H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 198)

26-3
3) If several DHCP servers send offers to the client, the client accepts the first received offer, and
broadcasts it in a DHCP-REQUEST message to request the IP address formally.
4) All DHCP servers receive the DHCP-REQUEST message, but only the server from which the client
accepts the offered IP address returns a DHCP-ACK message to the client, confirming that the IP
address has been allocated to the client, or a DHCP-NAK unicast message, denying the IP
address allocation.
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 the specified time, the client can use this IP address. Otherwise, the client sends a
DHCP-DECLINE message to the server and requests an IP address again.
IP addresses offered by other DHCP servers are still assignable to other clients.
IP address lease extension
The IP address dynamically allocated by a DHCP server to a client has a lease. When the lease expires,
the DHCP server will reclaim the IP address. If the client wants to use the IP address longer, it has to
extend the lease duration.
When the half lease duration elapses, the DHCP client sends to the DHCP server a DHCP-REQUEST
unicast 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 no reply, 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 26-3 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.