H3C Technologies H3C SecPath F1000-E - Manual (Page 178)

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Configuring DHCP server
Introduction to DHCP server
The DHCP server is well suited to the network where:
• Manual configuration and centralized management are difficult to implement.
• Many hosts need to acquire IP addresses dynamically. This may be because the number of hosts
exceeds the number of assignable IP addresses, so it is impossible to assign a fixed IP address to
each host. For example, an ISP has a limited number of host addresses.
• A few hosts need fixed IP addresses.
DHCP address pool
Address pool structure
In response to a client's request, the DHCP server selects an idle IP address from an address pool and
sends it together with other parameters such as lease and DNS server address to the client.
The address pool database is organized as a tree. The root of the tree is the address pool for natural
networks, branches are address pools for subnets, and leaves are addresses statically bound to clients.
For the same level address pools, a previously configured pool has a higher selection priority than a new
one.
At the very beginning, subnets inherit network parameters and clients inherit subnet parameters.
Therefore, common parameters, for example a DNS server address, should be configured at the highest
(network or subnet) level of the tree.
After establishment of the inheritance relationship, the new configuration at the higher level (parent) 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.
NOTE:
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 such an address pool, see " Creating a static address pool for
the DHCP server ."
2. If the receiving interface has an extended address pool referenced, the DHCP server assigns an IP
address from this address pool. If no IP address is available in the address pool, the DHCP server
fails to assign an address to the client.