CCNA RnS Article #21: Switch Interface Configuration

In this article, we are going to cover what goes into configuring an interface. In Cisco network world Interface represent a port on any device. So, when we say interface configuration, we are talking about configuring switch ports.

As you can see in Figure 1, a switch can have multiple ports where user devices are connected. Majorly there are two types of switches –

Fixed Switches
Modular Chassis Based Switch

In fixed switches, we get 12 ports, 24 ports, and 48 ports models. Figure 1 is an example of a 24 port switch model.

Figure 1: Cisco Catalyst 2960 switch

Whenever there is a requirement of more than 48 ports we have two solutions – either stack multiple switches of 48 and 24 port models or use Modular Chassis Based Switch.

Modular Chassis based switches can hold multiple switches (called line cards) of different port configurations – 24, 48 ports.

Irrespective of the type of switch we use, this article talks about port or Interface Configuration settings.

To start the switch Interface configuration, we need to know what interfaces are available on the switches. The show interface status command displays what are the interface available on the switch.

Let’s discuss the command output before we go into configuration details. The first column is the Interface number. This switch has 24 Fast Ethernet ports and 2 Gigabit Ethernet ports. All 24 ports are also known as downlinks where end-users or devices like printers, cameras, Access Points, and IP phones are connected in a network. 2x Gigabit Ethernet Interfaces are called the uplink ports which are normally used to connect to another switch. It is always a high-speed port compared to downlink ports as it carries traffic from multiple host ports to the uplink switch.

In the output in the first column above, Fa represents Fast Ethernet; 0 represents to slot number, and 1 represents the port number. As this is a fixed configuration switch, there on no slot that’s the reason it is 0.

The second column is the name – which by default is blank (nothing is configured). You can configure a description to a port that can be used to identify what is the use of the Interface. The interface description command is used to set a name for an interface.

Example – we have configured Interface Fa0/1 name to “** IP Phone 1 **”. To configure an interface first we need to go to configuration mode using the config t command and then to the interface that we want to configure using the Interface interface_name command.

The next column is the status which talks about whether this interface is connected to some device or not. From the status above, nothing is connected to any of the interfaces, so the status is not connect means not a working port.

The VLAN column shows the VLAN membership of the port. By default, all the interfaces are in VLAN 1 which makes it a true plug-n-play device. Where all devices connected to a switch in the same IP subnet can talk to each other without configuring anything on a switch.

When you see a value trunk in the VLAN column, you think it is connected to another switch or a server and carries traffic from multiple ports that are in different vlans.

The Duplex column shows that is the duplex configured on the interface. If you are not aware of duplex please click this article covers the duplex information. Duplex setting auto means when a device is connected to the switch on the port, the switch is going to negotiate the duplex setting and configure the one automatically.

The Speed column tells the speed of the interface. The switch interface can support multiple speeds. See the type of column it tells fa0/1 to fa0/24 can work on 10Mbps or 100Mbps speed. Speed setting auto means, the switch interface configures itself based on the speed negotiated with the connected device.

The speed and duplex can be changed with speed and duplex commands. Interface speed varies based on the interface capability for our lab switches it supports 10, 100, or auto value. The duplex command allows you to configure full, half, or auto for full-duplex, half-duplex, and automatic respectively.

Note: a question mark “?” after any command give you a hint of what the next parameter in the command and possible values is. Network admins use “?” when typing the long complex command, they are not aware what is the correct sequence.

Let’s change the speed to 100 and duplex to full for interface fa0/1.

Negate a command

You can negate a command by prefixing it with a “no” word. Any command that we used till now, if we start with the “no” word will revert the interface configuration to default settings. For example, the default setting for our lab switch is as follows - Name is blank, Duplex auto, and speed auto.

Output before changes

Negate the commands – description, duplex, and speed.

The show command output after changes – no description, speed, and duplex

Range Command

One of the command that is quite useful when configuring a switch is range. Consider a scenario where you need to change the description of all the ports on the switch to “Available to Use”. One way is to change the description of each interface one by one which is time consuming task. The range command helps to change the configuration of the multiple interfaces by executing the command once.

Usage

interface range start-interface - end-interface

The output below shows how easy to configure the interfaces using the range command.

At last let’s talk about one of the command that is rarely used. Let’s say you have configured an interface and you want this interface to revert to its default configuration. The default global configuration command helps you to reset the interface configuration to default.

In the below example, interface fa0/1 is set to default settings and you can see now there is no description in the show interface status command output.

I hope you find this informative!

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CCNA RnS Article #21: Switch Interface Configuration