Dhcp Relay - Huawei Quidway S3500 Series Operation Manual

Operation Manual - Network Protocol
Quidway S3500 Series Ethernet Switches
Offer stage where DHCP servers offer IP addresses. Upon the receipt of the
DHCP_Discover message from the client, each DHCP server sends a
DHCP_Offer message carrying an unassigned IP address selected from its IP
address pool and other settings to the client.
Selecting stage where the DHCP client picks one IP address out of all the offers. If
several DHCP servers return DHCP_Offer messages, the client accepts only the
one reaching first. Then, it broadcasts to all the DHCP servers a DHCP_Request
message containing the IP address for which it will request the selected DHCP
server.
Acknowledgement stage where the selected DHCP server acknowledges the
offered IP address. In response to the received DHCP_Request message, the
DHCP server sends a DHCP_ACK message carrying the provided IP address and
other settings to the client. Then the DHCP client binds its TCP/IP protocol
components to its MAC address.
Except for the selected DHCP server, all other DHCP servers can allocate their offered
IP addresses to other requesting clients.
2) The non-first login of DHCP client
If it is not the first time for the DHCP client to log into the network, it undergoes the
following stages in order to set up a connection with a DHCP server.
When the DHCP client logs into the network again after the first successful login, it
only needs to broadcast a DHCP_Request message containing the IP address
assigned to it the last time instead of sending a DHCP_Discover message.
Upon the receipt of the DHCP_Request message, the DHCP server sends back a
DHCP_ACK message allowing the client to use the requested address if it is still
available.
If the IP address is not available because it has been assigned for example, the
DHCP server returns a DHCP_NAK message. Upon the receipt of the message,
the client sends a DHCP_Discover message requesting a new IP address.
3) IP address lease renewal
DHCP server takes back the dynamic IP address allocated to a DHCP client when the
lease expires. If the DHCP client still wants to use this address, it must renew the IP
address lease.
In practice, when half of the address lease period passes, the DHCP client by default
automatically sends a DHCP_Request message to renew the lease. If the current IP
address is still valid, the DHCP server sends back a DHCP_ACK message notifying the
DHCP client that it has extended the IP address lease.

5.1.2 DHCP Relay

In the early days, the DHCP was only suitable for the case, when the DHCP Client and
DHCP Server locate on the same subnet, and could not work across the network
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5-3
Chapter 5 DHCP Configuration