H3C S9500 Series Operation Manual page 91

Operation Manual – DHCP
H3C S9500 Series Routing Switches
The address check is disabled on a relay agent-enabled VLAN interface by default.
Caution:
After the address check feature is enabled on a DHCP relay agent enabled VLAN
interface, the client that has already obtained an IP address will lose its access right
and has to apply for an IP address again. Therefore, it is recommended that the
administrator should conduct this configuration before any user obtains an IP address.
VII. Enabling refreshing of dynamic user address entries and specifying the
refreshing interval
When a DHCP client wants to release its IP address, it sends a DHCP-RELEASE
unicast to the DHCP server through the DHCP relay agent. The DHCP relay agent
simply conveys the message to the server without removing the entry of the client from
the client entry table. To solve this problem, the system supports refreshing dynamic
user address entries at a configurable interval.
With this feature enabled, the DHCP relay agent sends DHCP-REQUEST packets
periodically to the DHCP server using the recorded IP addresses and its own MAC
address.
A DHCP-ACK packet returned by the DHCP server indicates that the
corresponding IP address is available for being assigned and therefore the DHCP
relay agent ages out the corresponding entry in the user address table.
A DHCP-NAK packet returned by the DHCP server indicates that the lease of the
corresponding IP address has not expired, and therefore the DHCP relay agent
does not age out the corresponding entry.
The maximum times for a DHCP relay agent to send a DHCP-REQUEST packet
to the DHCP server is three. If no response from the server is received after
sending a handshake packet three times, the DHCP relay agent considers the IP
address is available and ages out the corresponding entry in the user address
table.
If DHCP relay handshake is disabled, the DHCP relay agent does not send
DHCP-REQUEST packets to the DHCP server periodically and the entries in the
user address table do not age. As a result, the expired entries are unable to be
removed in time. So, you can disable DHCP relay handshake only when
absolutely needed.
Follow these steps to configure the function of refreshing dynamic user address entries
and the refreshing interval:
Chapter 3 DHCP Relay Agent Configuration
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