3com 8807 User Guide
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PIM-DM C
ONFIGURATION
PIM-DM Overview
Introduction to PIM-DM
PIM-DM (Protocol Independent Multicast, Dense Mode) belongs to dense mode
multicast routing protocols. PIM-DM is suitable for small networks. Members of
multicast groups are relatively dense in such network environments.
multicast routing protocols. PIM-DM is suitable for small networks. Members of
multicast groups are relatively dense in such network environments.
PIM-DM Working
Principle
The working procedures of PIM-DM include neighbor discovery, flood & prune and
graft.
graft.
Neighbor discovery
The PIM-DM router needs to use Hello messages to perform neighbor discovery
when it is started. All network nodes running PIM-DM keep in touch with one
another with Hello messages, which are sent periodically.
when it is started. All network nodes running PIM-DM keep in touch with one
another with Hello messages, which are sent periodically.
Flood&Prune
PIM-DM assumes that all hosts on the network are ready to receive multicast data.
When a multicast source "S" begins to send data to a multicast group "G", after
the router receives the multicast packets, the router will perform RPF check
according to the unicast routing table first. If the RPF check is passed, the router
will create an (S, G) entry and then flood the data to all downstream PIM-DM
nodes. If the RPF check is not passed, that is, multicast packets enter from an error
interface, the packets will be discarded. After this process, an (S, G) entry will be
created in the PIM-DM multicast domain.
When a multicast source "S" begins to send data to a multicast group "G", after
the router receives the multicast packets, the router will perform RPF check
according to the unicast routing table first. If the RPF check is passed, the router
will create an (S, G) entry and then flood the data to all downstream PIM-DM
nodes. If the RPF check is not passed, that is, multicast packets enter from an error
interface, the packets will be discarded. After this process, an (S, G) entry will be
created in the PIM-DM multicast domain.
If the downstream node has no multicast group members, it will send a Prune
message to the upstream nodes to inform the upstream node not to forward data
to the downstream node. Receiving the prune message, the upstream node will
remove the corresponding interface from the outgoing interface list corresponding
to the multicast forwarding entry (S, G). In this way, a SPT (Shortest Path Tree)
rooted at Source S is built. The pruning process is initiated by leaf routers first.
message to the upstream nodes to inform the upstream node not to forward data
to the downstream node. Receiving the prune message, the upstream node will
remove the corresponding interface from the outgoing interface list corresponding
to the multicast forwarding entry (S, G). In this way, a SPT (Shortest Path Tree)
rooted at Source S is built. The pruning process is initiated by leaf routers first.
This process is called "flood & prune" process. In addition, nodes that are pruned
provide timeout mechanism. Each router re-starts the "flood & prune" process
upon pruning timeout. The consistent "flood & prune" process of PIM-DM is
performed periodically.
provide timeout mechanism. Each router re-starts the "flood & prune" process
upon pruning timeout. The consistent "flood & prune" process of PIM-DM is
performed periodically.
During this process, PIM-DM uses the RPF check and the existing unicast routing
table to build a multicast forwarding tree rooted at the data source. When a
packet arrives, the router will first judge the correctness of the path. If the
interface that the packet arrives is the one indicated by the unicast routing to the
multicast source, the packet is regarded to be from the correct path. Otherwise,
the packet will be discarded as a redundancy packet without the multicast
table to build a multicast forwarding tree rooted at the data source. When a
packet arrives, the router will first judge the correctness of the path. If the
interface that the packet arrives is the one indicated by the unicast routing to the
multicast source, the packet is regarded to be from the correct path. Otherwise,
the packet will be discarded as a redundancy packet without the multicast