3Com Switch 7750 Configuration Guide Guide

572 CHAPTER 54: DHCP SERVER CONFIGURATION
Prerequisites Before configuring DHCP security, you should first complete the DHCP server
configuration (either global address pool-based or interface address pool-based
DHCP server configuration).
Configuring Private
DHCP Server Detecting
A private DHCP server on a network also answers IP address request packets and
assigns IP addresses to DHCP clients. However, the IP addresses they assigned may
conflict with those of other hosts. As a result, users cannot normally access
networks. This kind of DHCP servers are known as private DHCP servers.
With the private DHCP server detecting function enabled, when a DHCP client
sends the DHCP-REQUEST packet, the DHCP server tracks the information (such as
the IP addresses and interfaces) of DHCP servers to enable the administrator to
detect private DHCP servers in time and take proper measures.
Configuring IP Address
Detecting
To avoid IP address conflicts caused by assigning the same IP address to multiple
DHCP clients simultaneously, you can configure a DHCP server to detect an IP
address before it assigns the address to a DHCP client.
IP address detecting is achieved by performing ping operations. To detect whether
an IP address is currently in use, the DHCP server sends an ICMP packet with the IP
address to be assigned as the destination and waits for a response. If the DHCP
server receives no response within a specified time, it resends an ICMP packet. This
procedure repeats until the DHCP server receives a response or the number of the
sent ICMP packets reaches the specified maximum number. The DHCP server
assigns the IP address to the DHCP client only when no response is received during
the whole course, thus ensuring that an IP address is assigned to one DHCP client
exclusively.
Tabl e 446 Enable detection of a private DHCP server
Operation Command Description
Enter system view system-view -
Enable the private DHCP
server detecting function
dhcp server detect
Required
By default, the private DHCP
server detecting function is
disabled
Tabl e 447 Configure IP address detecting
Operation Command Description
Enter system view system-view -
Set the maximum number of
ICMP packets a DHCP server
sends in a ping test
dhcp server ping packets
number
Optional
By default, a DHCP server
performs the ping operation
twice to test an IP address
Set the response timeout time
of each ICMP packet
dhcp server ping timeout
milliseconds
Optional
The default timeout time is
500 milliseconds