H3C Technologies H3C S12500 Series Switches - Manual (Page 40)

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Dynamic IP address allocation process
Figure 14 Dynamic IP address allocation process
As shown in Figure 14 , a DHCP client obtains an IP address from a DHCP server by using the four steps:
1. The client broadcasts a DHCP-DISCOVER message to locate a DHCP server.
2. A DHCP server offers configuration parameters such as an IP address to the client in a
DHCP-OFFER message. The sending mode of the DHCP-OFFER is determined by the flag field in
the DHCP-DISCOVER message. For related information, see " DHCP message format ."
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 formally request the IP address.
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 returns a DHCP-NAK unicast message, denying the IP
address allocation.
After the client receives the DHCP-ACK message, it broadcasts a gratuitous ARP packet to verify whether
the IP address assigned by the server is in use. 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 IP address is reclaimed by the DHCP server. To continue using the IP address, the client must extend
the lease duration.
After half the lease duration, the DHCP client sends a DHCP-REQUEST unicast to the DHCP server to
extend the lease duration. Depending on 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 broadcasts another DHCP-REQUEST for lease extension after 7/8 lease
duration. Again, depending on availability of the IP address, the DHCP server returns a DHCP-ACK
unicast confirming that the client's lease has been extended, or a DHCP-NAK unicast denying the
request.