3Com S7906E - Switch, Руководство по настройке

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How MD-VPN Works
This section describes the implementation principle of the MD-VPN technology, including establishment
of a share-MDT, packet delivery over it, and implementation of multi-AS MD-VPN.
For a VPN instance, multicast data transmission in the public network is transparent. The MTIs at the
local PE device and the remote PE device form a channel for the seamless transmission of private
network data over the public network. All that is known to the VPN instance is that the VPN data is sent
out the MTI and then the remote site can receive the data through the MTI. Actually, the multicast data
transmission process (the MDT transmission process) over the public network is very complicated.
Share-MDT Establishment
The multicast routing protocol running in the public network can be PIM-DM, PIM-SM, or PIM-SSM. The
process of creating a share-MDT is different in these three PIM modes.
Share-MDT establishment in a PIM-DM network
Figure 1-5 Share-MDT establishment in a PIM-DM network
As shown in Figure 1-5 , PIM-DM is enabled in the network and all the PE devices support VPN instance
A. The process of establishing a share-MDT is as follows:
The public instance on PE 1 initiates a flood-prune process in the entire public network, with the BGP
interface address (namely the interface address used to establish the BGP peer) as the multicast
source address and the share-group address as the multicast group address. All the other PE devices
that are running VPN instance A are group members, so that a (11.1.1.1, 239.1.1.1) state entry is
created on each device along the path in the public network. This forms an SPT with PE 1 as the root,
and PE 2 and PE 3 as leaves.
At the same time, PE 2 and PE 3 respectively initiate a similar flood-prune process. Finally, three
independent SPTs are established in the MD. In the PIM-DM network, these independent SPTs
constitute a share-MDT.