Configuring How the Extended DHCP Local Server Determines Which Address-Assignment Pool to Use

You can specify the match order in which the extended DHCP local server uses the client data to determine the address-assignment pool that provides the IP address and configuration for a DHCP client. You use the pool-match-order statement to specify the match order. If you do not specify the pool-match-order, the router uses the default ip-address-first matching to select the address pool. After the DHCP local server determines the address assignment pool to use, the server performs the matching based on the criteria you specified in the pool configuration.

In the default ip-address-first matching, the server selects the address-assignment pool to use by matching the IP address in the client DHCP request with the network address of the address-assignment pool. If the client request contains the gateway IP address (giaddr), the local server matches the giaddr to the address-assignment pool’s address. If there is no giaddr in the request, then the DHCP local server matches the IP address of the receiving interface to the address of the address-assignment pool.

For IPv4 address-assignment pools, you can optionally configure the extended DHCP local server to match the DHCP relay agent information option (option 82) in the client DHCP packets to a named range in the address-assignment pool used for the client. Named ranges are subsets within the overall address-assignment pool address range, which you can configure when you create the address-assignment pool.

Note: To use the DHCP local server option 82 matching feature with an IPv4 address-assignment pool, you must ensure that the option-82 statement is included in the dhcp-attributes statement for the address-assignment pool.

You can optionally include the option-82-strict method in the pool-match-order statement for matching the address-assignment pool. If you include the option-82-strict method, the extended DHCP local server applies the ip-address-first matching method first and then uses the option-82 method to select the address-assignment pool. The ip-address-first method must be specified before the option-82-strict method in the pool-match-order list.

Note: If the option-82-strict method is configured in the pool-match-order list, any DHCP discover packet with either unknown or no option-82 information will be denied with an IP address. A DHCP client is allocated with an IP address only when the DHCP packet matches the option-82 information configured as a part of the dhcp-attributes statement. If dhcp-attributes to match for specific option-82 value is not configured as a part of the selected pool configuration, the option-82-strict match order option will have no effect and IP addresses are allocated by the DHCP local server by matching the subnet of the incoming interface.

1. Access the pool-match-order configuration.
   [edit system services dhcp-local-server]

   user@host# edit pool-match-order
2. Specify the pool matching methods in the order in which the router performs the methods. You can specify the methods in any order. All methods are optional—the router uses the ip-address-first method by default.
   • Configure the router to use the ip-address-first method.
     [edit system services dhcp-local-server pool-match-order]

     user@host# set ip-address-first
   • (IPv4 address-assignment pools only) Specify the option-82 matching method.
     [edit system services dhcp-local-server pool-match-order]

     user@host# set option-82
   • (IPv4 address-assignment pools only) Specify the option-82-strict matching method.
     [edit system services dhcp-local-server pool-match-order]

     user@host# set option-82-strict