Brocade Multi-Service IronWare Multicast Configuration Guide (Supporting R05.6.00) - Manual (Page 37)

Multi-Service IronWare Multicast Configuration Guide 25
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PIM Dense
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In Figure 5 , the root node (R1) is forwarding multicast packets for group 229.225.0.1, which it
receives from the server, to its downstream nodes, R2, R3, and R4. Device R4 is an intermediate
device with R5 and R6 as its downstream devices. Because R5 and R6 have no downstream
interfaces, they are leaf nodes. The receivers in this example are those workstations that are
resident on devices R2, R3, and R6.
Pruning a multicast tree
As multicast packets reach these leaf devices, the devices check their IGMP databases for the
group. If the group is not in the IGMP database of the device, the device discards the packet and
sends a prune message to the upstream device. The device that discarded the packet also
maintains the prune state for the source, group (S,G) pair. The branch is then pruned (removed)
from the multicast tree. No further multicast packets for that specific (S,G) pair will be received
from that upstream device until the prune state expires. You can configure the PIM Prune Timer
(the length of time that a prune state is considered valid).
For example, in Figure 5 the sender with address 207.95.5.1 is sending multicast packets to the
group 229.225.0.1. If a PIM device receives any groups other than that group, the device discards
the group and sends a prune message to the upstream PIM device.
In Figure 6 , device R5 is a leaf node with no group members in its IGMP database. Therefore, the
device must be pruned from the multicast tree. R5 sends a prune message upstream to its
neighbor device R4 to remove itself from the multicast delivery tree and install a prune state, as
seen in
Figure 6 . Device 5 will not receive any further multicast traffic until the prune age interval
expires.
When a node on the multicast delivery tree has all of its downstream branches (downstream
interfaces) in the prune state, a prune message is sent upstream. In the case of R4, if both R5 and
R6 are in a prune state at the same time, R4 becomes a leaf node with no downstream interfaces
and sends a prune message to R1. With R4 in a prune state, the resulting multicast delivery tree
would consist only of leaf nodes R2 and R3.
FIGURE 5 Transmission of multicast packets from the source to host group members