Principles for selecting an address pool

The DHCP server observes the following principles to select an address pool for a client:

1. If there is an address pool where an IP address is statically bound to the MAC address or ID of the client, the DHCP server selects this address pool and assigns the statically bound IP address and other configuration parameters to the client.

2. If the receiving interface has an address pool applied, the DHCP server selects an IP address and other configuration parameters from this address pool.

3. If the receiving interface has a DHCP policy and the DHCP client matches a user class, the DHCP server selects the address pool that is bound to the matching user class. If no matching user class is found, the server assigns an IP address and other parameters from the default DHCP address pool. If no default address pool is specified or the default address pool does not have assignable IP addresses, the address assignment fails.

4. If the above conditions are not met, the DHCP server selects an address pool depending on the client location.

   • Client on the same subnet as the server—The DHCP server compares the IP address of the receiving interface with the primary subnets of all address pools.

     • If a match is found, the server selects the address pool with the longest-matching primary subnet.

     • If no match is found, the DHCP server compares the IP address with the secondary subnets of all address pools. The server selects the address pool with the longest-matching secondary subnet.

   • Client on a different subnet than the server—The DHCP server compares the IP address in the giaddr field of the DHCP request with the primary subnets of all address pools.

     • If a match is found, the server selects the address pool with the longest-matching primary subnet.

     • If no match is found, the DHCP server compares the IP address with the secondary subnets of all address pools. The server selects the address pool with the longest-matching secondary subnet.

For example, two address pools 1.1.1.0/24 and 1.1.1.0/25 are configured but not applied to any DHCP server's interfaces.

• If the IP address of the receiving interface is 1.1.1.1/25, the DHCP server selects the address pool 1.1.1.0/25. If the address pool has no available IP addresses, the DHCP server will not select the other pool and the address allocation will fail.

• If the IP address of the receiving interface is 1.1.1.130/25, the DHCP server selects the address pool 1.1.1.0/24.

To ensure correct address allocation, keep the IP addresses used for dynamic allocation on one of the subnets:

• Clients on the same subnet as the server—Subnet where the DHCP server receiving interface resides.

• Clients on a different subnet than the server—Subnet where the first DHCP relay interface that faces the clients resides.

  	NOTE: As a best practice, configure a minimum of one matching primary subnet in your network. Otherwise, the DHCP server selects only the first matching secondary subnet for address allocation. If the network has more DHCP clients than the assignable IP addresses in the secondary subnet, not all DHCP clients can obtain IP addresses.