HP FlexNetwork NJ5000, User Manual

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233
IP address allocation process
Figure 239 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 is determined by the flag
field in the DHCP-DISCOVER message. For more information about the DHCP message
format, 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 request the IP address formally. (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 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 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 uses this IP address.
Otherwise, the client sends a DHCP-DECLINE message to the server and requests 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 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
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 seven eighths of the 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.