H3C Technologies H3C S12500 Series Switches - Manual (Page 388)

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Figure 101 Assert mechanism
As shown in Figure 101 , after Router A and Router B receive an (S, G) IPv6 multicast packet from the
upstream node, both of them 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
downstream interfaces, receive a duplicate IPv6 multicast packet that the other has forwarded. After
detecting this condition, both routers send an assert message to all IPv6 PIM routers on the local subnet
through the interface that received the packet. The assert message contains the multicast source address
(S), the multicast group address (G), and the preference and metric of the IPv6 unicast route/IPv6 MBGP
route/IPv6 multicast static 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 preference to the source wins.
2. If both routers have the same preference to the source, the router with a smaller metric to the source
wins.
3. If a tie exists in the route metric to the source, the router with a higher IPv6 link-local address on the
downstream interface wins.
IPv6 PIM-SM overview
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-sized 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-sized 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 will 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.
• When a receiver is interested in the IPv6 multicast data addressed to a specific IPv6 multicast group,
the router connected to this receiver sends a join message to the RP that corresponds to that IPv6