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a. DHCP matches the client against DHCP user classes in the order they are configured.
b. If the client matches a user class, the DHCP server selects an IP address from the address range
of the user class.
c. If the matching user class has no assignable addresses, the DHCP server matches the client
against the next user class. If all the matching user classes have no assignable addresses, the
DHCP server selects an IP address from the common address range.
d. If the DHCP client does not match any DHCP user class, the DHCP server selects an address in
the IP address range specified by the address range command. If the address range has no
assignable IP addresses or it is not configured, the address allocation fails.
NOTE:
ll address ranges must belong to the primary subnet. If an address ran
e does not reside in the primar
subnet, DHCP cannot assign the addresses in the address range.
• Method 2—Specify a primary subnet and multiple secondary subnets in an address pool.
The DHCP server selects an IP address from the primary subnet first. If there is no assignable IP
address in the primary subnet, the DHCP server selects an IP address from secondary subnets in the
order they are configured.
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 there is no static address pool and the receiving interface has no address pool applied, the
DHCP server selects an address pool in the following way:
{ If the client and the server reside on the same subnet, the DHCP server matches the IP address
of the receiving interface against the primary subnets of all address pools, and selects the
address pool with the longest-matching primary subnet. If no matching primary subnet is found,
the DHCP server matches the IP address against the secondary subnets of all address pools,
and selects the address pool with the longest-matching secondary subnet.
{ If the client and the server reside on different subnets (a DHCP relay agent is in-between), the
DHCP server matches the IP address in the giaddr field of the DHCP request against the primary
subnets of all address pools, and selects the address pool with the longest-matching primary
subnet. If no matching primary subnet is found, the DHCP server matches the IP address against
the secondary subnets of all address pools, and 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 on the DHCP server.
If the IP address of the interface receiving DHCP requests is 1.1.1.1/25 and no address pool is
applied on the interface, the DHCP server selects IP addresses for clients from the address pool
1.1.1.0/25. If no IP address is available in the address pool, the DHCP server fails to assign
addresses. If the IP address of the receiving interface is 1.1.1.130/25, the DHCP server selects IP
addresses for clients from the address pool 1.1.1.0/24.
It is a good practice to make sure the primary subnet can be matched so the DHCP server turns to
the secondary subnets only when the matching primary subnet has no assignable IP addresses. If
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