Basic question on community behavior, BGP

Answered Question
Sep 6th, 2009

Hi,

Please find attached detailed diagram and question.

Can someone explain why "R2" would see the aforementioned route 10.2.0.0?

If the 'noexport' option is on, and R2 is on a different AS, I thought that only R1 could see such route 10.2.0.0. How can the route get propagated to the neighbor AS R2 is in?

I have this problem too.
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Correct Answer by Peter Paluch about 7 years 3 months ago

Hello,

Note that the network 10.2.0.0/16 has the NO-EXPORT community set only when advertised to the R2. In other words, in R1's BGP database, the network 10.2.0.0/16 does not have any community so its propagation is not limited. The NO-EXPORT community attribute will be added to it only when advertising that network to R2 but again, it will not be stored in R1's database. An outgoing route-map modifies BGP attributes on the fly but it does not modify the actual BGP database on the router - it modifies only the advertisements. Of course, if R2 accepts this community then it will not propagate the network to another AS, exactly as NO-EXPORT suggests.

I hope this clarifies things a bit.

Best regards,

Peter

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Correct Answer
Peter Paluch Sun, 09/06/2009 - 23:16

Hello,

Note that the network 10.2.0.0/16 has the NO-EXPORT community set only when advertised to the R2. In other words, in R1's BGP database, the network 10.2.0.0/16 does not have any community so its propagation is not limited. The NO-EXPORT community attribute will be added to it only when advertising that network to R2 but again, it will not be stored in R1's database. An outgoing route-map modifies BGP attributes on the fly but it does not modify the actual BGP database on the router - it modifies only the advertisements. Of course, if R2 accepts this community then it will not propagate the network to another AS, exactly as NO-EXPORT suggests.

I hope this clarifies things a bit.

Best regards,

Peter

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