H3C Technologies H3C S12500-X Series Switches - Manual (Page 41)

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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:
All address ranges must belong to the primary subnet. If an address range does not reside in the
primary 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.
NOTE:
To make sure correct address allocation, keep the IP addresses used for dynamic allocation in the subnet
where the interface of the DHCP server or DHCP relay agent resides as possible as you can.