H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 202)

26-7
At the very beginning, subnetworks inherit network parameters and clients inherit subnetwork
parameters. Therefore, common parameters, for example a DNS server address, should be configured
at the highest (network or subnetwork) level of the tree.
After establishment of the inheritance relationship, the new configuration at the higher level (father) of
the tree will be:
Inherited if the lower level (child) has no such configuration, or
Overridden if the lower level (child) has such configuration.
The IP address lease does not enjoy the inheritance attribute.
Principles for selecting an address pool
The DHCP server observes the following principles to select an address pool when assigning an IP
address to 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 will select this address pool and assign the statically bound IP address to
the client.
2) Otherwise, the DHCP server will select the smallest address pool that contains the IP address of
the receiving interface (if the client and the server reside on the same network segment), or the
smallest address pool that contains the IP address specified in the giaddr field of the client’s
request (if a DHCP relay agent is in-between). If no IP address is available in the address pool, the
DHCP server will fail to assign an address to the client because it cannot assign an IP address from
the father address pool to the client.
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, the DHCP server will select IP
addresses for clients from address pool 1.1.1.0/25. If no IP address is available in the address pool, the
DHCP server will fail to assign addresses to clients. If the IP address of the interface receiving DHCP
requests is 1.1.1.130/25, the DHCP server will select IP addresses for clients from the 1.1.1.0/24
address pool.
IP Address Allocation Sequence
A DHCP server assigns an IP address to a client according to the following sequence:
1) The IP address manually bound to the client’s MAC address or ID
2) The IP address that was ever assigned to the client
3) The IP address designated by the Option 50 field in the DHCP-DISCOVER message
4) The first assignable IP address found in a proper DHCP address pool
5) The IP address that was a conflict or passed its lease duration
If no IP address is assignable, the server does not respond.