CCIE Blog

Current as of Jan 2008.

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IOS 12.1 (3550)

Brian,

When I use the debug ip mpacket command I’m not getting any output. Any idea why?

First off it’s important to understand that multicast traffic is fast switched with the exception of interfaces using X.25 encapsulation. Since the multicast traffic is fast switched it will not be sent to the processor which in turn means the debug will not “see” the multicast packets. You need to disable fast switching for multicast traffic under the interfaces where the multicast traffic will transit. You can do this by using the “no ip mroute-cache” interface level command.

The exception to this is when the router has an IGMP join configured (ip igmp join-group) for a particular group. Multicast traffic destined for the joined group will be process switched and in turn the debug will be able to “see” these packets.