Use Of Data Mdts - Alcatel-Lucent 7750 SR Manual

Virtual Private Routed Network Services

Use of Data MDTs

Using the above method, all multicast data offered by a given CE is always delivered to all other
CEs that are part of the same multicast. It is possible that a number of CEs do not require the
delivery of a particular multicast stream because they have no downstream receivers for a specific
multicast group. At low traffic volumes, the impact of this is limited. However, at high data rates
this could be optimized by devising a mechanism to prune PEs from the distribution tree that
although forming part of the customer multicast have no need to deliver a given multicast stream
to the CE attached to them. To facilitate this optimization, the Rosen draft specifies the use of data
MDTs. These data MDTs are signaled once the bandwidth for a given SG exceeds the configurable
threshold.
Once a PE detects it is transmitting data for the SG in excess of this threshold, it sends an MDT
join TLV (at 60 second intervals) over the default MDT to all PEs. All PEs that require the SG
specified in the MDT join TLV will join the data MDT that will be used by the transmitting PE to
send the given SG. PEs that do not require the SG will not join the data MDT, thus pruning the
multicast distribution tree to just the PEs requiring the SG. After providing sufficient time for all
PEs to join the data MDT, the transmitting PE switches the given multicast stream to the data
MDT.
PEs that do not require the SG to be delivered, keep state to allow them to join the data MDT as
required.
When the bandwidth requirement no longer exceeds the threshold, the PE stops announcing the
MDT join TLV. At this point the PEs using the data MDT will leave this group and transmission
resumes over the default MDT.
Sampling to check if an s,g has exceeded the threshold occurs every ten seconds. If the rate has
exceeded the configured rate in that sample period then the data MDT is created. If during that
period the transmission rate has not exceeded the configured threshold then the data MDT is not
created. If the data MDT is active and the transmission rate in the last sample period has not
exceeded the configured rate then the data MDT is torn down and the multicast stream resumes
transmission over the default MDT.
7750 SR OS Services Guide Page 1085