MVPN establishes a
static default multicast distribution tree (MDT) for each multicast domain. The
default MDT defines the path used by PE routers to send multicast data and
control messages to every other PE router in the multicast domain.
If Source Specific
Multicast (SSM) is used as the core multicast routing protocol, the multicast
IP addresses used for the default and data MDT must be configured within the
SSM range on all PE routers.
MVPN also supports
the dynamic creation of MDTs for high-bandwidth transmission. Data MDTs are a
feature unique to Cisco IOS software. Data MDTs are intended for high-bandwidth
sources such as full-motion video inside the VPN to ensure optimal traffic
forwarding in the MPLS VPN core. The threshold at which the data MDT is created
can be configured on a per-router or a per-VRF basis. When the multicast
transmission exceeds the defined threshold, the sending PE router creates the
data MDT and sends a UDP message, which contains information about the data
MDT, to all routers on the default MDT. The statistics to determine whether a
multicast stream has exceeded the data MDT threshold are examined once every
second. After a PE router sends the UDP message, it waits 3 more seconds before
switching over; 13 seconds is the worst case switchover time, and 3 seconds is
the best case.
Data MDTs are created
only for (S, G) multicast route entries within the VRF multicast routing table.
They are not created for (*, G) entries regardless of the value of the
individual source data rate.
In the following
example, a service provider has a multicast customer with offices in San Jose,
New York, and Dallas. A one-way multicast presentation is occurring in San
Jose. The service provider network supports all three sites associated with
this customer, in addition to the Houston site of a different enterprise
The default MDT for
the enterprise customer consists of provider routers P1, P2, and P3 and their
associated PE routers. PE4 is not part of the default MDT, because it is
associated with a different customer. The figure shows that no data flows along
the default MDT, because no one outside of San Jose has joined the multicast.
Figure 1. Default Multicast
Distribution Tree Overview
An employee in New
York joins the multicast session. The PE router associated with the New York
site sends a join request that flows across the default MDT for the multicast
domain of the customer. PE1, the PE router associated with the multicast
session source, receives the request. The figure depicts that the PE router
forwards the request to the CE router associated with the multicast source
Figure 2. Initializing the Data
The CE router (CE1a)
begins to send the multicast data to the associated PE router (PE1), which
sends the multicast data along the default MDT. Immediately sending the
multicast data, PE1 recognizes that the multicast data exceeds the bandwidth
threshold for which a data MDT should be created. Therefore, PE1 creates a data
MDT, sends a message to all routers using the default MDT, which contains
information about the data MDT, and, three seconds later, begins sending the
multicast data for that particular stream using the data MDT. Only PE2 has
interested receivers for this source, so only PE2 will join the data MDT and
receive traffic on it.
PE routers maintain a
PIM relationship with other PE routers over the default MDT and a PIM
relationship with directly attached PE routers.