Huawei Quidway S3500 Series Operation Manual page 109

Operation Manual - Network Protocol
Quidway S3500 Series Ethernet Switches
Offer stage, the stage when the DHCP server allocates the IP address. After
receiving the DHCP_Discover message from the client, the DHCP server chooses
an IP address still available in IP address pool for the client, and sends to the client
the DHCP_Offer message containing the leased IP address and other settings.
Select stage, the stage when the client selects the IP address. If several DHCP
servers send DHCP_Offer messages to the client, the client only accepts the first
received one and then broadcasts DHCP_Request messages respectively to
those DHCP servers. The message contains the information of IP address request
from the selected DHCP server.
Acknowledge stage, the stage when the DHCP server acknowledges the IP
address. When receiving the DHCP_Request message from the client, the DHCP
server sends the DHCP_ACK message containing the allocated IP address and
other settings back to the client. Then the DHCP client binds its TCP/IP
components to the NIC (network interface card).
Other DHCP servers not selected still can allocate their IP addresses to other clients
later.
2) DHCP client logs into the network for a second time
When DHCP client logs into the network for a second time, its communication with the
DHCP server includes these stages:
When the DHCP client logs into the network at the first time, then at later login the
client only needs to broadcast the DHCP_Request message containing the IP
address obtained last time, other than the DHCP_Dsicover message.
After the reception of the DHCP_Request message, the DHCP server returns the
DHCP_ACK message if the requested IP address is still not allocated, to indicate
the client to continue use of the IP address.
If the requested IP address becomes unavailable (for example, having been
allocated to another client), the DHCP server returns the DHCP_NAK message.
After receiving the DHCP_NAK message, the client sends the DHCP_Discover
message to request another new IP address.
3) DHCP client extends its IP lease period
There is time limit for the IP addresses leased to DHCP clients. The DHCP server shall
withdraw the IP addresses when their lease period expires. If the DHCP client wants to
continue use of the old IP address, it has to extend the IP lease.
In practice, the DHCP client, by default, shall originate the DHCP_Request message to
the DHCP server right in the middle of the IP lease period, to update the IP lease. If the
IP address is still available, the DHCP server responds with the DHCP_ACK message,
notifying the client that it has got the new IP lease.
The DHCP client implemented on the switch supports automatic IP lease update.
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