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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 3 Configuring the Switch IP Address and Default Gateway
Understanding How Automatic IP Configuration Works
Understanding DHCP
There are three methods for obtaining an IP address from the DHCP server:
•
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.
In addition to the sc0 interface IP address, the switch can obtain the subnet mask, broadcast address, and
default gateway address. 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.
Table 3-1
shows the supported DHCP options.
Other options that are specified in the DHCPOFFER message are ignored.
If a BOOTP response is received from a BOOTP server, the switch sets the in-band (sc0) interface IP
address to the address that is specified in the BOOTP response.
If no DHCPOFFER message or BOOTP response is received in reply, the switch rebroadcasts the request
using an exponential backoff algorithm (the amount of time between requests increases exponentially).
If no response is received after 10 minutes, the sc0 interface IP address remains set to 0.0.0.0 (if the
BOOTP and RARP requests also fail).
Table 3-1
Supported DHCP Options
Code
Option
1
Subnet mask
2
Time offset
3
Router
6
Domain name server
12
Host name
15
Domain name
28
Broadcast address
33
Static route
42
NTP servers
51
IP address lease time
52
Option overload
61
Client-identifier
66
TFTP server name