Using DHCP and DHCP Option 82
DHCP is short for Dynamic Host Configuration Protocol. We know that DHCP is used in LAN environments to dynamically assign host IP addresses from a centralized server, which reduces the overhead of administrating IP addresses.
I’ve read an article “DHCP Snooping and DHCP Snooping Configuration” that is about a CCIE’s experience. In that article it also shares the DHCP Option 82 concept.
In this article we will share some info of using the DHCP Option 82.
DHCP also helps conserve limited IP address space because IP addresses no longer need to be permanently assigned to client devices; only those client devices that are connected to the network require IP addresses. The DHCP relay agent information feature (option 82) enables the DHCP relay agent (Catalyst switch) to include information about itself and the attached client when forwarding
DHCP requests from a DHCP client to a DHCP server. This basically extends the standard DHCP process by tagging the request with the information regarding the location of the requestor. (See the Figure “DHCP Option 82 Operation”)
The following are key elements required to support the DHCP option 82 feature:
• Clients supporting DHCP
• Relay agents supporting option 82
• DHCP server supporting option 82
The relay agent information option is inserted by the DHCP relay agent when forwarding the client-initiated DHCP request packets to a DHCP server. The servers recognizing the relay agent information option may use the information to assign IP addresses and to implement policies such as restricting the number of IP addresses that can be assigned to a single circuit ID. The circuit ID in relay agent option 82 contains information identifying the port location on which the request is arriving.
1. The DHCP option 82 feature is supported only when DHCP snooping is globally enabled and on the VLANs to which subscriber devices using this feature are assigned.
2. DHCP and the DHCP option 82 feature have not been validated in the lab for EttF version 1.1. At this time, Cisco recommends considering only DHCP with option 82 for the application servers at level 3.