DHCP relay (option-82) circuit id wrong (sometimes)

[mrobbert]

We have had dhcp snooping turned on and working on our Catalyst 3750 switches for some time now. I just recently added logging of the option-82 info on our ISC DHCP server. As I was checking the data that ISC DHCP was logging I found some ports were reported incorrectly. I know that the ports are numbered starting with 3 so I know to subtract 2 from the logged number (If anybody can explain the reasoning for this I'd love to hear that as well), but some of the ports are 4 higher than their actual port number. I checked the database on the switch and it shows the correct port number. Can anybody explain this or should I open a TAC case?

Thanks,

Mike

[dathelen]

Mike,

I think there are different formats of the what the switch is inserting into the option 82 portion of the dhcp packet. If you are using the the vlan-mod-port, which is the default actually uses the SNMP index for the mod/port number information. However due bug:

CSCeg69592 which was introduced into 12.2(25)SEA the ifindex was no longer a one-to-one corresponce. So the higher port numbers will not look like their real port numbers they could be offset by a bit. Unfortunetly this is no work around for that. The only redeeming factor is that there should be a pattern to it, so using some arthimetic you should be able to figure the port number.

[mrobbert]

OK, I think that I finally found the pattern. There is a shift between port 24 and 25. Port 24 shows up in the DHCP logs as 26 and all ports less than that are 2 arpart, but port 25 shows up as 29 and all above that are 4 apart. While this is nice to know it makes using this feature incredibly difficult and convoluted.

You mention a different format for the data. Can you (or anybody) tell me how to implement that. I thought I read about it somewhere, but can't find anywhere in the documentation to change it. Ultimately I'd like to also change the remote-id field from the mac address to the IP address or hostname of the switch if anybody knows how to do that.

Any idea if/when this bug will be fixed?

Thanks,

Mike

[dathelen]

Mike,

Well I can tell you that the bug is in closed state, which means there is no plans to fix this in softwhere. With that being said the patteren that you have found out should remain the same.

The other option 82 insert mode is what is called remote id:

ip dhcp snooping information option format remote-id [string ASCII-string | hostname]

(Optional) Configure the remote-ID suboption.

You can configure the remote ID to be:

?String of up to 63 ASCII characters (no spaces)

?Configured hostname for the switch

Note If the hostname is longer than 63 characters, it is truncated to 63 characters in the remote-ID configuration.

The default remote ID is the switch MAC address.

that is a snip from the 3550 config guide:

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225see/scg/swdhcp82.htm#wp1148846

thanks,

Dave