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If so, it joins the data-MDT rooted at PE 1. Otherwise, it caches the message and will join the
data-MDT when it has attached receivers.
4. After sending the MDT switchover message, PE 1 waits a certain period of time (known as the
data-delay period). After this period of time, PE 1 starts using the default-group address to
encapsulate the VPN multicast data. The multicast data is then forwarded down the data-MDT.
5. After the multicast traffic is switched from the default-MDT to the data-MDT, PE 1 continues sending
MDT switchover messages periodically. Subsequent PE devices with attached receivers can then
join the data-MDT. When a downstream PE device no longer has active receivers attached to it, it
leaves the data-MDT.
For a given VPN instance, the default-MDT and the data-MDT are both forwarding tunnels in the same
MD. A default-MDT is uniquely identified by a default-group address, and a data-MDT is uniquely
identified by a data-group address. Each default-group is uniquely associated with a data-group address
range.
Backward switching from data-MDT to default-MDT
After the VPN multicast traffic is switched to the data-MDT, the multicast traffic conditions might change
and no longer meet the switchover criterion. In this case, PE 1, as in the preceding example, initiates a
backward MDT switchover process when any of the following criteria are met:
• The traffic rate of the VPN multicast data has dropped below the switchover threshold and stayed
lower than the threshold for a certain period of time. This period of time is known as the
data-holddown period.
• The associated data-group address range is changed, and the data-group address for
encapsulating the VPN multicast data is out of the new address range.
• The ACL rule for controlling the switchover from the default-MDT to the data-MDT has changed, and
the VPN multicast data fails to pass the new ACL rule.
Inter-AS MD VPN
If the nodes of a VPN network are allocated in multiple ASs, these VPN nodes must be interconnected.
To implement inter-AS VPN, VRF-to-VRF PE interconnectivity and multi-hop EBGP interconnectivity are
available.
VRF-to-VRF PE interconnectivity
As shown in Figure 64:
• A VPN involves AS 1 and AS 2.
• PE 3 and PE 4 are 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.
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