3Com 3C13636 Configuration Manual page 764

3Com Router 3000 Ethernet Family
Configuration Guide
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.
IV. Assert mechanism
As shown in Figure 4-1, Routers A and B are on the same LAN network 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.
Router A Router A Router A Router A
Figure 4-1 Assert mechanism diagram
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, decide by the IP address of the forwarding routers along the paths. 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.
V. Grafting
When a pruned downstream node needs to be restored to the forwarding state, it may
send a graft packet to the upstream node.
4.1.2 PIM-SM
PIM-SM (Protocol Independent Multicast, Sparse Mode) is a kind of multicast routing
protocol mainly used in the large-scaled networks where group members are scattered
sparsely.
As opposed to PIM-DM, PIM-SM assumes that no host needs the multicast packets,
unless there is an explicit request.
PIM-SM sends multicast information to all PIM-SM routers by configuring the RP
(Rendezvous Point) and the BSR (Bootstrap Router), and builds the RP-rooted shared
tree (RPT) using the join/prune information of the routers to reduce the bandwidth
occupied by data packets and control packets as well as the processing costs. Multicast
Multicast datagram forwarded by the upstream node Multicast datagram forwarded by the upstream node Multicast datagram forwarded by the upstream node Multicast datagram forwarded by the upstream node
Router C Router C Router C Router C
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4-2
Chapter 4 PIM Configuration
Router B Router B Router B Router B
Receiver Receiver Receiver Receiver