4000 Router Series IP Multicast Configuration Guide

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Figure 48 IPv6 multicast source registration

As shown in Figure 48, the IPv6 multicast source registers with the RP as follows:

1. The IPv6 multicast source S sends the first multicast packet to the IPv6 multicast group G. When

receiving the multicast packet, the source-side DR that directly connects to the IPv6 multicast source

encapsulates the packet in an register message and unicasts the message to the RP.

2. After the RP receives the register message, it decapsulates it and forwards it down to the RPT.

Meanwhile, it sends an (S, G) source-specific join message hop by hop toward the IPv6 multicast

source. The routers along the path from the RP to the IPv6 multicast source constitute an SPT branch,

and each router on this branch creates an (S, G) entry in its forwarding table. The SPT is rooted at

the source-side DR, and has the RP as its leaf.

3. The subsequent IPv6 multicast data from the IPv6 multicast source are forwarded to the RP along

the established branch, and the RP forwards the data to the receivers along the RPT. When the IPv6

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 an IPv6 PIM-SM domain, only one RP and one RPT provide services for a specific IPv6 multicast group.

Before the switchover to SPT occurs, the source-side DR encapsulates all IPv6 multicast data addressed to

the IPv6 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 an IPv6 multicast packet might not be the shortest one.

• The RP might be overloaded by IPv6 multicast traffic bursts.