[c-nsp] DHCP Option 82 Oddities

[sthaug at nethelp.no]

> We just turned on DHCP Option 82 on our Cisco switches and we are seeing
> some oddities in the logs.
> The switch seems to relay the incorrect port number to the logs.
> For instance, if a device is connected to 1/0/4, the logs will show 1/0/6.
> The difference of 2 is consistent until we get to higher port numbers,
> around port 24.  At that point the difference increases to 4.  So if a
> device is connected to 1/0/40, the log says it's connected to 1/0/44.
> 
> Has anyone else encountered this error? Is there anything that has to be
> configured on the switch to fix this?
> It seems to be an error in the way the switch relays the information.

This is documented (well, at least part of it), and therefore it's a
feature not a bug :-)

See for instance 

    http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_guide_chapter09186a00808e0574.html#wp1069615

"In the port field of the circuit-ID suboption, the port numbers start
at 3. For example, on a switch with 24 10/100 ports and small
form-factor pluggable (SFP) module slots, port 3 is the Fast Ethernet
x/0/1 port, port 4 is the Fast Ethernet x/0/2 port, and so forth,
where x is the stack member number. Port 27 is the SFP module slot
x/0/1, and so forth."

Steinar Haug, Nethelp consulting, sthaug at nethelp.no