MDT switchover
Switching from default-MDT to data-MDT
When a multicast packet of a VPN is transmitted through the default-MDT on the public network, the packet is forwarded to all PEs that support that VPN instance. This occurs whether or not any active receivers exist in the attached sites. When the VPN multicast traffic is high, multicast data might get flooded on the public network. This increases the bandwidth use and brings extra burden on the PEs.
To optimize multicast transmission of large VPN multicast traffic that enters the public network, the MDT-based MVPN solution introduces a dedicated data-MDT. The data-MDT is built between the PEs that connect VPN multicast receivers and multicast sources. When specific network criteria are met, a switchover from the default-MDT to the data-MDT occurs to forward VPN multicast traffic to receivers.
The process of default-MDT to data-MDT switchover is as follows:
The source-side PE (PE 1, for example) starts a data-delay timer when receiving the first multicast VPN packet that matches the switchover criterion.
When the data-delay timer expires, PE 1 sends an MDT switchover message to all the other PE devices down the default-MDT. This message contains the multicast source address, the multicast group address, and the least-used data-group address in the data-group range.
Each PE that receives this message examines whether it interfaces with a VPN that has receivers of that VPN multicast stream.
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.
After sending the MDT switchover message, PE 1 starts the data-delay timer. When the timer expires, PE 1 uses the default-group address to encapsulate the VPN multicast data. The multicast data is then forwarded down the data-MDT.
After the multicast traffic is switched from the default-MDT to the data-MDT, PE 1 continues sending MDT switchover messages periodically. Subsequent PEs with attached receivers can then join the data-MDT. When a downstream PE 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 MVPN. 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 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 associated data-group 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.