• Extended branches to VLANs with hosts that currently belong to the group.
• Pruned branches to VLANs with no hosts that belong to the group.
Figure 11: Example of multicast tree for a given flow
When the routing switch detects a new multicast flow, it initially floods the traffic throughout the PIM-DM domain,
then it prunes the traffic on the branches (network paths) where joins have not been received from individual
hosts. This creates the tree structure shown in the preceding figure. The routing switch maintains individual
branches in the multicast tree as long as there is at least one host maintaining a membership in the multicast
group. When all of the hosts in a particular VLAN drop out of the group, PIM-DM prunes that VLAN from the
multicast tree. Similarly, if the routing switch detects a join from a host in a pruned VLAN, it adds that branch back
into the tree.
NOTE:
Where the multicast routers in a network use one or more multinetted VLANs, there must be
at least one subnet common to all routers on the VLAN. This is necessary to provide a continuous
forwarding path for the multicast traffic on the VLAN. See
PIM VLAN (interface) configuration
on page 55.
Multicast flow management
This section provides details on how the routing switch manages forwarding and pruned flows. This information is
useful when you plan topologies to include multicast support and when viewing and interpreting the
show
command output for PIM-DM features.
Initial flood and prune
When a router running PIM-DM receives a new multicast flow, it initially floods the traffic to all downstream
multicast routers. PIM-DM then prunes the traffic on paths to VLANs that have no host joins for that multicast
address. (PIM-DM does not re-forward traffic back to its source VLAN.)
Maintaining the prune state
For a multicast group "X" on a given VLAN, when the last host belonging to group "X" leaves the group, PIM
places that VLAN in a prune state; this means that the group "X" multicast traffic is blocked to that VLAN. The
Chapter 3 PIM-DM (Dense Mode)
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