4000 Router Series IP Multicast Configuration Guide

173

Default-MDT establishment in a PIM-SM network

Figure 61 Default-MDT establishment in a PIM-SM network

As shown in Figure 61, PIM-SM is enabled in the network and all the PE devices support VPN instance A.

The process of establishing a default-MDT is as follows:

1. The public network interface of PE 1 initiates a join to the public network RP by specifying the

multicast group address as the default-group address in the join message, and a (*, 239.1.1.1)

state entry is created on each device along the path on the public network.

2. At the same time, PE 2 and PE 3 separately initiate a similar join process.

Finally, an RPT is established in the MD, with the public network RP as the root and PE 1, PE 2, and

PE 3 as leaves.

3. To establish PIM neighboring relationships with PE 2 and PE 3 through the MTI in VPN instance A,

PE 1 encapsulates the PIM protocol packet of the private network into a public network multicast

data packet by specifying the source address as the IP address of the MD source interface and the

multicast group address as the default-group address, and then send it to the public network. The

public network interface of PE 1 first registers the multicast source with the public network RP and

the public network RP initiates a join to PE 1. A (11.1.1.1, 239.1.1.1) state entry is created on

each device along the path on the public network.

4. At the same time, PE 2 and PE 3 separately initiate a similar register process.

Finally, three SPTs between the PE devices and the RP are established in the MD.

In the PIM-SM network, the RPT, or the (*, 239.1.1.1) tree, and the three independent SPTs constitute a

default-MDT.

Default-MDT characteristics

No matter what PIM mode is running on the public network, the default-MDT has the following

characteristics:

• All PE devices that support the same VPN instance join the default-MDT.

• All VPN multicast packets that belong to this VPN are forwarded along the default-MDT to every PE

device on the public network, even if they have no active receivers downstream.