Page 404

364

Chapter 9

Multicast

the fact that bidirectional distribution eliminates the need to forward data
upstream. You may ask, "How do you define upstream?" It is easy to clarify.
By means of the routing protocols, routers are aware of which interface leads
to the source(s) of the multicast group. That interface is considered
upstream.

The Reverse Path Forwarding process is based on the upstream informa-

tion. When it receives an incoming multicast packet, the router verifies that
the packet came in on an interface that leads back to the source. The router
forwards the packet if the verification is positive, otherwise the packet is dis-
carded. This check stops potential loops. To avoid increased overhead on the
router's processor, a multicast forwarding cache is implemented for the RPF
lookups.

Time to Live (TTL)

You can also control the delivery of IP multicast packets through the TTL
counter and TTL thresholds. The Time-to-Live counter is decremented by
one every time the packet hops a router. Once the TTL counter is set to zero,
the packet is discarded.

Thresholds are used to achieve higher granularity and greater control

within one's own network. Thresholds are applied to specified interfaces of
multicast-enabled routers. The router compares the threshold value of the
multicast packet with the value specified in the interface configuration. If the
TTL value of the packet is greater than or equal to the TTL threshold con-
figured for the interface, the packet will be forwarded through that interface.

TTL thresholds allow network administrators to bound their network

and limit the distribution of multicast packets beyond the boundaries. This
is accomplished by setting high values for outbound external interfaces. The
maximum value for the TTL threshold is 255. Refer to Figure 9.15 to see
how network boundaries can be set to limit distribution of multicast traffic.

www.sybex.com