cisco C3560 encoding for option agent.circuit-id

Claus Holm Christensen iscdhcp at
Thu Jun 4 13:33:06 UTC 2009

Niall O'Reilly wrote:
> 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.

We had to infer the ordering a year or so ago, it's not exactly easy (we 
even asked Cisco Denmark, if my memory is correct, then they had trouble 
figuring it out as well).

(1st)	You have to count the two internal VLAN interfaces as the first 
two interfaces in the CID numbering (or just accept, that the interfaces 
counts from 3).

(2nd)	Count the first 24 interfaces into the CID numbering.

(3th)	Count the first one or two SFP uplink ports count as the next 
couple of interfaces in the CID numbering.

(4th)	Count the last 24 interfaces (if you have a 48 port switch).

(5th)	Now count the other one or two SFP uplink ports (again only if you 
have a 48 port switch).

The trick about the last two steps really hit us hard.  Please test 
extensively and on multiple ports on multiple switches.

We had a lot of trouble inferring these rules from testing, and it's 
really irritating that the 3550, 3560 and 4500 series follows different 
rules.  I can only recommend that you configure your own installation id 
onto the CID of each port and forget about vlan-mod-port.

Claus Holm Christensen

More information about the dhcp-users mailing list