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Data MDT Creation Overview

Initially, the PE routers discover each other in a VPN routing and forwarding (VRF) instance using the default MDT. Each PE router configuration includes in its VRF instance various parameters to control the creation of a data MDT, such as when the source traffic in the VRF instance exceeds the configured threshold rate. The PE router monitors the rate during its periodic statistics-collection cycles. If the source locally attached to the PE router in the VPN exceeds this limit, the source PE advertises the new data MDT group and new MDT with a User Datagram Protocol (UDP) type-length-vector (TLV) packet called an MDT join TLV. The MDT join TLV describes the source and group pair (S,G) in the VRF instance and the new data MDT group address used in the provider space. The source PE 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. Only the PE routers with receivers in the VRF instance for that multicast group can join the new group, and 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 in the provider space. 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). SSM using (S,G) is possible with data MDT instead of the default MDT any-source multicast (ASM) (*,G) because the source address is known from the UDP signaling used with data MDT.

The source PE router starts encapsulating the multicast traffic for the entries in the VRF instance using the new data MDT group after 3 seconds, allowing time for the remote PE routers to switch to the new group. The source PE router then halts the flow of multicast packets over the default MDT, and the packet flow for the entries in the VRF instance source shifts to the newly created data MDT, joined only by PE routers with interested receivers.

When the preconfigured conditions, such as the rate threshold, are no longer met by the source because the source stops sending or the rate falls below the threshold, the source PE stops announcing the MDT join TLVs and the PE router switches to sending on the default MDT for that VRF instance again.

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