ISC DHCP Relay Agent Information Options Configuration
In addition to populating the GIAddr field within a DHCP message relay agents may also specify additional parameters about the client within the relay agent information option (82). This option is commonly used in broadband service provider environments for example to determine which and how many IP addresses to assign to a given subscriber. Thus a service provider edge router or access server can provide more detail than simply the GIAddr which only provides a subnet address which all subscribers on that interface share. Given the various broadband technologies differing option types or suboptions have been defined:
- Circuit ID - this suboption specifies unique circuit information and was originally used in DSL environments to identify the ATM circuit on which the given subscriber resided though other forms of identification may be used.
- Remote ID - this suboption specifies the hardware address (MAC) of the subscriber device which can be used in Ethernet or cable environment though other forms of identification may be used.
- DOCSIS device class - this suboption used in cable environments provides information about the class of customer premises device connected to the cable network
- Link selection - this suboption enables a device to provide a preferred subnet on which it would like an IP address assignment.
- Subscriber ID - this suboption enables a subscriber ID value to be populated in the relay agent information option to the DHCP server
Other suboptions have also been defined within the IETF. The DHCP server receives the option and places the same value in the return message; it does not specify a value. However the DHCP server can make address and parameter assignment decisions based on this setting. As we mentioned earlier when discussing configuration of classes and subclasses the ISC DHCP server can provide such processing as well as limiting the number of leases provided to a given subscriber (by relay agent suboption value). Currently the ISC DHCP server supports the matching of the following suboptions in the following syntax:
- agent.circuit-id string - matches circuit ID suboption value of string.
- agent.remote-id string - matches remote ID suboption value of string.
- agent.DOSCIS-device-class integer - matches DOCSIS device class suboption value of integer.
- agent.link-selection IP-address - indicates the subnet to which the client is connected to influence address assignment.