Cisco 7604 Configuration Manual page 377
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Chapter 25
Configuring IPv4 Multicast VPN Support
To create data MDTs, each PE router that is forwarding multicast streams to the backbone periodically
examines the traffic being sent in each default MDT as follows:
1.
Each PE router periodically samples the multicast traffic (approximately every 10 seconds for
software switching, and 90 seconds for hardware switching) to determine whether a multicast stream
has exceeded the configured threshold. (Depending on when the stream is sampled, this means that
in a worst-case scenario, it could take up to 180 seconds before a high-bandwidth stream is
detected.)
Note
If a particular multicast stream exceeds the defined threshold, the sending PE router dynamically
2.
creates a data MDT for that particular multicast traffic.
3.
The sending PE router then transmits a DATA-MDT JOIN request (which is a User Datagram
Protocol (UDP) message to port 3232) to the other PE routers, informing them of the new data MDT.
4.
Receiving PE routers examine their VRF routing tables to determine if they have any customers
interested in receiving this data stream. If so, they use the PIM protocol to transmit a PIM JOIN
message for this particular data MDT group (in the global table PIM instance) to accept the stream.
Routers that do not currently have any customers for this stream still cache the information, in case
any customers request it later on.
Three seconds after sending the DATA-MDT JOIN message, the sending PE router removes the
5.
high-bandwidth multicast stream from the default MDT and begins transmitting it over the new data
MDT.
6.
The sending PE router continues to send a DATA-MDT JOIN message every 60 seconds, as long as
the multicast stream continues to exceed the defined threshold. If the stream falls below the
threshold for more than 60 seconds, the sending PE router stops sending the DATA-MDT JOIN
messages, and moves the stream back to the default MDT.
Receiving routers age out the cache information for the default MDT when they do not receive a
7.
DATA-MDT JOIN message for more than three minutes.
Data MDTs allow for high-bandwidth sources inside the VPN while still ensuring optimal traffic
forwarding in the MPLS VPN core.
For technical information about the DATA-MDT JOIN message and other aspects of the data MDT
Note
creation and usage, see the Internet-Draft, Multicast in MPLS/BGP IP VPNs, by Eric C. Rosen et al.
In the following example, a service provider has a multicast customer with offices in San Jose, New
York, and Dallas. The San Jose site is transmitting a one-way multicast presentation. 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. Although PE4 is interconnected to these other routers in the MPLS core, PE4 is
associated with a different customer and is therefore not part of the default MDT.
Figure 25-1
broadcast, which means that no data is flowing along the default MDT. Each PE router maintains a PIM
relationship with the other PE routers over the default MDT, as well as a PIM relationship with its
directly attached PE routers.
OL-4266-08
Data MDTs are created only for (S, G) multicast route entries within the VRF multicast
routing table. They are not created for (*, G) entries.
shows the situation in this network when no one outside of San Jose has joined the multicast
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
Understanding How MVPN Works
25-3
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