H3C Technologies H3C SecPath F1000-E - Manual (Page 172)

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• Dynamic allocation—DHCP assigns an IP address to a client for a limited period of time, which is
called a lease. Most DHCP clients obtain their addresses in this way.
Dynamic IP address allocation process
Figure 80 Dynamic IP address allocation process
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 message is determined by the flag
field in the DHCP-DISCOVER message. For more 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 responds. The server 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.
NOTE:
•
After the client receives the DHCP-ACK message, it broadcasts gratuitous ARP packet to verify whether
the IP address assigned by the server is in use. If the client receives no response within a specified time,
the client uses 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. 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 message for lease extension after
7/8 lease duration elapses.