cisco C3560 encoding for option agent.circuit-id

Hans Liss Hans at
Thu Jun 4 13:06:40 UTC 2009

Niall O'Reilly wrote:
> Hello.
> After some Googling and visiting the archives, I'm no wiser, so I'm 
> asking this list.
> I'll appreciate it if anyone can refer me to where I can find a 
> definition for how the vlan-module-port information is encoded
> for option agent.circuit-id by Cisco C3560 switches.
> Here's what I'm seeing.
> VLAN 2, port Fa0/1: option agent.circuit-id 0:4:0:2:1:3;
> VLAN 2, port Fa0/3: option agent.circuit-id 0:4:0:2:1:5;
> VLAN 2, port Fa0/17: option agent.circuit-id 0:4:0:2:1:13;
> I've seen mentioned in the archives an off-by-one issue, but I
> read it as low-by-one; here I'm seeing high-by-one for the module,
> and high-by-two for the port.
Google for "In the port field of the circuit-ID suboption, the port 
numbers start at 3" and you will find plenty of references, mostly from 
official Cisco docs for various switch models.


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