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IPv6 Multicast Flow

IPv6 Multicast Flow Overview

The IPv6 multicast flow adds or enhances the following features:

  • IPv6 transit multicast which includes the following packet functions:

    • Normal packet handling

    • Fragment handling

    • Packet reordering

  • Protocol-Independent Multicast version 6 (PIMv6) flow handling

  • Other multicast routing protocols, such as Multicast Listener Discovery (MLD)

The structure and processing of IPv6 multicast data session are the same as those of IPv4. Each data session has the following:

  • One template session

  • Several leaf sessions.

The reverse path forwarding (RPF) check behavior for IPv6 is the same as that for IPv4. Incoming multicast data is accepted only if the RPF check succeeds. In an IPv6 multicast flow, incoming Multicast Listener Discovery (MLD) protocol packets are accepted only if MLD or PIM is enabled in the security zone for the incoming interface. Sessions for multicast protocol packets have a default timeout value of 300 seconds. This value cannot be configured. The null register packet is sent to rendezvous point (RP).

In IPv6 multicast flow, a multicast router has the following three roles:

  • Designated router

    This router receives the multicast packets, encapsulates them with unicast IP headers, and sends them for multicast flow.

  • Intermediate router

    There are two sessions for the packets, the control session, for the outer unicast packets, and the data session. The security policies are applied to the data session and the control session, is used for forwarding.

  • Rendezvous point

    The RP receives the unicast PIM register packet, separates the unicast header, and then forwards the inner multicast packet. The packets received by RP are sent to the pd interface for decapsulation and are later handled like normal multicast packets.

On a Services Processing Unit (SPU), the multicast session is created as a template session for matching the incoming packet's tuple. Leaf sessions are connected to the template session. On the Customer Premise Equipment (CPE), only the template session is created. Each CPE session carries the fan-out lists that are used for load-balanced distribution of multicast SPU sessions.


IPv6 multicast uses the IPv4 multicast behavior for session distribution.

The network service access point identifier (nsapi) of the leaf session is set up on the multicast text traffic going into the tunnels, to point to the outgoing tunnel. The zone ID of the tunnel is used for policy lookup for the leaf session in the second stage. Multicast packets are unidirectional. Thus for multicast text session sent into the tunnels, forwarding sessions are not created.

When the multicast route ages out, the corresponding chain of multicast sessions is deleted. When the multicast route changes, then the corresponding chain of multicast sessions is deleted. This forces the next packet hitting the multicast route to take the first path and re-create the chain of sessions; the multicast route counter is not affected.


The IPv6 multicast packet reorder approach is same as that for IPv4.

For the encapsulating router, the incoming packet is multicast, and the outgoing packet is unicast. For the intermediate router, the incoming packet is unicast, and the outgoing packet is unicast.