DVMRP Configuration Commands

We have already seen some of the commands that can be used to configure the route exchange process between a DVMRP and a PIM router. This section will present the rest of the commands that can be used to fine-tune this process.

ip dvmrp metric-offset [in | out] increment

in

Optional. The value of increment is added to routes in incoming DVMRP route reports. The default increment for in is 1.

out

Optional. The value of the increment is added to routes in outgoing DVMRP reports. The default increment for out is 0.

increment

Value added to the routes in a DVMRP route report.

Use this interface command to adjust the metric of DVMRP routes being received on an interface (in) or reported to a neighbor (out). The default value when applied to incoming routes is 1 and the default value applied to outgoing routes is 0. But be careful, this command adds the same metric to all incoming or outgoing routes.

ip dvmrp output-report-delay delay-time [burst]

delay-time

Number of milliseconds between DVMRP route reports.

burst

Optional. Number of packets in a set of route reports. The default value is 2.

Use this interface command to send the route reports to a neighbor. DVMRP typically runs as mrouted on UNIX machines, and if the Cisco router has a large DVMRP routing table, then it is possible for the route reports to overload the DVMRP router, preventing some of the routes from being received. Any missed routes consequently expire and are placed in hold-down. Subsequent reports may fix this problem for the routes missed in previous route reports, but other routes may be dropped in subsequent reports, causing route flapping to occur. The delay-time parameter is the time to wait between sending route report packets to the neighbor and the Burst parameter indicates how many reports to send. For example, if we use

ip dvmrp output-report-delay 300 3

and nine reports must be sent, then the sequence listed on the following page will be executed.

1.   Send three reports.

2.   Wait 300 milliseconds.

3.   Send three reports.

4.   Wait 300 milliseconds.

5.   Send three reports.

The default value for delay-time is 100 milliseconds and the default value of burst is 2.

ip dvmrp route-limit count

count

Number of DVMRP routes that can be advertised. The default value is 7000.

This global command limits the number of routes that can be advertised on an interface that has DVMRP enabled. When the first interface is configured with ip dvmrp unicast-routing, when a DVMRP tunnel is configured, or when a PIM interface hears a DVMRP neighbor, this command is automatically configured with a default limit of 7000 routes. This prevents flooding routes into DVMRP when the ip dvmrp metric command is accidentally misused.

ip dvmrp route-hog-notification count

count

Number of routes allowed before a syslog message is sent. Default is 10,000 routes.

This global command places a limit on the number of routes that can be advertised over a DVMRP-enabled interface, including tunnels during a one-minute interval. If the number is exceeded, a syslog message is sent. This is another method for determining if a misconfigured router is injecting too many routes. The default value is 10,000.

ip dvmrp reject-non-pruners

This is an interface command that prevents peering with a DVMRP neighbor that does not support pruning and grafting.

ip dvmrp default-information {originate | only}

originate

Routes more specific than the default route (0.0.0.0) can be advertised

only

Only the default route (0.0.0.0) is advertised

Here we have an interface command used to advertise the default network 0.0.0.0. to the DVMRP neighbor on the interface. The originate option allows more specific routes to be advertised. The only keyword prevents other routes from being advertised. Do not use this command to inject a default route into the MBONE.

ip dvmrp auto-summary

This interface command is enabled by default. Auto-summarization is when subnets are advertised as a classful network number. To turn off this feature, use the no form of the command.

ip dvmrp summary-address address mask [metric value]

address

The summary IP address that is advertised.

mask

The mask for the summary address.

metric value

Optional. The metric that is advertised for the summary address. The default metric is 1.

This command is used on an interface to summarize addresses in a route report. The summarization applies only to routes in the unicast routing table, not the DVMRP routing table, and the default metric assigned to the summarized route is one of the unicast routing table routes. This command can be used multiple times on an interface. cp9