Naming and Numbering Interfaces

Interfaces are configured with the interface command, followed by an interface name, followed by a port number. The space between the interface name and the port number is optional. For example:

interface serial 0 Serial port 0
interface serial 1 Serial port 1
interface ethernet 0 Ethernet port 0
interface ethernet 1 Ethernet port 1

On high-end routers, the interface cards are in slots; each slot has a series of ports. To specify these interfaces, use the slot/port naming scheme. For example, the Ethernet interface on port 5 of the card in slot 4 would be called:

interface ethernet 4/5

The interface command is followed by other commands that perform the actual configuration. If you're entering commands at the console, the interface command changes the prompt to Router(config-if)#.

VIP2 (Versatile Interface Processor) cards have two Ethernet ports per card. To accommodate these cards, use the syntax card/slot/port to specify a particular Ethernet interface. For example, Fast Ethernet card 2 on slot 1 on port 0 would be called:

interface fastethernet 2/1/0


5.1.1. Subinterfaces

Subinterfaces provide a way to have multiple logical configurations for the same interface; they are most commonly used in Frame Relay, ATM, and Fast Ethernet in switched environments. To specify a subinterface, add a period and the subinterface number to the regular interface name. For example:

interface serial 1.1
interface serial 1.2

On a high-end router that uses the slot/port notation, append the subinterface number to the port number:

interface serial 1/2.1
interface serial 1/2.2

Subinterface zero (0) refers to the actual interface; i.e., serial1 is equal to serial1.0.

Here's a simple example that shows how subinterfaces are typically used. Frame Relay permits subinterfaces in both point-to-point and multipoint modes. Each mode can have its own IP address and subnet mask. This multiple-IP address configuration can be accomplished only with the subinterface commands:

interface serial 1
 no shutdown
interface serial 1.1 point-to-point
 ip address
interface serial 1.2 point-to-multipoint
 ip address

In this example, we apply the no shutdown command to serial 1, which includes both subinterfaces. We then assign a different IP address to subinterface 1 and subinterface 2, each with a subnet mask of

Most commands that apply to interfaces can also be applied to subinterfaces. However, a few commands can be applied only to an interface or to a subinterface.

Getting Started

IOS Images and Configuration Files

Basic Router Configuration

Line Commands

Interface Commands

Networking Technologies

Access Lists

IP Routing Topics

Interior Routing Protocols

Border Gateway Protocol

Quality of Service

Dial-on-Demand Routing

Specialized Networking Topics

Switches and VLANs

Router Security

Troubleshooting and Logging

Quick Reference

Appendix A Network Basics


Cisco IOS in a Nutshell
Cisco IOS in a Nutshell (In a Nutshell (OReilly))
ISBN: 0596008694
EAN: 2147483647
Year: 2006
Pages: 1031
Authors: James Boney © 2008-2020.
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