What is DHCP Option 82?
To put it simply, DHCP Option 82 is the “DHCP Relay Agent Information Option”. Wasn’t that easy?
Option 82 was designed to allow a DHCP Relay Agent to insert circuit specific information into a request that is being forwarded to a DHCP server. Specifically the option works by setting two sub-options: Circuit ID and Remote ID.
The Circuit ID sub-option is supposed to include information specific to which circuit the request came in on. It’s an identifier that is specific to the relay agent, so what kind of circuit is described will vary depending on the relay agent. In an Ethernet-based network this is probably a port on a switch. In other networks, this might be a wireless modem or an ATM virtual circuit.
The Remote ID sub-option was designed to carry information relating to the remote host end of the circuit. In practice, this sub-option usually contains information that identifies the relay agent. In the case of an Ethernet network, this is the MAC address of the relay agent. In a wireless network, this would likely be a unique identifier of the wireless access point. It’s worth noting that the Remote ID sub-option was designed to be globally unique. In other words, Remote ID “7″ should not have different meanings depending on some other context.
In it’s default configuration, the DHCP Relay Agent Information Option passes along port and agent information to a central DHCP server. It is useful in statistical analysis, as well as, indicating where an assigned IP address physically connects to the network. It may also be used to make DHCP decisions based on where the request is coming from or even which user is making the request.
Outside Resources:
- Wikipedia - Dynamic Host Configuration Protocol
- RFC 3046 - DHCP Relay Agent Information Option
- Recommended Operation for Switches Running Relay Agent and Option 82
Related Reading:
June 13th, 2007 at 10:12 am
[…] you’re interested in DHCP Option 82 (”DHCP Relay Agent Information Option”) in your Cisco Catalyst-based network, […]
December 21st, 2007 at 10:30 am
Do you have any experience on the server-side of option 82? I like the examples you gave, but I have yet to see it in action. Like I said in an earlier reply, I usually see people removing this option in most cases due to the fact that they are trying to use DHCP snooping and don’t want option 82 confusing their servers.
December 21st, 2007 at 10:38 am
I have worked with the server side, specifically ISC’s dhcpd. I’ll post my experiences in the near future.
March 3rd, 2008 at 7:20 am
hi
how can i enable dhcp option 82 in windows 2003 server and how server will use the relay agent information to assign the ip address to the client.
Thanks & Regards,
Padma R
March 3rd, 2008 at 7:34 am
@Padma -
From my understanding, Windows 2003 doesn’t support DHCP Option 82. I’ve never attempted it though, so my information is only second-hand.
I have successfully used DHCP Option 82 with ISC’s DHCP server for quite a while, and it works just fine.