3Com SuperStack 4 Configuration Manual page 83

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.
A DHCP client logs into the network for a second time
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When DHCP client logs into the network for a second time, its communication
with the DHCP server includes these stages:
The client broadcasts the DHCP_Request message containing the IP address
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obtained last time, other than the DHCP_Discover message.
After the reception of the DHCP_Request message, the DHCP server returns
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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
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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.
A DHCP client extends its IP lease period
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There is a 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.
DHCP Relay
The DHCP described above applies only when DHCP clients and server(s) are in the
same subnet, and it does not support trans-segment networking. To achieve
dynamic address configuration, you would have to configure a DHCP server for
each subnet, which is not a practical solution. Introduction of DHCP relay has
solved this problem: the clients in a LAN can communicate with DHCP servers in
another subnet through DHCP relay, to get valid IP addresses. Then DHCP clients
of multiple different networks can share a DHCP server, which saves networking
cost, as well as facilitating centralized management. A typical DHCP relay
application is shown in Figure 23.
DHCP Configuration 95