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Verify BGP prepended routes with Looking Glass?

I have 3 BGP paths to my headquarters. When I look at the path via a looking glass site, only the primary shows up. THe ones I have prepended don't show up..Did I misconfigure something or do the prepended networks not show up?

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: Verify BGP prepended routes with Looking Glass?

Hello Andy,

a looking glass offers a specific point of view over the internet.

Different looking glass can provide different results.

Notice that if none of the ISP receiving your prepended paths has a direct eBGP session with the looking glass this will explain what you see.

You should go on the looking glass of the provider that receives your prepended paths to verify if it accepts them.

Also be aware that if you have three BGP sessions with the same provider it is correct that in the internet you will see only the non-prepended best path.

If you are connecting to different ISPs and they accept prepending you should find the prepended paths at least on the provider's looking glass (if any)

A good source of looking glasses is:

www.traceroute.org

Hope to help

Giuseppe

3 REPLIES
Hall of Fame Super Silver

Re: Verify BGP prepended routes with Looking Glass?

Hello Andy,

a looking glass offers a specific point of view over the internet.

Different looking glass can provide different results.

Notice that if none of the ISP receiving your prepended paths has a direct eBGP session with the looking glass this will explain what you see.

You should go on the looking glass of the provider that receives your prepended paths to verify if it accepts them.

Also be aware that if you have three BGP sessions with the same provider it is correct that in the internet you will see only the non-prepended best path.

If you are connecting to different ISPs and they accept prepending you should find the prepended paths at least on the provider's looking glass (if any)

A good source of looking glasses is:

www.traceroute.org

Hope to help

Giuseppe

Hall of Fame Super Gold

Re: Verify BGP prepended routes with Looking Glass?

Andy

It is not necessarily a problem and does not necessarily mean that anything is misconfigured. You advertise your routes with prepends to the other providers to provide an alternate path. When a BGP router at your other provider receives different advertisements for the same prefix (your normal advertisement through your primary and then your prepended advertisement) it will select the best path (shortest AS path) and that is what it will advertise to its other peers. So it is not likely that the looking glass would see your prepended routes.

HTH

Rick

New Member

Please feel free to check up

Please feel free to check up your prefix visibility with our BGP Looking Glass servers

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