Data MDT Characteristics
A data multicast distribution tree (MDT) solves the problem of routers flooding unnecessary multicast information to PE routers that have no interested receivers for a particular VPN multicast group.
The default MDT uses multicast tunnel (mt-) logical interfaces. Data MDTs also use multicast tunnel logical interfaces. If you administratively disable the physical interface that the multicast tunnel logical interfaces are configured on, the multicast tunnel logical interfaces are moved to a different physical interface that is up. In this case the traffic is sent over the default MDT until new data MDTs are created.
The maximum number of data MDTs for all VPNs on a PE router is 1024, and the maximum number of data MDTs for a VRF instance is 1024. The configuration of a VRF instance can limit the number of MDTs possible. No new MDTs can be created after the 1024 MDT limit is reached in the VRF instance, and all traffic for other sources that exceed the configured limit is sent on the default MDT.
Tear-down of data MDTs depends on the monitoring of the multicast source data rate. This rate is checked once per minute, so if the source data rate falls below the configured value, data MDT deletion can be delayed for up to 1 minute until the next statistics-monitoring collection cycle.
Changes to the configured data MDT limit value do not affect existing tunnels that exceed the new limit. Data MDTs that are already active remain in place until the threshold conditions are no longer met.
In a draft-rosen MVPN in which PE routers are already configured to create data MDTs in response to exceeded multicast source traffic rate thresholds, you can change the group range used for creating data MDTs in a VRF instance. To remove any active data MDTs created using the previous group range, you must restart the PIM routing process. This restart clears all remnants of the former group addresses but disrupts routing and therefore requires a maintenance window for the change.
 | Caution: Never restart any of the software processes unless instructed to do so by a customer support engineer. |
Multicast tunnel (mt) interfaces created because of exceeded thresholds are not re-created if the routing process crashes. Therefore, graceful restart does not automatically reinstate the data MDT state. However, as soon as the periodic statistics collection reveals that the threshold condition is still exceeded, the tunnels are quickly re-created.
Data MDTs are supported for customer traffic with PIM sparse mode, dense mode, and sparse-dense mode. Note that the provider core does not support PIM dense mode.
