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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 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
the ellipse 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
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
de-encapsulates 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 private data through the MTI. As shown in
, you can think of an MD as a private
data 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).
Figure 69 Relationship between PIM on the public network and an MD in a VPN instance
•
Each VPN instance is assigned a unique share-group address. The VPN data is transparent to
the public network. A PE device encapsulates any VPN multicast packet within a normal public
network multicast packet, no matter what multicast group the VPN packet is destined for and
whether it is a protocol packet or a data packet. The PE device uses the share-group as the
public network multicast group for the packet. Then, the PE sends the public network multicast
packet onto the public network.
•
A share-group corresponds to a unique MD. For each share-group, a unique share-MDT is
constructed through the public network resources for multicast data forwarding. All the VPN
multicast packets transmitted in this VPN are forwarded along this share-MDT, no matter at
which PE device they entered the public network.
•
A share-group is assigned a unique switch-group-pool for the MDT switchover. When the rate of
a VPN multicast stream that entered the public network at a PE device reaches or exceeds the
switchover threshold, the PE does the following:
{
Selects an address that is least used from the switch-group-pool.
{
Uses the address to encapsulate the multicast packets for that VPN.
•
All the PE devices on the network monitor the forwarding rate on the share-MDT.
