TCP/IP Tutorial and Technical Overview - IBM Redbooks

6.9.1 MBONE routing
MBONE will most likely become obsolete as IPv6 becomes more widely
implemented.
Multicast traffic does not flow to every Internet location. Until that occurs,
MBONE will consist of a set of multicast network islands. These islands are
interconnected through virtual tunnels. The tunnels bridge through areas that do
not support multicast traffic.
A router that needs to send multicast packets to another multicast island
encapsulates the packets in unicast packets. These encapsulated packets are
transmitted through the standard Internet routers. The destination address
contained in the unicast packets is the endpoint of the tunnel. The router at the
remote end of the tunnel removes the encapsulation header and forwards the
multicast packets to the receiving devices.
Figure 6-15 shows an overview of an MBONE tunnel’s structure.
Figure 6-15 MBONE tunnel
MBONE tunnels have associated metric and threshold parameters. The metric
parameter is used as a cost in the multicast routing algorithm. The routing
algorithm uses this value to select the best path through the network. Figure 6-16
on page 271 depicts an environment containing four multicast sites
interconnected through MBONE tunnels. The tunnels have been assigned
different metric values to skew traffic forwarding through the network.
270 TCP/IP Tutorial and Technical Overview