Multiprotocol Extensions for BGP

Multiprotocol BGP (MBGP), also referred to as BGP-4+ and often erroneously expanded to Multicast BGP, is defined in RFC 2283 and is negotiated via BGP capabilities. MBGP provides backward-compatible extensions to the BGP-4 protocol that let it carry information for network layer protocols other than IPv4, such as IPv6 and IPX. Although we won't spend a great deal of time on MBGP, we will cover the new attribute types, as well as touch on where MBGP is most often used today.

In order to support multiprotocol capabilities in BGP-4, two new attributes were introduced: Multiprotocol Reachable NLRI (MP_REACH_NLRI) and Multiprotocol Unreachable NLRI (MP_UNREACH_NLRI).

Multiprotocol Reachable NLRI (MP_REACH_NLRI) is an optional nontransitive attribute that can be used for three purposes:

  • To advertise a feasible route to a peer

  • To permit a router to advertise the network layer address of the router that should be used as the next hop to the destination listed in the NLRI field of the MP_NLRI attribute

  • To allow a given router to report some or all of the Subnetwork Points of Attachment (SNPAs) that exist within the local system

Multiprotocol Unreachable NLRI (MP_UNREACH_NLRI) is an optional nontransitive attribute that can be used to withdraw one or more unfeasible routes from service.

These new attributes introduced by MBGP provide the capability to associate a particular network layer protocol with the next-hop information, as well as associate a particular network layer protocol with NLRI. The Address Family information defined in RFC 1700[6] is used to identify particular network layer protocols.

Interdomain multicast routing is the most common use of BGP's multiprotocol extensions today. It's probably the reason that folks often expand MBGP to Multicast BGP rather than Multiprotocol BGP. When using MBGP for multicast, BGP carries two sets of routes: one for unicast routing and one for multicast routing. The routes associated with multicast routing are then used by PIM (Protocol-Independent Multicast) for RPF (Reverse Path Forwarding) checks to build data distribution trees.

Prior to MBGP, interdomain multicast routing used the unicast BGP infrastructure and required that unicast and multicast topologies be congruent. MBGP introduces greater flexibility to interdomain multicasting and gives the network administrator more control over network resources.

For more information on IP multicast routing, associated protocols, and their behaviors, refer to Beau Williamson's Developing IP Multicast Networks[7].

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