from the internet.

Can anyone pinpoint any reasons why the internet bgp routers won't see both paths in the same time? the shortest path and the prepended one for my prefixes?

If both would be present, the switchover to backup will be much faster in case of a failure, as this path won't have to be learned by each and every AS in the internet.

I'm thinking of some reasons, but i might be wrong. It would be good to have a second oppinon.


Cisco Employee

Re: BGP prepending - visibility from internet


This is normal behavior as BGP only advertises its best path to its neighbors. The non prepended is the preferred path and only this one get propagated.


Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
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