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PIM-DM Overview 333
Displaying and
debugging IGMP
After the above configuration, execute display command in any view to display the
running of IGMP configuration, and to verify the effect of the configuration.
Execute
debugging command in user view for the debugging of IGMP.
PIM-DM Overview 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.
The working procedures of PIM-DM include:
■ Neighbor discovery
■ Graft
■ Flood&Prune
Neighbor discovery
The PIM-DM router uses Hello messages to perform neighbor discovery when it is
started. All network nodes running PIM-DM stay in touch with one another by
periodically sending Hello messages.
Graft
When the pruned downstream node needs to be restored to the forwarding state,
the node will send a graft packet to inform the upstream node.
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.
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.
Table 340 Displaying and debugging IGMP
Operation Command
Display the information about
members of IGMP multicast
groups
display igmp group [ group_address |
interface interface_type interface_number ]
Display the IGMP configuration
and running information about
the interface
display igmp interface [ interface_type
interface_number ]
Enable the IGMP information
debugging
debugging igmp { all | event | host |
packet | timer }
Disable the IGMP information
debugging
undo debugging igmp { all | event | host |
packet | timer }