177
2. At the same time, PE 2 and PE 3 separately initiate a similar flood-prune process.
Finally, three independent SPTs are established in the MD, constituting a default-MDT in the
PIM-DM network.
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. 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 does the following:
a. Encapsulates the PIM protocol packet of the private network into a public network multicast
data packet. PE 1 does this by specifying the source address as the IP address of the MD
source interface and the multicast group address as the default-group address.
b. Sends the multicast data packet to the public network.
The public network interface of PE 1 registers the multicast source with the public network RP. 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.
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