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Configuring DVMRP
Routes
In DVMRP, source network routing information is exchanged in the same basic manner as it is in RIP.
That is to say, periodic Route Report messages are sent between DVMRP neighbors. A Route Report
contains the sender's current routing table. The routing table contains entries that advertise a source
network (with a mask) and a hop-count that is used as the routing metric. (The key difference between the
way routing information is exchanged in DVMRP and in RIP is that DVMRP routes are advertised with a
subnet mask, which makes DVMRP effectively a classless protocol.)
The routing information stored in a DVMRP routing table is separate from the unicast routing table and is
used to build source distribution trees and to perform multicast forwarding (that is, Reverse Path Forward-
ing checks).
The ip dvmrp report-interval command enables you to specify the number of seconds between transmis-
sion of Route Report messages. For example, the following command specifies that a Route Report
message be sent every 60 seconds:
-> ip dvmrp report-interval 60
The ip dvmrp flash-interval command enables you to specify the number of seconds between transmis-
sion of Routing Table Change messages. Routing Table Change messages are sent between transmissions
of the complete routing tables contained in Route Report messages. For this reason, the Flash Interval
value must be lower than the Route Report interval. For example:
-> ip dvmrp flash-interval 5
The ip dvmrp route-timeout command enables you to specify the route expiration timeout value. The
route expiration timeout value determines the number of seconds before a route to an inactive network is
aged out. For example, the following command specifies that the route to an inactive network age out in
140 seconds:
-> ip dvmrp route-timeout 140
The ip dvmrp route-holddown command enables you to specify the number of seconds that DVMRP
routes are kept in a hold-down state. A hold-down state refers to the period of time that a route to an inac-
tive network continues to be advertised as unreachable. When a route is deleted (because it expires, the
neighbor it was learned from goes down, etc.) a router may be able to reach the source network described
by the route through an alternate gateway. However, in the presence of complex topologies, often the
alternate gateway may only be echoing back the same route learned via a different path. If this occurs, the
route will continue to be propagated long after it is no longer valid.
In order to prevent this, it is common in distance vector protocols to continue to advertise a route that has
been deleted with a metric of infinity for one or more report intervals. This is a hold-down. While it is in
hold-down, a route must only be advertised with an infinity metric. The hold down period is usually two
report intervals.
For example, the following command specifies that the route to an inactive network continue to be adver-
tised for 120 seconds:
-> ip dvmrp route-holddown 120
Note. Current global DVMRP parameter values—including the ip dvmrp report-interval, ip dvmrp
flash-interval, ip dvmrp route-timeout, and ip dvmrp route-holddown values—can be viewed via the
show ip dvmrp
command. The DVMRP routes that are being advertised to other routers can be viewed
via the
show ip dvmrp route
OmniSwitch AOS Release 7 Advanced Routing Configuration Guide
command.
March 2011
Configuring DVMRP
page 5-13
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