Cisco Catalyst 3560-X Software Configuration Manual page 1325

Chapter 1 Implementing IPv6 Multicast
PIM-SM is used in a multicast network when relatively few switches are involved in each multicast and
these switches do not forward multicast packets for a group, unless there is an explicit request for the
traffic. PIM-SM distributes information about active sources by forwarding data packets on the shared
tree. PIM-SM initially uses shared trees, which requires the use of an RP.
Requests are accomplished via PIM joins, which are sent hop by hop toward the root node of the tree.
The root node of a tree in PIM-SM is the RP in the case of a shared tree or the first-hop switch that is
directly connected to the multicast source in the case of a shortest path tree (SPT). The RP keeps track
of multicast groups and the hosts that send multicast packets are registered with the RP by that host's
first-hop switch.
As a PIM join travels up the tree, switches along the path set up multicast forwarding state so that the
requested multicast traffic will be forwarded back down the tree. When multicast traffic is no longer
needed, a switch sends a PIM prune up the tree toward the root node to prune (or remove) the
unnecessary traffic. As this PIM prune travels hop by hop up the tree, each switch updates its forwarding
state appropriately. Ultimately, the forwarding state associated with a multicast group or source is
removed.
A multicast data sender sends data destined for a multicast group. The designated switch (DR) of the
sender takes those data packets, unicast-encapsulates them, and sends them directly to the RP. The RP
receives these encapsulated data packets, de-encapsulates them, and forwards them onto the shared tree.
The packets then follow the (*, G) multicast tree state in the switches on the RP tree, being replicated
wherever the RP tree branches, and eventually reaching all the receivers for that multicast group. The
process of encapsulating data packets to the RP is called registering, and the encapsulation packets are
called PIM register packets.
Designated Switch
Cisco switches use PIM-SM to forward multicast traffic and follow an election process to select a
designated switch when there is more than one switch on a LAN segment.
The designated switch is responsible for sending PIM register and PIM join and prune messages toward
the RP to inform it about active sources and host group membership.
If there are multiple PIM-SM switches on a LAN, a designated switch must be elected to avoid
duplicating multicast traffic for connected hosts. The PIM switch with the highest IPv6 address becomes
the DR for the LAN unless you choose to force the DR election by use of the ipv6 pim dr-priority
command. This command allows you to specify the DR priority of each switch on the LAN segment
(default priority = 1) so that the switch with the highest priority will be elected as the DR. If all switches
on the LAN segment have the same priority, then the highest IPv6 address is again used as the tiebreaker.
If the DR should fail, the PIM-SM provides a way to detect the failure of Switch A and elect a failover
DR. If the DR (Switch A) became inoperable, Switch B would detect this situation when its neighbor
adjacency with Switch A timed out. Because Switch B has been hearing MLD membership reports from
Host A, it already has MLD state for Group A on this interface and would immediately send a join to the
RP when it became the new DR. This step reestablishes traffic flow down a new branch of the shared
tree via Switch B. Additionally, if Host A were sourcing traffic, Switch B would initiate a new register
process immediately after receiving the next multicast packet from Host A. This action would trigger the
RP to join the SPT to Host A via a new branch through Switch B.
Note •
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OL-25303-03
Two PIM switches are neighbors if there is a direct connection between them. To display your PIM
neighbors, use the show ipv6 pim neighbor privileged EXEC command.
The DR election process is required only on multiaccess LANs.
Information About Implementing IPv6 Multicast
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
1-5