Avaya 8800 Planning And Engineering, Network Design page 194

Multicast network design
public network. This time, Router A has only one multicast-capable interface connected to the
private network. Because one interface precludes the possibility of intradomain multicast transit
traffic, private multicast streams do not need to be forwarded to Router A. In this case, it is inefficient
to use an announce policy on the public interface because private streams are forwarded to Router
A and then are dropped (and pruned) by Router A. In such circumstances, it is appropriate to use an
accept policy on the private interface of Router A. Public multicast streams are forwarded to the
private network as desired.
Figure 88: Accept policy on a border router
Accept policies are useful when you cannot control routing updates on the neighboring router. For
example, a service provider cannot directly control the routes advertised by its neighboring router,
so the provider can configure an accept policy to only accept certain agreed-on routes.
You can use an accept policy to receive a default route over an interface. If a neighbor supplies a
default route, you can accept only that route and discard all others, which reduces the size of the
routing table. In this situation, the default route is accepted and poison-reversed, whereas the more
specific routes are filtered and not poison-reversed.
You can also use announce or accept policies (or both) to implement a form of traffic engineering for
multicast streams based on the source subnet. The following figure shows a network where multiple
potential paths exist through the network. According to the default settings, all multicast traffic in this
network follows the same path to the receivers. Load balancing can distribute the traffic to the other
available links. To make the path between Routers B and D more preferable, use announce policies
on Router A to increase the advertised metric of certain routes. Thus, traffic that originates from
those subnets takes the alternate route between B and D.
June 2016 Planning and Engineering — Network Design
Comments on this document? infodev@avaya.com
194