3Com 400 Family, Configuration Manual

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82 C
HAPTER
5: N
ETWORK
P
ROTOCOL
O
PERATION
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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.
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A DHCP client extends its IP lease period
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 22 .
Figure 22 Typical DHCP Relay Application
DHCP Relay works on the following principle:
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When the DHCP client starts and initializes DHCP, it broadcasts the request
message to the local network.
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If there is a DHCP server on the local network, it can begin DHCP configuration
without requiring a DHCP relay function. If not, the local network device
configured for DHCP relay, upon receiving the broadcast message, will forward
the message to the DHCP server on the specified network.
Ethernet Internet
DHCP client
DHCP client DHCP client
DHCP client
Switch ( DHCP Relay)
DHCP Server