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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 4 Assigning the Switch IP Address and Default Gateway
Assigning Switch Information

DHCP Client Request Process

When you boot your switch, the sw itch automat ically requests configurati on information from a DHCP
server only if a co nfiguration file is not prese nt on the sw itch.
DHCP autoconfiguration does not occur under these conditions:
•When a c onfigurati on file is presen t and th e service config global c onfigurati on c om mand i s
disabled o n the swit ch.
•When a c onfigurati on file is presen t and th e service config global c onfigurati on c om mand i s
enabled on the switch. In this case, the switch broadcasts TFTP requests for the configuration file.
Figure 4-1 shows th e sequen ce of mess age s that ar e e xchanged betw een the DHCP c lient an d the DHC P
server.
Figure 4-1 DHCP Client and Server Message Exchange
The client, Switch A, broadcasts a DHCPDISCOVER message to locate a DHCP server. The DHCP
server offers configuration parameters (such as an IP address, subnet mask, gateway IP address, DNS IP
address, a lease for the IP address, and so forth) to the client in a DHCPOFFER unicast message.
In a D HCPRE QUEST br oadc ast me ssage , th e clie nt r etu rns a fo rmal r eque st f or the offered
configuration information to the DHCP server. The formal request is broadcast so that all other DHCP
servers that received the DHCPDISCOVER broadcast message from the client can reclaim the IP
addresses that they offered to the client.
The DHCP server confirms that the IP address has been allocated to the client by returning a DHCPA CK
unicas t message to the client. With thi s message, th e client an d server are boun d, and the clie nt uses
configuration information received from the server. The amount of informatio n the switch receives
depends on how you configure the DHCP server. For more information, see the “Configuring the DHCP
Server” section on page 4-5.
If the configuration parameters sent to the client in the DHCPOFFER unicast message are invalid (a
conf ig ura tion er ror exists), the clien t returns a DHCPDECL INE broa dcast me ssage to the DHCP server.
The DHCP serv er sends the client a DHCPN A K denial broadcast message, which means that the of fered
configuration parameters have not been assigned, that an error has occurred during the negotiation of the
parameters, or that the client has been slow in responding to the DHCPOFFER message (the DHCP
server assign ed t he p ar ame ters t o anot her cl ien t).
A DHCP client might receive offers from multiple DHCP or BOOTP servers and can accept any of the
off ers; however, the client usually accepts the first offer it receives. The off er from the DHCP server is
not a guarantee that the IP address is allocated to the client; however, the server usually reserves the
address until the client has had a chance to formally request the address. If the switch accepts replies
from a BOO TP server and configures itself, the switch broadcasts, instead of unicasts, TFTP requests to
obtain th e switch co nfiguration file.
Switch A
DHCPACK (unicast)
DHCPREQUEST (broadcast)
DHCPOFFER (unicast)
DHCPDISCOVER (broadcast)
DHCP server
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