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Implementing Automatic Multicast Routing:
Implementing AMT
The Multicast source sends traffic to the firsthop. Multicast traffic flows through the network until it reaches
the lasthop (receivers) or AMT Relays. AMT relays are typically routers with native IP multicast connectivity
that aggregate a potentially large number of AMT tunnels.
Figure 13: Implementing Automatic Multicast Tunneling
In the above diagram, Relay1 and Relay2 are two AMT Relays, which encapsulate the traffic into AMT
tunnels, and send one copy to each of the AMT Gateways.
AMT Message Exchanges
The AMT protocol defines seven message types for control and encapsulation. The message exchanges happen
in this particular order (as discussed here):
1 Relay Discovery - Gateway sends an AMT discovery message to an anycast address that represents the
AMT relay.
2 Relay Advertisement – Relay responds with an advertisement message, which includes the relay's unique
IP address.
3 Relay Request - Gateway sends an AMT Request message to the relay using the unique IP address as the
destination, along with a nonce to be used for security.
4 Membership Query – Relay responds with an AMT query that includes the nonce from the AMT request
and an opaque security code.
5 Membership Update- Gateway responds with a membership update that includes an encapsulated
IGMPv3/MLDv2 packet.
6 Teardown - Gateway sends a message to stop the delivery of multicast data messages requested in an
earlier membership update message.
Cisco IOS XR Multicast Configuration Guide for the Cisco CRS Router, Release 5.2.x
Implementing AMT
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