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.
Источник: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/software/release/12-2_55_se/configuration/guide/scg3750/swdhcp82.html