HP A6600 Configuration Manual page 227

Figure 61 Relationship between PIMon 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
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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
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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 MDT switchover. When the rate of a VPN
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multicast stream that entered the public network at a PE device exceeds the switchover threshold, the
PE chooses an idle address (namely, switch-group) from the switch-group-pool, and encapsulates the
multicast packets for that VPN using that address.
All the ingress PE devices in the network monitor the forwarding rate on the share-MDT. When the
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rate of a VPN multicast stream that entered the public network at a specific PE device exceeds the
threshold, the PE device initiates an MDT switchover message to the downstream along the
share-MDT, so that a switch-MDT is built using the switch-group between that PE device and the
remote PE devices with receivers downstream. Then, after a switch-delay period has passed, an
MDT switchover process starts. All VPN multicast packets that have entered the public network at
that PE device are encapsulated into public network multicast packets using the switch-group,
instead of using the share-group, so that they are switched from the share-MDT to the switch-MDT.
A VPN uniquely corresponds to an MD and an MD serves only one VPN, which is called a one-to-one
relationship. Such a relationship exists between VPN, MD, MTI, share-group, and switch-group-pool.
For more information about MDT switchover, see "MDT switchover."
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