Owner's Manual

Chapter 27. Multicast Routing and Switching | 727

NETGEAR 8800 User Manual

Note: For additional information on PIM-DM, see RFC 3973, Protocol

Independent Multicast - Dense Mode (PIM-DM): Protocol

Specification.

PIM-DM Without State Refresh

PIM-DM is a broadcast and prune protocol, which means that multicast servers initially

broadcast traffic to all destinations, and then switches later prune paths on which there are

no receivers.

Figure 82 shows a dense mode multicast tree with an active branch and a pruned branch.

Figure 82. PIM-DM Operation

In Figure 82, multicast traffic is flowing from VLAN V1 connected to switch S1. S1 floods

multicast traffic to both neighbors S2 and S3 which in turn flood multicast traffic to S4 and S5.

S4 has IGMP members, so it floods multicast traffic down to VLAN V6. S5, which has no

multicast members, sends a prune upstream towards the source. The green line shows the

flow of traffic on the active branch, and the red line shows the prune sent upstream for the

pruned branch. After outgoing interface V2 is pruned from the multicast tree, subsequent

multicast traffic from S1 flows only through S2 and S4 and is not forwarded to S3.

After S3 sends a prune upstream, S3 starts a prune hold time timer on outgoing interface V5.

When this timer expires, S3 adds V5 back to the multicast egress list and sends a graft

upstream to pull multicast traffic down again. When multicast traffic arrives from S1, it is

forwarded to S5, which repeats the upstream prune message because it still has no

members. This prune, time-out, and flood process repeats as long as the traffic flow exists

and no members are on the pruned branch, and this process consumes bandwidth during

every cycle.

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