BGP-Basic Complication

Unanswered Question

Hai,

I would like to clear about a basic BGP concept with VPN.We are having a VPN set up (CE-PE-P-P-PE-CE)so for a particular customer connectd to pe which is having a particular vrf and particulad RD and there these PEs are having iBGP peering with the RR and in betwen that vpnv4(ipv4+RD) and also with the other PE aswell,so ultimately there the PE-PE is MPiBGP peers and they are sending the MPiBGP up date (IPV4+RD+RT+SOO).I came to know that the RD is for making the private overlapping adddress to make the unique

As my knowldge RD is only having the local significance ,then what is the need of attaching RD with IPV4(VPNV4) as the RT itself can do this function

Hope u people help us

Thanx in advance

Mahesh

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Giuseppe Larosa Thu, 09/04/2008 - 05:41

Hello Mahesh,

RD and RT are not the same thing and can have different values even if in some examples they look like equal.

A VPNv4 prefix is a 96 bit quantity that is made of the join of the 64 bit RD and the 32bit IPv4 prefix.

RD helps to make the prefix unique in the service provider network, it can help to solve address overlapping in a single PE but it is usefuf if different for managing a VRF site that is connected to two or more PEs for redundancy and load-balancing.(it allows to have the RR to propagate both advertisements from both PE because they look like different)

RT is a 64 bit quantity and it is an extended community attribute.

RD is integral part of the VPNv4 prefix and RT (one or multiple are supported like for BGP communities) is the color of the VPNv4 route.

RT is used as the criteria to decide to import a prefix inside a VRF routing table or not.

By using multiple RTs in export and import different connectivity models can be built and extranets as well.

Hope to help

Giuseppe

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