Cisco Systems 3750E manual Configuring a Dvmrp Summary Address, Route-count, 42-48

Chapter 42 Configuring IP Multicast Routing

Configuring Advanced DVMRP Interoperability Features

Beginning in privileged EXEC mode, follow these steps to change the threshold number of routes that trigger the warning. This procedure is optional.

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	ip dvmrp routehog-notification	Configure the number of routes that trigger a syslog message.
	route-count	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.

To return to the default setting use the no ip dvmrp routehog-notificationglobal 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 42-9shows 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 port 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-addressaddress 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 42-9and Figure 42-10, you configure the ip dvmrp summary-addresscommand 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.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
42-48	OL-9775-02