H3C Technologies H3C SecPath F1000-E - Manual (Page 167)

4
Assert
The assert mechanism is used to shutoff duplicate IPv6 multicast flows onto the same multi-access network,
where more than one multicast routers exists, by electing a unique IPv6 multicast forwarder on the
multi-access network.
Figure 2 Assert mechanism
As shown in
Figure 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:
• 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