H3C Technologies H3C S5120 Series Switches - Manual (Page 246)

1-6
z If the server returns a DHCP-ACK message or does not return any message within a specified
interval, which means the IP address is assignable now, the DHCP relay agent will age out the
client entry with this IP address.
z If the server returns a DHCP-NAK message, which means the IP address is still in use, the relay
agent will not age it out.
Follow these steps to configure periodic refresh of dynamic client entries:
To do… Use the command… Remarks
Enter system view system-view —
Enable periodic refresh of
dynamic client entries dhcp relay security
refresh enable Optional
Enabled by default.
Configure the refresh
interval dhcp relay security
tracker { interval | auto }
Optional
auto by default. (auto interval is
calculated by the relay agent according
to the number of client entries.)
Enabling unauthorized DHCP servers detection
There are unauthorized DHCP servers on networks, which reply DHCP clients with wrong IP
addresses.
With this feature enabled, upon receiving a DHCP request, the DHCP relay agent will record the IP
address of the DHCP server which assigned an IP address to the DHCP client and the receiving
interface. The administrator can use this information to check out any DHCP unauthorized servers.
Follow these steps to enable unauthorized DHCP server detection:
To do… Use the command… Remarks
Enter system view system-view —
Enable unauthorized DHCP
server detection
dhcp relay server-detect
Required
Disabled by default.
With the unauthorized DHCP server detection enabled, the device puts a record once for each DHCP
server. The administrator needs to find unauthorized DHCP servers from the log information. After the
information of recorded DHCP servers is cleared, the relay agent will re-record server information
following this mechanism.
Configuring the DHCP Relay Agent to Send a DHCP-Release Request
This task allows you to release a client’s IP address manually on the DHCP relay agent. After you
configure this task, the DHCP relay agent actively sends a DHCP-RELEASE request that contains the
client’s IP address to be released. Upon receiving the DHCP-RELEASE request, the DHCP server then
releases the IP address for the client; meanwhile, the client’s IP-to-MAC binding entry is removed from
the DHCP relay agent.