H3C Technologies H3C S7500E Series Switches - Manual (Page 90)

4-6
After you configure a multicast static route on Router C and Router D, specifying Router B as
the RPF neighbor of Router C and specifying Router C as the RPF neighbor of Router D, the
receivers can receive multicast data sent by the multicast source.
z A multicast static route only affects RPF check; it cannot guide multicast forwarding.
Therefore, a multicast static route is also called an RPF static route.
z A multicast static route is effective only on the multicast router on which it is configured,
and will not be advertised throughout the network or redistributed to other routers.
Application of GRE Tunnel in Multicast Forwarding
There may be routers that do not support multicast protocols in a network. As multicast traffic
from a multicast source is forwarded hop by hop by multicast routers along the forwarding tree,
when the multicast traffic is forwarded to a next-hop router 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 router resides by establishing a generic
routing encapsulation (GRE) tunnel between the routers at both ends of the unicast subnet.
For details about GRE tunneling, refer to Tunneling Configuration in the Layer 3 - IP Services
Configuration Guide.
Figure 4-4 Multicast data transmission through a GRE tunnel
As shown in Figure 4-4 , with a GRE tunnel established between Router A and Router B, Router
A encapsulates multicast data in unicast IP packets, which are then forwarded by unicast
routers to Router B across the GRE tunnel. Then, Router B strips off the unicast IP header and
continues forwarding the multicast data down towards the receivers.
If unicast static routes are configured across the tunnel, any unicast packet can be transmitted
through the tunnel. If you wish the tunnel to be dedicated to multicast traffic delivery, you can
configure only a multicast static route across the tunnel, so that unicast packets cannot be
transmitted through this tunnel.