There used to be a link to an ancient post with an unfinished tool on an antique forum here, but it went away. The concept was to use Cisco Discovery Protocol to figure out which switch port your clients are attached to.

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This concept looks promising... there's even a direct libpcap interface in Perl, so I could do my own special purpose program instead of using a port of tcpdump. That part where it'll take up to 60 seconds is concerning though. Maybe I could have ldms_auto_gateway do it on /ipaddr trigger, then write it to a registry value for ldms_client to pick up and report.