3Com CoreBuilder 3500, Implementation Manual, Page: 352 / 592

352 C
HAPTER
13: IP M
ULTICAST
R
OUTING
Configuring DVMRP
Default Routes
You can configure a default route for IP multicast traffic on any DVMRP
routing interface in the system.
How Default Routes
Work
If an interface is configured as a default route, it advertises source 0.0.0.0
to neighboring DVMRP routers. In their DVMRP routing tables, these
neighboring routers list 0.0.0.0 as a source and list the advertising router
interface as the
gateway to reach that source.
Thus, if a neighboring router receives an IP multicast packet for which it
has no normal routing information in its routing table, instead of filtering
the packet, the router forwards it to the router which advertises the
default route.
How to Configure A
Default Route
To configure a default route on an interface, follow these steps:
1 Specify the interface index number.
2 Set the default route metric (cost).
Enter a value from 1 through 32 to signify the cost of the route.
The value 0 indicates that no default route is configured.
3 Set the default route mode.
There are two options:
■
all — The interface advertises the default route plus all other known
routes to neighboring DVMRP routers.
■
only — The interface advertises only the default route to neighboring
DVMRP routers.
Important Considerations
■
If the system learns a default route, it propagates it no matter which
mode is set on a given interface.
■
The system allows you to configure an interface as a DVMRP default
route, even when DVMRP is disabled on the interface. If DVMRP is
disabled, the interface does not advertise itself as a default route.