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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 24 Configuring IP Multicast Routing
Configuring Advanced DVMRP Interoperability Features
Disabling DVMRP Autosummarization
By default, the Cisco IOS software automatically performs some level of DVMRP summarization.
Disable this function if you want to advertise all routes, not just a summary. In some special cases, you
can use the neighboring DVMRP router with all subnet information to better control the flow of
multicast traffic in the DVMRP network. One such case might occur if the PIM network is connected to
the DVMRP cloud at several points and more specific (unsummarized) routes are being injected into the
DVMRP network to advertise better paths to individual subnets inside the PIM cloud.
If you configure the ip dvmrp summary-address interface configuration command and did not
configure no ip dvmrp auto-summary, you get both custom and autosummaries.
Beginning in privileged EXEC mode, follow these steps to disable DVMRP autosummarization:
To re-enable auto summarization, use the ip dvmrp auto-summary interface configuration command.
Adding a Metric Offset to the DVMRP Route
By default, the multilayer switch increments by 1 the metric (hop count) of a DVMRP route advertised
in incoming DVMRP reports. You can change the metric if you want to favor or not favor a certain route.
For example, a route is learned by multilayer switch A, and the same route is learned by multilayer
switch B with a higher metric. If you want to use the path through switch B because it is a faster path,
you can apply a metric offset to the route learned by switch A to make it larger than the metric learned
by switch B, and you can choose the path through switch B.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the interface connected
to the DVMRP router.
Step 3
no ip dvmrp auto-summary
Disable DVMRP autosummarization.
Step 4
end
Return to privileged EXEC mode.
Step 5
show running-config
Verify your entries.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
