Principles For Selecting An Address Pool - H3C SR8800-F Configuration Manual

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