Understanding Data MDTs

In a draft-rosen Layer 3 multicast virtual private network (MVPN) configured with service provider tunnels, the VPN is multicast-enabled and configured to use the Protocol Independent Multicast (PIM) protocol within the VPN and within the service provider (SP) network. A multicast-enabled VPN routing and forwarding (VRF) instance corresponds to a multicast domain (MD), and a PE router attached to a particular VRF instance is said to belong to the corresponding MD. For each MD there is a default multicast distribution tree (MDT) through the SP backbone, which connects all of the PE routers belonging to that MD. Any PE router configured with a default MDT group address can be the multicast source of one default MDT.

To provide optimal multicast routing, you can configure the PE routers 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 the new MDT group address. An advertisement of a new MDT group address 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 as well as the new data MDT group address used in the provider space. The PE router to which the source site is attached sends the MDT join TLV over the default MDT for that VRF instance every 60 seconds as long as the source is active.

All PE routers in the VRF instance receive the MDT join TLV because it is sent over the default MDT, but not all the PE routers join the new data MDT group:

• PE routers connected to receivers in the VRF instance for the current multicast group cache the contents of the MDT join TLV, adding a 180-second timeout value to the cache entry, and also join the new data MDT group.

• PE routers not connected to receivers listed in the VRF instance for the current multicast group also cache the contents of the MDT join TLV, adding a 180-second timeout value to the cache entry, but do not join the new data MDT group at this time.

After the source PE stops sending the multicast traffic stream over the default MDT and uses the new MDT instead, only the PE routers that join the new group receive the multicast traffic for that group.

When a remote PE router joins the new data MDT group, it sends a PIM join message for the new group directly to the source PE router from the remote PE routers by means of a PIM (S,G) join.

If a PE router that has not yet joined the new data MDT group receives a PIM join message for a new receiver for which (S,G) traffic is already flowing over the data MDT in the provider core, then that PE router can obtain the new group address from its cache and can join the data MDT immediately without waiting up to 59 seconds for the next data MDT advertisement.

When the PE router to which the source site is attached sends a subsequent MDT join TLV for the VRF instance over the default MDT, any existing cache entries for that VRF instance are simply refreshed with a timeout value of 180 seconds.

To display the information cached from MDT join TLV packets received by all PE routers in a PIM-enabled VRF instance, use the show pim mdt data-mdt-joins operational mode command.

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 multicast traffic, 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.