H3C S3600 Series Operation Manual page 616

Operation Manual – DHCP
H3C S3600 Series Ethernet Switches-Release 1510
III. Configuring the dynamic user address entry updating function
When a DHCP client obtains an IP address from a DHCP server with the help of a
DHCP relay, the DHCP relay creates an entry (dynamic entry) in the user address table
to track the binding information about the IP address and MAC address of the DHCP
client. But as a DHCP relay does not process DHCP-RELEASE packets, which are
sent to DHCP servers by DHCP clients through unicast when the DHCP clients release
IP addresses, the user address entries maintained by the DHCP cannot be updated in
time. The dynamic user address entry updating function is developed to resolve this
problem.
The dynamic user address entry updating function works as follows: at regular intervals,
the DHCP relay sends a DHCP-REQUEST packet that carries the IP address assigned
to a DHCP client and its own bridge MAC address to the corresponding DHCP server. If
the DHCP server answers with a DHCP-ACK packet, the IP address is available (it can
be assigned again) and the DHCP relay ages the corresponding entry in the user
address table. If the DHCP server answers with a DHCP-NAK packet, the IP address is
still in use (the lease is not expired) and the DHCP relay remains the corresponding
user address entry unchanged.
Table 3-6 Configure the dynamic user address entry updating function
Enter system view
Enable
handshake
Set the interval at which
the
dynamically updates the
user address entries
IV. Configuring private DHCP server detection function
If there is an unauthorized DHCP server in the network, when a client applies for an IP
address, the unauthorized DHCP server interconnects with the DHCP client. As a result,
the DHCP client obtains an incorrect IP address. Such unauthorized DHCP server is
called a private DHCP server.
After the private DHCP server detection function is enabled on a DHCP relay, when a
DHCP client sends the DHCP-REQUEST packet, the DHCP relay can obtain from the
packet the information (such as the IP address and interface receiving the packet) of
the DHCP server that assigns an IP address to the client. As a result, the administrator
can find and deal with the private DHCP server.
Operation
system-view
DHCP relay
dhcp relay hand enable
DHCP relay dhcp-security
{ interval | auto }
Chapter 3 DHCP Relay Configuration
Command
tracker
3-8
Description
—
Required
Optional
Only S3600-EI series
switches among S3600
series switches support
this configuration.