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How MD VPN works
This section describes default-MDT establishment, multicast traffic delivery based on the
default-MDT, and inter-AS MD VPN implementation.
For a VPN instance, multicast data transmission on the public network is transparent. The VPN data
is exchanged between the MTIs of the local PE and the remote PE. This implements the seamless
transmission of the VPN data over the public network. However, the multicast data transmission
process (the MDT transmission process) over the public network is very complicated.
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.
Default-MDT establishment in a PIM-DM network
Figure 61 Default-MDT establishment in a PIM-DM network
As shown in Figure 61, PIM-DM is enabled on the network, and all the PE devices 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 the 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.
For other PE devices 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 MD, constituting the default-MDT in the
PIM-DM network.
BGP: 11.1.1.1/24
Public instance BGP peers
MD
P
PE 3
PE 2PE 1
BGP: 11.1.2.1/24
SPT (11.1.1.1, 239.1.1.1)
SPT (11.1.2.1, 239.1.1.1)
SPT (11.1.3.1, 239.1.1.1)
BGP: 11.1.3.1/24
Default-group: 239.1.1.1
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