BGP Community /MPLS

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Mar 26th, 2008
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Dear all, Question - if incoming routes from a CE are tagged with a community a:b, is this tagging kept as it passes through the Service Provider? What does the command set community 12456:9351 additive mean (this command is applied in the Service Providers PE for my customers VRF).

Any help appreciated?



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Overall Rating: 4.5 (2 ratings)
royalblues Wed, 03/26/2008 - 02:07
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Yes the community tagging would be kept as it is on the MPLS cloud.

The additive keyword appends the community string to the exisiting value. For eg you might tag ur routes using community 1:1. when it reaches the PE they append the community string so that it looks like 1:1 12456:9351. Policies can be set using these strings

If the additive keyword is removed, the PE community string would replace the community string set by you



marikakis Wed, 03/26/2008 - 02:14
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Hello Mary,

Community is an optional transitive attribute. According to documentation, "A BGP speaker that sees multiple community attributes in a prefix can act based on one, some or all the attributes. A router has the option to add or modify a community attribute before the router passes the attribute on to other peers."

If you set the community and the additive keyword is not specified, then the community received from the CE is overwritten in the PE. If the additive keyword is there, then both communities will be maintained.

Kind Regards,


Joseph W. Doherty Wed, 03/26/2008 - 04:32
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BTW: As Maria notes: "Community is an optional transitive attribute." My experience has been some MPLS/VPN (BGP) providers will pass community attributes by default, some you must request them to do so.

r.cheung Wed, 01/06/2010 - 07:12
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The defaults for sending standard/extended community between BGP peers is disabled. For standard communities to be propagated from one site to another in that VRF, send-community both will need to be enabled on all the PE routers the LSP transits, for IBGP and VPNV4 address-families. Extended communities should already be enabled to support MPLS VPNs.

You'll also want to enable ip bgp new-format to support the ASN:IP Address, or the inverse, for the community to not appear as a set of numbers instead.


Marwan ALshawi Wed, 01/06/2010 - 17:08
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if you add a community with additive keyword it will be passed throughout the SP network but it will not be passed when its exit the other end PE to the other CE unless they enable send community in their PE toward the other end CE

as mentioned by the pevous post maybe some SPs have it and others you need to ask for it

good luck

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