Operation Manual - Multicast Protocol
Quidway S6500 Series Ethernet Switches Chapter 6
PIM Configuration
Huawei Technologies Proprietary
6-2
towards the multicast source, the path is considered as correct. Otherwise, the
multicast packet will be discarded as a redundant one. The concerned unicast routing
information can be of any unicast routing protocol such as RIP or OSPF.
3) Assert mechanism
As shown in
Figure 6-1, Routers A and B are on the same LAN segment, but have
different paths to the multicast source S. Therefore, both will forward the multicast
packet received from S over the LAN, and the multicast router C at the downstream
node will receive two identical multicast packets.
Receiver
Router A
Router C
Multicast packets forwarded by
upstream node
Router B
the
Figure 6-1 Assert mechanism
Assert mechanism helps in choosing a single forwarder in that case. By sending Assert
packet, a router can find the shortest path. If two or more paths have the same priority
and cost, the one with greater IP address is to be selected as the upstream neighbor of
the (S, G) entry, which is responsible for forwarding the (S, G) multicast packet.
4) Grafting
When a pruned downstream node needs to be restored to the forwarding state, it may
send a graft packet to the upstream node.
6.1.2 PIM-SM
I. Introduction
Protocol independent multicast-sparse mode (PIM-SM) is the sparese-mode multicast
routing protocol. It is applicable to large-size networks, where multicast group members
are rather sparse over a wide area.
As a PIM protocol opposed to PIM-DM, PIM-SM assumes that no host needs the
multicast packets, unless there is an explicit request.
PIM-SM protocol uses rendezvous points (RPs) and bootstrap routers (BSRs) to
advertise multicast messages to all PIM-SM routers. It aso uses the joint/prune
messages to create the RP-rooted shared tree (RPT). These measures help a lot in
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