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Assert
Where more than one multicast routers exists, the assert mechanism shuts off duplicate IPv6 multicast
flows onto the same multi-access network. It does this by electing a unique IPv6 multicast forwarder on
the multi-access network.
Figure 93 Assert mechanism
As shown in Figure 93, 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 downstream
interface, 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
downstream 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 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 of 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 IPv6 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.
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