Avaya 8800 Planning And Engineering, Network Design page 196

Multicast network design
Default route policy examples
Use a default route policy to reduce the size of the multicast routing table for parts of the network
that contain only receivers. You can configure an interface to supply (inject) a default route to a
neighbor.
The default route does not appear in the routing table of the supplier. You can configure an interface
to not listen for the default route. When a default route is learned from a neighbor, it is placed in the
routing table and potentially advertised to its other neighbors, depending on whether or not you
configure the outgoing interfaces to advertise the default route. Advertising a default on an interface
is different from supplying a default on an interface. The former only advertises a default if it has
learned a default on another interface, whereas the latter always advertises a default. The default
setting for interfaces is to listen and advertise, but not supply a default route.
The metric assigned to an injected default route is 1 by default. However, you can alter it. Changing
metrics is useful in situations where two or more routers are advertising the default route to the
same neighbor, but one link or path is preferable over the other. For example, in the following figure,
Router A and B both advertise the default route to Router C. Because Router A is the preferred path
for multicast traffic, configure it with a lower metric (a value of 1 in this case) than that of Router B,
which is configured with a value of 2. Router C then chooses the lower metric and poison-reverses
the route to Router A.
Figure 91: Default route
Avaya recommends that you configure announce policies on Routers A and B to suppress the
advertisement of all other routes to Router C. Alternatively, you can configure accept policies on
Router C to prevent all routes from Router A and Router B, other than the default, from installation in
the routing table.
DVMRP passive interfaces
A DVMRP passive interface acts like an IGMP interface: no DVMRP neighbors, and hence no
DVMRP routes, are learned on that interface. However, multicast sources and receivers exist on the
interface.
The passive interface feature is useful if you wish to use IGMP Snoop and DVMRP on the same
switch. IGMP Snoop and Layer 3 IGMP (with DVMRP and PIM) operate independently of each
other. If you configure DVMRP on interface 1 and IGMP Snoop on interface 2 on Switch A, multicast
data with sources from interface 1 is not forwarded to the receivers learned on interface 2 (and vice
versa). To overcome this communication problem, use a DVMRP passive interface.
June 2016 Planning and Engineering — Network Design
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