R3102-R3103-HP 6600/HSR6600 Routers IP Multicast Configuration Guide

Table Of Contents

Router C and Router D, to specify Router B as the RPF neighbor of Router C and Router C as the RPF

neighbor of Router D, the receiver hosts will receive the multicast data from the multicast source.

Multicast forwarding across unicast subnets

Some networking devices might not support multicast protocols in a network. Multicast devices forward

multicast traffic from a multicast source hop by hop along the forwarding tree. When the multicast traffic

is forwarded to a next-hop device that does not support IP multicast, the forwarding path is blocked. In

this case, you can enable multicast traffic forwarding across the unicast subnet where the

non-multicast-capable device resides by establishing a GRE tunnel between the devices at both ends of

the unicast subnet.

For more information about GRE tunneling, see Layer 3—IP Services Configuration Guide.

Figure 21 Multicast data transmission through a GRE tunnel

As shown in Figure 21, with a GRE tunnel established between Router A and Router B, Router A

encapsulates multicast data in unicast IP packets. Unicast routers then forward the packets to Router B

across the GRE tunnel. Then, Router B strips off the unicast IP header and continues to forward the

multicast data down toward the receivers.

If unicast static routes are configured across the tunnel, any unicast packet can be transmitted through the

tunnel. If you want to dedicate this tunnel to multicast traffic delivery, configure only a static multicast

route across the tunnel so that only multicast packets are transmitted through this tunnel.

Multicast traceroute

You can use the multicast traceroute utility to trace the path that a multicast traffic flows down from the

first-hop router to the last-hop router.

Concepts in multicast traceroute

• Last-hop router—If one of the interfaces of a router connects to the subnet that contains the given

destination address, and if the router can forward multicast streams from the given multicast source

onto that subnet, that router is called the "last-hop router".

• First-hop router—The router that directly connects to the multicast source is called the "first-hop

router".

• Querier—The router that sends multicast traceroute requests is called the "querier".

GRE tunnel

Unicast router

Multicast router

Router ASource ReceiverRouter B