KB.15.15

For the flow of a given multicast group, PIM-DM creates a tree structure between the source and

the VLANs where hosts have joined the group, see Figure 10 (page 57). The tree structure consists

of:

• 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 10 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 Figure 10 (page 57).

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

context” (page 39).

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 prune state remains until a host on the same VLAN issues a join for

PIM-DM operation 57