[c-nsp] Multicast Issue

[Jay Ford]

On Wed, 26 Apr 2006, Tim Stevenson wrote:
> You can also always try a span session to a sniffer of gig 4/16 in
> the tx direction to see if the packets are going out.

Be advised that port monitoring on a sup720 (& maybe the sup2) results in
some multicast traffic (at least PIM) being sent out the monitor which didn't
really appear on the port being monitored.  This has something to do with the
way the box handles such traffic internally & Cisco says it can't be fixed or
worked around.  It's damn confusing the first time you see it, so I figured a
warning might save you some of the consternation we went through while trying
to use port monitoring to troubleshoot a multicast issue.  If you see stuff
in the monitor output with L2/L3 addresses which shouldn't be there, consider
this Cisco "restriction" & the possibility that the packets aren't really on
the port you're trying to monitor.

________________________________________________________________________
Jay Ford, Network Engineering Group, Information Technology Services
University of Iowa, Iowa City, IA 52242
email: jay-ford at uiowa.edu, phone: 319-335-5555, fax: 319-335-2951