Default-MDT establishment

The multicast routing protocol running on the public network can be PIM-DM, PIM-SM, BIDIR-PIM, or PIM-SSM. The process of creating a default-MDT is different in these PIM modes.

As shown in Figure 69, PIM-DM is enabled on the network, and all the PEs support VPN instance A. The process of establishing a default-MDT is as follows:

1. To establish PIM neighboring relationships with PE 2 and PE 3 through the MTI for VPN instance A, PE 1 does the following:

   1. 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 MVPN source interface and the multicast group address as the default-group address.

   2. Sends the multicast data packet to the public network.

   For other PEs that support VPN instance A as default-group members, PE 1 of VPN instance A initiates a flood-prune process in the entire public network. A (11.1.1.1, 239.1.1.1) state entry is created on each device along the path on the public network. This forms an SPT with PE 1 as the root, and PE 2 and PE 3 as leaves.

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 MVPN, constituting the default-MDT in the PIM-DM network.

As shown in Figure 70, PIM-SM is enabled on the network, and all the PEs support VPN instance A. The process of establishing a default-MDT is as follows:

1. 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 MVPN, 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 for VPN instance A, PE 1 does the following:

   1. 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 MVPN source interface and the multicast group address as the default-group address.

   2. 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, 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 PEs and the RP are established in the MVPN.

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

As shown in Figure 71, BIDIR-PIM runs on the network, and all the PEs support VPN instance A. The process of establishing a default-MDT is as follows:

1. 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.

   At the same time, PE 2 and PE 3 separately initiate a similar join process. Finally, a receiver-side RPT is established in the MVPN, with the public network RP as the root and PE 1, PE 2, and PE 3 as leaves.

2. PE 1 sends a multicast packet with the default-group address as the multicast group address. The DF of each network segment on the public network forwards the multicast packet to the RP. Each device on the path creates a (*, 239.1.1.1) state entry.

   At the same time, PE 2 and PE 3 separately initiate a similar process. Finally, three source-side RPTs are established in the MVPN, with PE 1, PE 2, and PE 3 as the roots and as the public network RP as the leave.

3. The receiver-side RPT and the three source-side RPTs constitute the default-MDT in the BIDIR-PIM network.

As shown in Figure 72, PIM-SSM runs on the network, and all the PEs support VPN instance A. The process of establishing a default-MDT is as follows:

1. PE 1, PE 2, and PE 3 exchange MDT route information (including BGP interface address and the default-group address) through BGP.

2. PE 1 sends a subscribe message to PE 2 and PE 3. Each device on the public network creates an (S, G) entry. An SPT is established in the MVPN with PE 1 as the root and PE 2 and PE 3 as the leaves.

   At the same time, PE 2 and PE 3 separately initiate a similar process, and establish an SPT with itself as the root and the other PEs as the leaves.

3. The three independent SPTs constitute the default-MDT in the PIM-SSM network.

In PIM-SSM, the term "subscribe message" refers to a join message.

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

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

• All multicast packets that belong to this VPN are forwarded along the default-MDT to every PE on the public network, even if no active downstream receivers exist.