Configuration Guide Configuring DHCPConfiguring DHCP
Related Commands
 Configuring Excluded IP Address
ip dhcp excluded-address low-ip-address [ high-ip-address ]
low-ip-address: Indicates a start IP address.
high-ip-address: Indicates an end IP address.
Global configuration mode
Unless otherwise specified, a DHCP server assigns all the addresses from an IP address pool to DHCP
clients. To reserve some addresses(e.g., addresses already assigned to the server or devices), you
need to configure these addresses as excluded addresses. To configure a DHCP server, it is
recommended to configure excluded addresses to avoid address conflict and shorten detection time
during address assignment.
 Configuring Compulsory NAK Reply
Global configuration mode
In a WLAN, a DHCP client often moves from one network to another. When a DHCP server receives a
lease renewal request from a client but finds that the client crosses the network segment or that the
lease is expired, it replies with a NAK packet to require the client to obtain an IP address again. This
prevents the client from sending request packets continually before obtaining an IP address again after
timeout.
The server sends a NAK packet only when it finds the client's lease record. When a DHCP client crosses
the network, a DHCP server cannot find lease record of the client and will not reply with a NAK packet.
The client sends request packets continually before obtaining an IP address again after timeout.
Consequently, it takes a long to obtain an IP address. This also occurs when a DHCP server loses a
lease after restart and a client requests lease renewal. In this case, you may configure a command to
force the DHCP server to reply with a NAK packet even though it cannot find the lease record so that the
client may obtain an IP address rapidly. Please note that the command is disabled by default. To enable
it, only one DHCP server can be configured in a broadcast domain.
 Configuring Ping Times
ip dhcp ping packets [ number ]
number: (Optional) Ranges from 0 to 10. 0 indicates the ping function is disabled. The default is two
pings.
Global configuration mode
By default, when a DHCP server assigns an IP address from a pool, it runs the Ping command twice
(one packet per time). If there is no reply, the server takes the address as idle and assigns it to a client. If
there is a reply, the server takes the address as occupied and assigns another address.
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