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b.
If both routers have the same preference to the source, the router with a smaller metric to the
source wins.
c.
If a tie exists in route metric to the source, the router with a higher IP address on the
downstream interface wins.
PIM-SM overview
PIM-DM uses the flood-and-prune principle to build SPTs for 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.
PIM-SM is a type of sparse mode multicast protocol. It uses the pull mode for multicast forwarding, and
is suitable for large-sized and medium-sized networks with sparsely and widely distributed multicast
group members.
The basic implementation of PIM-SM is as follows:
•
PIM-SM assumes that no hosts need to receive multicast data. In the PIM-SM mode, routers must
specifically request a particular multicast stream before the data is forwarded to them. The core task
for PIM-SM to implement multicast forwarding will build and maintain RPTs. An RPT is rooted at a
router in the PIM domain as the common node, or RP, through which the multicast data travels along
the RPT and reaches the receivers.
•
When a receiver is interested in the multicast data addressed to a specific multicast group, the
router connected to this receiver sends a join message to the RP associated with that multicast group.
The path along which the message goes hop-by-hop to the RP forms a branch of the RPT.
•
When a multicast source sends multicast streams to a multicast group, the source-side DR first
registers the multicast source with the RP by sending register messages to the RP by unicast until it
receives a register-stop message from the RP. The arrival of a register message at the RP triggers the
establishment of an SPT. Then, the multicast source sends subsequent multicast packets along the
SPT to the RP. After reaching the RP, the multicast packet is duplicated and delivered to the receivers
along the RPT.
Multicast traffic is duplicated only where the distribution tree branches, and this process automatically
repeats until the multicast traffic reaches the receivers.
The operating mechanism of PIM-SM is summarized as follows:
•
Neighbor discovery
•
DR election
•
RP discovery
•
RPT building
•
Multicast source registration
•
Switchover to SPT
•
Assert
Neighbor discovery
PIM-SM uses a similar neighbor discovery mechanism as PIM-DM does. For more information, see
"
DR election
PIM-SM also uses hello messages to elect a DR for a shared-media network (such as Ethernet). The
elected DR will be the only multicast forwarder on this shared-media network.
