HP A6600 Configuration Manual page 226

Concept
Switch-group
Switch-group-pool
Implementation of MD-VPN
Main points in the implementation of MD-VPN are as follows:
The public network of the service provider supports multicast. The PE devices must support the public
1.
network and multiple VPN instances. Each instance runs PIM independently. VPN multicast traffic
between the PE devices and the CE devices is transmitted on a per-VPN-instance basis, but the
public network multicast traffic between the PE devices and the P devices is transmitted through the
public network.
Logically, an MD defines the transmission range of the multicast traffic of a specific VPN over the
2.
public network. Physically, an MD identifies all the PE devices that support that VPN in the public
network. Different VPN instances correspond to different MDs. As shown in
area in the center of each VPN instance plane represents an MD, which serves that particular VPN.
All the VPN multicast traffic in that VPN is transmitted within that MD.
Inside an MD, all the private traffic is transmitted through the MT. The process of multicast traffic
3.
transmission through an MT is as follows. The local PE device encapsulates the VPN data into a
public network packet, which is then forwarded in the public network. The remote PE device
decapsulates the packet to turn it back into a private packet.
The local PE device sends out VPN data through the MTI, and the remote PE devices receive the
4.
private data through the MTI. As shown in
transmission pool, and you can think of an MTI as an entrance or exit of the pool. The local PE
device puts the private data into the transmission pool (the MD) through the entrance (MTI), and the
transmission pool automatically duplicates the private data and transmits the data to each exit (MTI)
of the transmission pool, so that any remote PE device that needs the data can get it from the
respective exit (MTI).
Description
When the multicast traffic of a VPN reaches or exceeds a threshold,
the ingress PE device assigns it an independent multicast address
called switch-group, and notifies the other PE devices that they
should use that address to forward the multicast traffic for that VPN.
This initiates a switchover to the switch-MDT.
The switch-group-pool is a range of multicast addresses. At MDT
switchover, an address (namely, switch-group address) is chosen
from the switch-group-pool. The multicast packets for the VPN that
enter the public network at the PE device are to be encapsulated
using that address. The switch-group-pool of a VPN must not overlap
that of another VPN, and must not contain the share-group of
another VPN.
Figure 61, you can think of an MD as a private data
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Figure 61, the ellipse