Ip address allocation process, Ip address lease extension – H3C Technologies H3C S5560 Series Switches User Manual
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IP address allocation process
Figure 10 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 "
."
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. However, only the server selected by the
client performs the following operations:
{
Returns a DHCP-ACK message to confirm that the IP address has been allocated to the client.
{
Returns a DHCP-NAK message to deny the IP address allocation.
After receiving the DHCP-ACK message, the client verifies the following details before using the assigned
IP address:
•
The assigned IP address is not in use. To verify this, the client broadcasts a gratuitous ARP packet.
The assigned IP address is not in use if no response is received within the specified time.
•
The assigned IP address is not on the same subnet as any IP address in use on the client.
Otherwise, the client sends a DHCP-DECLINE message to the server to request an IP address again.
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 about half of the 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 one
of the following messages:
•
A DHCP-ACK unicast confirming that the client's lease duration has been extended.
•
A DHCP-NAK unicast denying the request.
If the client receives no reply, it broadcasts another DHCP-REQUEST message for lease extension when
about seven-eighths of the lease duration elapses. Again, depending on the availability of the IP address,