3Com 4800G Series, Руководство по настройке

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798 C
HAPTER
57: DHCP S
ERVER
C
ONFIGURATION
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, 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.
n
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 to
assign IP addresses to clients:
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, refer to section “Configuring manual address allocation” on page 800.
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 in 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 such 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 the configuration of such address pool,
refer to section “Configuring dynamic address allocation” on page 801.
For example, two address pools are configured on the DHCP server. The ranges of
IP addresses that can be dynamically assigned are 1.1.1.0/24 and 1.1.1.0/25
respectively. 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 the 1.1.1.0/25
address pool. If no IP address is available in the 1.1.1.0/25 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.
n
Keep the IP addresses for dynamic allocation within the subnet where the
interface of the DHCP server resides to avoid wrong IP address allocation.
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 a DHCP-DISCOVER message
4 The first assignable IP address found in a proper DHCP address pool