4000 Router Series IP Multicast Configuration Guide

multicast data reaches the RP along the SPT, the RP unicasts a register-stop message to the

source-side DR to prevent the DR from unnecessarily encapsulating the data.

Switchover to SPT

CAUTION:

If the router is an RP, disablin

switchover to SPT mi

ht cause multicast traffic forwardin

failures on the

source-side DR. When disabling switchover to SPT, be sure you fully understand its impact on your

network.

In a PIM-SM domain, only one RP and one RPT provide services for a specific multicast group. Before the

switchover to SPT occurs, the source-side DR encapsulates all multicast data addressed to the multicast

group in register messages and sends them to the RP. After receiving these register messages, the RP

decapsulates them and forwards them to the receivers-side DR along the RPT.

Switchover to SPT has the following weaknesses:

• Encapsulation and decapsulation are complex on the source-side DR and the RP.

• The path for a multicast packet might not be the shortest one.

• The RP might be overloaded by multicast traffic bursts.

To eliminate these weaknesses, PIM-SM allows an RP or the receiver-side DR to initiate a switchover to

SPT when the traffic rate exceeds a specific threshold.

• The RP initiates a switchover to SPT:

The RP periodically checks the multicast packet forwarding rate. If the RP finds that the traffic rate

exceeds the specified threshold, it sends an (S, G) source-specific join message hop by hop toward

the multicast source. The routers along the path from the RP to the multicast source constitute an SPT

branch. The subsequent multicast data for the multicast group can be forwarded to the RP along

the branch without being encapsulated.

For more information about the switchover to SPT initiated by the RP, see "Multicast source

regi

stration."

• The receiver-side DR initiates a switchover to SPT:

The receiver-side DR periodically checks the forwarding rate for the multicast packets that the

multicast source S sends to the multicast group G. If the forwarding rate exceeds the specified

threshold, the DR initiates a switchover to SPT, as follows:

a. The receiver-side DR sends an (S, G) source-specific join message hop by hop toward the

multicast source. The routers along the path from the RP to the source-side DR create an (S, G)

entry in their forwarding table to constitute an SPT branch.

b. When the multicast packets for the multicast group are forwarded to the router where the RPT

and the SPT branches, the router drops the multicast packets that reach it along the RPT and

sends a prune message with the RP bit hop by hop to the RP. After receiving the prune message,

the RP forwards it toward the multicast source (supposed only one receiver exists). Thus, the

switchover to SPT is completed.

c. Finally, the multicast source sends the multicast packets for the multicast group to the receiver

along the SPT.

With the switchover to SPT, PIM-SM builds SPTs more economically than PIM-DM does.

Assert

PIM-SM uses a similar assert mechanism as PIM-DM does. For more information, see "Assert."