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Chapter 24 Configuring IP Multicast Routing
Configuring Advanced DVMRP Interoperability Features
Changing the DVMRP Route Threshold
By default, 10,000 DVMRP routes can be received per interface within a 1-minute interval. When that
rate is exceeded, a syslog message is issued, warning that there might be a route surge occurring. The
warning is typically used to quickly detect when devices have been misconfigured to inject a large
number of routes into the MBONE.
Beginning in privileged EXEC mode, follow these steps to change the threshold number of routes that
trigger the warning:
To return to the default route count, use the no ip dvmrp routehog-notification global configuration
command.
Use the show ip igmp interface privileged EXEC command to display a running count of routes. When
the count is exceeded, *** ALERT *** is appended to the line.
Configuring a DVMRP Summary Address
By default, a Cisco device advertises in DVMRP route-report messages only connected unicast routes
(that is, only routes to subnets that are directly connected to the router) from its unicast routing table.
These routes undergo normal DVMRP classful route summarization. This process depends on whether
the route being advertised is in the same classful network as the interface over which it is being
advertised.
Figure 24-15
shows an example of the default behavior. This example shows that the DVMRP report
sent by the Cisco router contains the three original routes received from the DVMRP router that have
been poison-reversed by adding 32 to the DVMRP metric. Listed after these routes are two routes that
are advertisements for the two directly connected networks (176.32.10.0/24 and 176.32.15.0/24) that
were taken from the unicast routing table. Because the DVMRP tunnel shares the same IP address as
Fast Ethernet 0/1 and falls into the same Class B network as the two directly connected subnets, classful
summarization of these routes was not performed. As a result, the DVMRP router is able to
poison-reverse only these two routes to the directly connected subnets and is able to only RPF properly
for multicast traffic sent by sources on these two Ethernet segments. Any other multicast source in the
network behind the Cisco router that is not on these two Ethernet segments does not properly RPF-check
on the DVMRP router and is discarded.
You can force the Cisco router to advertise the summary address (specified by the address and mask pair
in the ip dvmrp summary-address address mask interface configuration command) in place of any
route that falls in this address range. The summary address is sent in a DVMRP route report if the unicast
routing table contains at least one route in this range; otherwise, the summary address is not advertised.
In
Figure 24-15
, you configure the ip dvmrp summary-address command on the Cisco router tunnel
interface. As a result, the Cisco router sends only a single summarized Class B advertisement for
network 176.32.0.0.16 from the unicast routing table.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip dvmrp routehog-notification
route-count
Configure the number of routes that trigger a syslog message.
The default is 10,000 routes. The range is 1 to 4294967295.
Step 3
end
Return to privileged EXEC mode.
Step 4
show running-config
Verify your entries.
Step 5
copy running-config startup-config
(Optional) Save your entries in the configuration file.
