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Understanding Automatic IP Configuration
When you configure the IP address, subnet mask, and broadcast address (and, on the sc0 interface,
VLAN membership) of the sc0 or me1 interface, you can access the switch through Telnet or SNMP.
When you configure the SLIP (sl0) interface, you can open a point-to-point connection to the switch
through the console port from a workstation.
All IP traffic generated by the switch (for example, a Telnet session opened from the switch to a host) is
forwarded according to the entries in the switch IP routing table. For intersubnetwork communication to
occur, you must configure at least one default gateway for the sc0 or me1 interface. The switch IP routing
table is used to forward traffic originating on the switch only, not for forwarding traffic sent by devices
connected to the switch.
Because sc0 and me1 are two distinct interfaces, they potentially can have duplicate IP addresses or
overlapping subnets. Therefore, when you enter a command that causes sc0 and me1 to have the same
IP address or occupy the same subnet, the switch software brings one of the interfaces down.
In most cases, the switch software brings down the sc0 interface after you confirm the change. However,
when the switch boots with the IP address 0.0.0.0 configured on both the sc0 and me1 interfaces, the
me1 interface is brought down to allow BOOTP and RARP requests to broadcast out the sc0 interface.
Note
When the switch boots with the IP address 0.0.0.0 configured on both the sc0 and me1 interfaces, the
me1 interface is automatically brought down by the switch software. You are not asked to confirm the
change, and no console messages or traps are generated in this case.
Duplicate IP addresses and equal subnets are allowed on the sc0 and me1 interfaces provided that one
of the interfaces is configured down. Non-equal subnets are not allowed (for example, sc0 with IP
address 10.1.1.1 and subnet mask 255.0.0.0 and me1 with IP address 10.1.1.2 and subnet mask
255.255.255.0).
Understanding Automatic IP Configuration
These sections describe how the switch can obtain its IP configuration automatically:
•
•
•
Automatic IP Configuration Overview
The switch can obtain its IP configuration automatically using one of the following protocols:
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•
The switch makes DHCP and RARP requests only if the sc0 interface IP address is set to 0.0.0.0 when
the switch boots up. This address is the default for a new switch or a switch whose configuration file has
been cleared using the clear config all command. DHCP and RARP requests are only broadcast out the
sc0 interface.
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
3-2
Automatic IP Configuration Overview, page 3-2
Understanding How DHCP Works, page 3-3
Understanding How RARP Works, page 3-4
Dynamic Host Configuration Protocol (DHCP)
Reverse Address Resolution Protocol (RARP)
Chapter 3
Configuring the Switch IP Address and Default Gateway
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Troubleshooting
