Multicast extensions to BGP

Multicast extensions to BGP

This section describes the implementation of extensions to MBGP to support multicast. Instead of assuming that all unicast routes are multicast-capable, some routed environments, in some cases, some ISPs do not support or have limited support for multicast throughout their AS.

BGP is capable of supporting two sets of routing information, one set for unicast routing and the other for multicast routing. The unicast and multicast routing sets either partially or fully overlay one another. To achieve this, BGP has added support for IPv4 and mcast-IPv4 address families. Routing policies can be imported or exported.

The multicast routing information can subsequently be used by the Protocol Independent Multicast (PIM) protocol to perform its Reverse Path Forwarding (RPF) lookups for multicast-capable sources. Thus, multicast traffic can only be routed across a multicast topology and not a unicast topology.

MBGP multicast topology support

Recursive lookup for BGP next hops

The next hop for multicast RPF routes learned by MBGP is not always the address of a directly-connected neighbor. For unicast routing, a router resolves the directly-connected next-hop by repeating the IGP routes. For multicast RPF routes, there are different ways to find the real next-hops:

  • Scanning to see if a route encompasses the BGP next hop. If one exists, this route is used. If not, the tables are scanned for the best matching route.

  • Checking to see if the recursed next hop is taken from the protocol routing table with the lowest administrative distance (protocol preference). This means that the operating system algorithm must perform multiple lookups in the order of the lowest admin distance. Unlike recursion on the unicast routing table, the longest prefix match rule does not take effect; protocol preference is considered before prefix length. For example, the route learned via MBGP is selected over the route learned via BGP.