H3C Technologies H3C S7500E Series Switches - Manual (Page 356)

13-5
Figure 13-2 Assert mechanism
As shown in Figure 13-2 , after Router A and Router B receive an (S, G) IPv6 multicast packet
from the upstream node, they both forward the packet to the local subnet. As a result, the
downstream node Router C receives two identical multicast packets, and both Router A and
Router B, on their own local interface, receive a duplicate IPv6 multicast packet forwarded by
the other. Upon detecting this condition, both routers send an assert message to all IPv6 PIM
routers through the interface on which the packet was received. The assert message contains
the following information: the multicast source address (S), the multicast group address (G),
and the preference and metric of the IPv6 unicast route to the source. By comparing these
parameters, either Router A or Router B becomes the unique forwarder of the subsequent (S, G)
IPv6 multicast packets on the multi-access subnet. The comparison process is as follows:
1) The router with a higher IPv6 unicast route preference to the source wins;
2) If both routers have the same IPv6 unicast route preference to the source, the router with a
smaller metric to the source wins;
3) If there is a tie in the route metric to the source, the router with a higher IPv6 link-local
address wins.
Introduction to IPv6 PIM-SM
IPv6 PIM-DM uses the “flood and prune” principle to build SPTs for IPv6 multicast data
distribution. Although an SPT has the shortest path, it is built with a low efficiency. Therefore
the PIM-DM mode is not suitable for large-and medium-sized networks.
IPv6 PIM-SM is a type of sparse mode IPv6 multicast protocol. It uses the “pull mode” for IPv6
multicast forwarding, and is suitable for large- and medium-sized networks with sparsely and
widely distributed IPv6 multicast group members.
The basic implementation of IPv6 PIM-SM is as follows:
z IPv6 PIM-SM assumes that no hosts need to receive IPv6 multicast data. In the IPv6
PIM-SM mode, routers must specifically request a particular IPv6 multicast stream before
the data is forwarded to them. The core task for IPv6 PIM-SM to implement IPv6 multicast
forwarding is to build and maintain rendezvous point trees (RPTs). An RPT is rooted at a
router in the IPv6 PIM domain as the common node, or rendezvous point (RP), through
which the IPv6 multicast data travels along the RPT and reaches the receivers.