Use Of Data Mdts - Alcatel-Lucent 7750 SR OS Service Manual

VPRN Features
The demarcation of these domains is in the PE's (routers 5 through 10). The PE router participates
in both the customer multicast domain and the provider's multicast domain. The customer's CEs
are limited to a multicast adjacency with the multicast instance on the PE specifically created to
support that specific customer's IP-VPN. This way, customers are isolated from the provider's
core multicast domain and other customer multicast domains while the provider's core routers
only participate in the provider's multicast domain and are isolated from all customers' multicast
domains.
The PE for a given customer's multicast domain becomes adjacent to the CE routers attached to
that PE and to all other PE's that participate in the IP-VPN (or customer) multicast domain. This is
achieved by the PE who encapsulates the customer multicast control data and multicast streams
inside the provider's multicast packets. These encapsulated packets are forwarded only to the PE
nodes that are attached to the same customer's edge routers as the originating stream and are part
of the same customer VPRN. This prunes the distribution of the multicast control and data traffic
to the PEs that participate in the customer's multicast domain. The Rosen draft refers to this as the
default multicast domain for this multicast domain; the multicast domain is associated with a
unique multicast group address within the provider's network.

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.
Page 876 7750 SR OS Services Guide