Data MDTs Overview
For draft-rosen MVPNs with service provider tunnels operating in SSM mode, the PE routers initially discover one another in a VPN routing and forwarding (VRF) instance across the default MDT. Any PE router configured with a default MDT group address can be the multicast source of one default MDT.
The PE routers can be configured so that when the multicast source within a site exceeds a traffic rate threshold, the PE router to which the source site is attached creates a new data MDT and advertises a new MDT group address.
The advertisement is sent in a User Datagram Protocol (UDP) type length value (TLV) packet called an MDT join TLV. The MDT join TLV identifies the source and group pair (S,G) in the VRF instance and the new data MDT group address used in the provider space. The PE router to which the source site is attached periodically announces the MDT join TLV over the default MDT for that VRF instance as long as the source is active. All PE routers receive the MDT join TLV because it is sent over the default MDT.
The traffic from that source is sent over the new data MDT. PE routers that have interested receivers can join the multicast group and receive the stream over the new data MDT. PE routers that do not have interested receivers do not join the group. After the PE routers join the new data MDT, the multicast traffic for that group is no longer sent over the default MDT.
Only the PE routers with receivers in the VRF instance for that multicast group join the new group. The PE routers must join the new group to receive the multicast traffic now sent over the new MDT by the source PE. PE routers without interested receivers listed in the VRF instance ignore the MDT join TLVs.
When remote PE routers join the new data MDT group, they send a PIM join message for the new group. The PIM join message for the new group is sent directly to the source PE router from the remote PE routers by means of PIM source-specific multicast. SSM mode using (S,G) is possible with a data MDT because the source address is known from the signaling used with a data MDT.
The source PE router starts encapsulating the multicast traffic for the VRF instance using the new data MDT group after 3 seconds, allowing time for the remote PE routers to join the new group. The source PE router then halts the flow of multicast packets over the default MDT, and the packet flow for the VRF instance source shifts to the newly created data MDT.
The PE router monitors the traffic rate during its periodic statistics-collection cycles. If the traffic rate drops below the threshold or the source stops sending, the PE router to which the source site is attached stops announcing the MDT join TLVs and switches back to sending on the default MDT for that VRF instance.
