R3303-HP HSR6800 Routers IP Multicast Configuration Guide

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5. After the multicast traffic is switched from the share-MDT to the switch-MDT, PE 1 continues sending
MDT switchover messages periodically, so that subsequent PE devices with attached receivers can
join the switch-MDT. When a downstream PE device has no longer active receivers attached to it,
it leaves the switch-MDT.
For a given VPN instance, the share-MDT and the switch-MDT are both forwarding tunnels in the same
MD. A share-MDT is uniquely identified by a share-group address, and a switch-MDT is uniquely
identified by a switch-group address. Each share-group is uniquely associated with a set of switch-group
addresses, namely, a switch-group-pool.
Backward switching from switch-MDT to share-MDT
After the VPN multicast traffic is switched to the switch-MDT, the multicast traffic conditions might change
and no longer meet the aforesaid switchover criterion. In this case, PE 1, as in the preceding example,
initiates a backward MDT switchover process. When any of the following criteria is met, the multicast
traffic is switched from the switch-MDT back to the share-MDT:
• The associated switch-group-pool is changed and the switch-group address for encapsulating the
VPN multicast data is out of the new address pool.
• The ACL rule for controlling the switching of VPN multicast traffic from the share-MDT to the
switch-MDT is changed and the VPN multicast data fails to pass the new ACL rule.
Multi-AS MD VPN
If the nodes of a VPN network are allocated in multiple ASs, these VPN nodes must be interconnected.
To implement multi-AS VPN, VRF-to-VRF PE interconnectivity and multi-hop EBGP interconnectivity are
available.
VRF-to-VRF PE interconnectivity
As shown in Figure 72, a VPN involves AS 1 and AS 2. PE 3 and PE 4 are the autonomous system
boundary routers (ASBRs) for AS 1 and AS 2, respectively. PE 3 and PE 4 are interconnected through
their respective VPN instance and treat each other as a CE device.
Figure 72 VPN instance-VPN instance interconnectivity
In the VPN instance-to-VPN instance interconnectivity approach, a separate MD must be established
within each AS, and VPN multicast traffic between different ASs is transmitted between these MDs.
Because only VPN multicast traffic is forwarded between ASBRs, different PIM modes can run within
different ASs. However, the same PIM mode (PIM-DM, PIM-SM, or PIM-SSM) must run on all interfaces
that belong to the same VPN (including interfaces with VPN bindings on ASBRs).