H3C Technologies H3C S12500 Series Switches - Manual (Page 36)

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184B
Dynamic IP address allocation process
Figure 10 Dynamic IP address allocation process
1. The client broadcasts a DHCP-DISCOVER message to locate a DHCP server.
2. Each 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 "
666H
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. (IP addresses
offered by other DHCP servers can be assigned to other clients.)
4. All DHCP servers receive the DHCP-REQUEST message, but only the server selected by the client
returns a DHCP-ACK message to confirm that the IP address has been allocated to the client, or a
DHCP-NAK message to deny 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 already in use. If the client receives no response within the
specified time, the client uses the assigned IP address. Otherwise, the client sends a DHCP-DECLINE
message to the server to request an IP address again.
185B
IP address lease extension
A dynamically assigned IP address 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.
When 1/2 lease duration elapses, the DHCP client unicasts a DHCP-REQUEST to the DHCP server to
extend the lease. Depending on the availability of the IP address, the DHCP server returns either 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 when
7/8 lease duration elapses. Again, depending on the availability of the IP address, the DHCP server
returns either a DHCP-ACK unicast confirming that the client's lease duration has been extended, or a
DHCP-NAK unicast denying the request.