H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 376)

35-3
4) All DHCP servers receive the DHCP-REQUEST message, but only the server to which the client
sent a formal request for the offered IP address returns a DHCP-ACK message to the client,
confirming that the IP address has been allocated to the client, or returns a DHCP-NAK unicast
message, denying the IP address allocation.
z
After the client receives the DHCP-ACK message, it will probe whether the IP address assigned by
the server is in use by broadcasting a gratuitous ARP packet. If the client receives no response
within specified time, the client can use this IP address. Otherwise, the client sends a
DHCP-DECLINE message to the server to request an IP address again.
z
If there are multiple DHCP servers, IP addresses offered by other DHCP servers are assignable to
other clients.
IP Address Lease Extension
The IP address dynamically allocated by a DHCP server to a client has a lease. After the lease duration
elapses, the IP address will be reclaimed by the DHCP server. If the client wants to use the IP address
again, it has to extend the lease duration.
After the half lease duration elapses, the DHCP client will send the DHCP server a DHCP-REQUEST
unicast message to extend the lease duration. Upon availability of the IP address, the DHCP server
returns a DHCP-ACK unicast confirming that the client’s lease duration has been extended, or a
DHCP-NAK unicast denying the request.
If the client receives the DHCP-NAK message, it will broadcast another DHCP-REQUEST message for
lease extension after 7/8 lease duration elapses. The DHCP server will handle the request as above
mentioned.
DHCP Message Format
Figure 35-3 gives the DHCP message format, which is based on the BOOTP message format and
involves eight types. These types of messages have the same format except that some fields have
different values. The numbers in parentheses indicate the size of each field in bytes.