4000 Router Series IP Multicast Configuration Guide

102

Configuring IPv6 multicast routing and

forwarding

Overview

IPv6 multicast routing and forwarding uses the following tables:

• IPv6 multicast protocols' routing tables, such as the IPv6 PIM routing table.

• General IPv6 multicast routing table that summarizes the multicast routing information generated by

different IPv6 multicast routing protocols. The IPv6 multicast routing information from IPv6 multicast

sources to IPv6 multicast groups are stored in a set of (S, G) routing entries.

• IPv6 multicast forwarding table that guides IPv6 multicast forwarding. The optimal routing entries in

the IPv6 multicast routing table are added to the IPv6 multicast forwarding table.

RPF check mechanism

An IPv6 multicast routing protocol relies on the existing IPv6 unicast routing information in creating IPv6

multicast routing entries. When creating IPv6 multicast routing table entries, an IPv6 multicast routing

protocol uses the reverse path forwarding (RPF) check mechanism to ensure IPv6 multicast data delivery

along the correct path. The RPF check mechanism also helps avoid data loops.

A multicast routing protocol uses the IPv6 unicast routing table to perform the RPF check. The IPv6 unicast

routing table contains unicast routing information.

RPF check process

When performing an RPF check, the router searches its IPv6 unicast routing table by using the IPv6

address of the packet source as the destination address and automatically selects an optimal route. The

outgoing interface of the route is the RPF interface and the next hop is the RPF neighbor. The router

considers the path of the IPv6 multicast packet that the RPF interface receives from the RPF neighbor as

the shortest path that leads back to the source.

The term "packet source" means different things in different situations:

• For a packet that travels along the SPT from the multicast source to the receivers or the RP, the packet

source for RPF check is the multicast source.

• For a packet that travels along the RPT from the RP to the receivers, or along the source-side RPT from

the multicast source to the RP, the packet source for RPF check is the RP.

• For a bootstrap message from the BSR, the packet source for RPF check is the BSR.

For more information about the concepts of SPT, RPT, source-side RPT, RP, and BSR, see "Configuring IPv6

PIM."

RPF check implementation in IPv6 multicast

Implementing an RPF check on each received IPv6 multicast packet would heavily burden the router. The

use of an IPv6 multicast forwarding table is the solution to this issue. When the router creates an IPv6

multicast routing entry and an IPv6 multicast forwarding entry for an IPv6 multicast packet, it sets the RPF