Uses of BGP Communities

Answered Question
Jun 2nd, 2009

Well we do not use BGP communities here at work but I'm getting to know them for my BSCI exam.

1. Why are there separate well-known communities for no-export and local-AS? They both seem designed to do the same thing.

2. What's the use of non-well-known communities? It seems like they'd accomplish the same thing as route tagging.

Thanks.

I have this problem too.
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Correct Answer by Edison Ortiz about 7 years 7 months ago

Hi,

Local-AS will not advertise to other ASes within the confederation while No-Export will.

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Edison.

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Edison Ortiz Tue, 06/02/2009 - 12:21

1. Not the same thing.

No-Export:

Routes with this community are advertised to only peers in the same autonomous system or to only other subautonomous systems within a confederation. These routes are not advertised to external peers.

Local-AS:

Routes with community are advertised to only peers that are part of the local autonomous system or to only peers within a subautonomous system of a confederation. These routes are not advertised external peers or to other subautonomous systems within a confederation.

2. Yes, it's tagging of routes.

HTH,

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Edison.

CriscoSystems Tue, 06/02/2009 - 13:01

"No-Export:

Routes with this community are advertised to only peers in the same autonomous system or to only other subautonomous systems within a confederation. These routes are not advertised to external peers.

Local-AS:

Routes with community are advertised to only peers that are part of the local autonomous system or to only peers within a subautonomous system of a confederation. These routes are not advertised external peers or to other subautonomous systems within a confederation."

The difference then is....?

Correct Answer
Edison Ortiz Tue, 06/02/2009 - 13:08

Hi,

Local-AS will not advertise to other ASes within the confederation while No-Export will.

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Edison.

Edison Ortiz Tue, 06/02/2009 - 13:15

A confederation is a group of sub-ASes, so the correct term is 'ASes within a confederation'.

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Edison.

CriscoSystems Tue, 06/02/2009 - 13:27

Hmmm, and here I thought a confederation WAS a sub-AS.

BGP is not for tiny networks.

Thanks for the xplanation.

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