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• An MD is assigned a unique data-group address range for MDT switchover. When a PE device
keeps receiving a VPN multicast stream that entered the public network for a period of time, the PE
does the following:
{ Selects an address that is least referenced from the data-group address range.
{ Uses the address to encapsulate the multicast packets for that VPN.
• All the PE devices in the network monitor the forwarding traffic on the default-MDT.
a. When a specific PE device continuously receives the multicast traffic of a VPN, the PE device
initiates an MDT switchover message to the downstream along the default-MDT. This causes a
data-MDT to be built by using the data-group between that PE device and the remote PE
devices with downstream receivers.
b. After a data-delay period has passed, an MDT switchover process starts. All VPN multicast
packets that have entered the public network through that PE device are not encapsulated with
the default-group address. They are encapsulated into public network multicast packets with
the data-group address. Then they are switched from the default-MDT to the data-MDT.
For more information about MDT switchover, see "MDT switchover."
NOTE:
VPN uniquely corresponds to an MD and an MD provides services for only one VPN, which is called a
one-to-one relationship. Such a relationship exists between VPN, MD, MTI, default-group, and the
data-group address range.
PIM neighboring relationships in MD-VPN
Figure 59 PIM neighboring relationships in MD-VPN
PIM neighboring relationships are established between two or more directly interconnected devices on
the same subnet. As shown in Figure 5
9, the following types of PIM neighboring relationships exist in
MD-VPN:
• PE-P PIM neighboring relationship—Established between the public network interface on a PE
device and the peer interface on the P device over the link.
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