IP Multicast VPN Routing and Forwarding and Multicast Domains
IP Multicast VPN Routing and Forwarding and Multicast Domains
Multicast VPN introduces multicast routing information to the VPN
routing and forwarding table. When a PE router receives multicast data
or control packets from a customer-edge (CE) router, forwarding is
performed according to the information in the Multicast VRF (MVRF).
A set of Multicast VPN Routing and Forwarding instances that can send
multicast traffic to each other constitutes a multicast domain. For
example, the multicast domain for a customer that wanted to send certain
types of multicast traffic to all global employees would consist of all
CE routers associated with that enterprise.
Multicast Distribution Trees
Multicast VPN establishes a static default 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.
Multicast VPN 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 User Datagram Protocol (UDP) message
that contains information about the data MDT to all routers in the
default MDT. The statistics to determine whether a multicast stream has
exceeded the data MDT threshold are examined once every 10 seconds. If
multicast distributed switching is configured, the time period can be up
to twice as long.
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 customer.
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. Figure 1 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 whether it is
configured to use Sparse Mode, Bidir or SSM within a VRF which contains
both the Dallas and the San Jose sites. PE1, the PE router associated
with the multicast session source, receives the request. Figure 2 depicts that the PE router forwards the request to the CE router associated with the multicast source (CE1a).
Figure 2 Initializing the Data MDT
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 after sending the multicast data, PE1 recognizes that the
multicast data exceeds the bandwidth threshold at which a data MDT
should be created. Therefore, PE1 creates a data MDT, sends a message to
all routers using the default MDT that 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 its directly attached PE
routers.
Figure 3
depicts the final flow of multicast data sourced from the multicast
sender in San Jose to the multicast client in New York. Multicast data
sent from the multicast sender in San Jose is delivered in its original
format to its associated PE router (PE1) using either sparse mode, bidir
or SSM. PE1 then encapsulates the multicast data and sends it across
the data MDT using the configured MDT data groups. The mode used to
deliver the multicast data across the data MDT is determined by the
service provider and has no direct correlation with the mode used by the
customer. The PE router in New York (PE2) receives the data along the
data MDT. The PE2 router deencapsulates the packet and forwards it in
its original format toward the multicast client using the mode
configured by the customer.
Figure 3 Multicast Distribution Tree with VRFs
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