H3C Technologies H3C S10500 Series Switches - Manual (Page 47)

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NOTE:
The extended address pools on a DHCP server are independent of each other and no inheritance
relationship exists among them.
IP address lease durations are not inherited.
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. For the configuration of this address pool, see “
Configuring static address allocation .”
2. If the receiving interface has an extended address pool referenced, the DHCP server will assign an
IP address from this address pool. If no IP address is available in the address pool, the DHCP
server will fail to assign an address to the client. For the configuration of such an address pool, see
“ Configuring dynamic address allocation for an extended address pool .”
3. Otherwise, the DHCP server will select the smallest common address pool that contains the IP
address of the receiving interface (if the client and the server reside on the same subnet), or the
smallest common 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 parent address pool to the client. For the configuration of such an address pool,
see “ Configuring dynamic address allocation .”
For example, two common 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.
NOTE:
To avoid wrong IP address allocation, keep the IP addresses for dynamic allocation within the subnet
where the interface of the DHCP server or DHCP relay agent resides.
IP address allocation sequence
A DHCP server assigns an IP address to a client according to the following sequence:
1. The IP address statically 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 a DHCP-DISCOVER message
4. The first assignable IP address found in an extended or common address pool
5. The IP address that was a conflict or passed its lease duration
If no IP address is assignable, the server will not respond.