4-2
Layer 3 Switching Software Feature and Configuration Guide
78-6235-04, Cisco IOS Release 12.0(10)W5(18)
Chapter4 Configuring Interfaces
Overview of Interface Configuration
Overview of Interface Configuration
A router’smain function is to relay packets from one data link to another. To do that, the characteristics
of the interfaces through which the packets are received and sent must be defined. Interface
characteristics include, but are not limited to, IP address, address of the port, data encapsulation
method, and media type.
Many features are enabled on a per-interface basis. Interface configuration mode contains commands
that modify the interface operation, for example, of an Ethernet port. When you issue the interface
command, you must define the interface type and number.
The following general guidelines apply to all physical and virtual interface configuration processes.
•Each interface must be configured with an IP address and an IP subnet mask.
•The virtual interfaces supported by Cisco switch routers include subinterfaces and IP tunnels.
A subinterface is a mechanism that allows a single physical interface to support multiple logical
interfaces or networks—that is, several logical interfaces or networks can be associated with a
single hardware interface. Configuring multiple virtual interfaces, or subinterfaces, on a single
physical interface allows greater flexibility and connectivity on the network.
Layer 3 interfaces have both a Media Access Control (MAC) address and an interface port ID. The
router keeps track of these designators and uses them to route traffic.

Media Access Control Address

The MAC address, also referred to as the hardware address, is required for every port or device that
connects to a network. Other devices in the network use MACaddresses to locate specific ports in the
network and to create and update routing tables and data structures.
Tips To find the MAC address for a device, use the show interfaces command.

Interface Port Identifier

The interface port identifierdesignates the physical location of the Layer 3 interface within the chassis.
This is the name that you use to identify the interface when configuring it. The system software uses
interface port identifiers to control activity within the switch router and to display status information.
Interface port identifiersare not used by other devices in the network; they are specific to the individual
switch router and its internal components and software.
Youcan find the interface port identifier on the rear of the switch router. It is composed of three parts,
formatted as slot/subslot/interface as depicted in Figure 4-1.