Understanding Dhcp - Cisco WS-C2950SX-48-SI Configuration Manual

Chapter 3 Configuring the Switch IP Address and Default Gateway
If both the sc0 and me1 interfaces are unconfigured (IP address 0.0.0.0), the me1 interface is brought
down to allow the switch to broadcast requests on the sc0 interface. If the me1 interface is configured
and the sc0 interface is not, requests are not sent. Similarly, if the sc0 interface is not configured but the
interface is configured down, requests are not sent.

Understanding DHCP

In software release 5.2 and later releases, the switch can obtain an IP address and other IP configuration
information using DHCP.
There are three methods for obtaining an IP address from the DHCP server:
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•
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In addition to the sc0 interface IP address, the switch can obtain the subnet mask, broadcast address,
default gateway address, and other information. DHCP-learned values are not used if user-configured
values are present.
The switch broadcasts a DHCPDISCOVER message 1 to 10 seconds after all of the switch ports are
online. The switch always requests an infinite lease time in the DHCPDISCOVER message.
If a DHCP or Bootstrap Protocol (BOOTP) server responds to the request, the switch takes appropriate
action. If a DHCPOFFER message is received from a DCHP server, the switch processes all the
supported options that are contained in the message.
Other options that are specified in the DHCPOFFER message are ignored.
Table 3-1
Code
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2
3
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15
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33
42
51
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61
66
78-15486-01
Manual allocation—The network administrator maps the switch MAC address to an IP address at
the DHCP server.
Automatic allocation—The switch obtains an IP address when it first contacts the DHCP server. The
address is permanently assigned to the switch.
Dynamic allocation—The switch obtains a "leased" IP address for a specified period of time. The
IP address is revoked at the end of this period, and the switch surrenders the address. The switch
must request another IP address.
Supported DHCP Options
Option
Subnet mask
Time offset
Router
Domain name server
Hostname
Domain name
Broadcast address
Static route
NTP servers
IP address lease time
Option overload
Client-identifier
TFTP server name
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
Understanding How Automatic IP Configuration Works
Table 3-1 shows the supported DHCP options.
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Release 8.1
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