BGP: Multiprotocol BGP

An extension that was added to BGP-4 protocol allows the advertisement of customer VPN routes between PE devices that were injected into CE devices.  Service provider clouds run MP-BGP, routers communicating with MP-BGP are considered internal since these routers traditionally will belong to the same AS.

MP-BGP is required withing the MPLS/VPN architecture for its ability to carry IPv4 Address, MPLS Labels, and extended or standard BGP communities.  The basis of MP-BGP is to allows BGP more capabilities to carry additional information in updates.

the OPEN message is used to communicate other parameters to other BGP routers, the OPEN message is what defines/enables multiprotocol extensions.  Through MP-BGP information other than standard IPv4 addresses can be exchanged with BGP.  MP-BGP introduced some additional optional attributes to provide enhanced functionality i nthe management and injection of any non-IPv4 traffic

  • Multiprotocol Reachable NLRI (MP_REACH_NLRI) announces new multiprotocol routes
  • Multiprotocol Unreachable NLRI (MP_UNREACH_NLRI) serves to revoke the routes previously announced by MP_REACH_NLRI

MP_REACH_NLRI communicates a set of reachable prefixes together with their next hop information.

MP_UNREACH_NLRI carries the set of unreachable destinations

For MP-BGP speakers to exchange multi-protocol data they must agree on these capabilities during their capabilities exchange.

When a Pe router sends a MP-iBGP update to other PE routers the MP_REACH_NLRI attribute contains one or more of the following triples:

  • AFI – The address family information identifies the network layer protocol that is being carried within the update
  • Next Hop Information – the next hop information is the next hop address of hte next router in the path to the destination
  • NLRI – The NLRI manages the addition or withdrawal of multiprotocol routes and the next hop address, and the NLRI prefixes must be in the same address family.

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