Configuring Cisco Switch Ports

A Cisco 1900 is basically a plug-and-play switch. This means that you can treat a 1900 much like a hub by simply plugging computers (or other hubs) into its ports and allowing them to communicate. One reason this works is that all ports are part of the same VLAN by default. However, you may want to change some of the switch’s default settings. For example, all 10baseT ports are configured for half-duplex by default. You may want to change some to full duplex, or configure some ports to be part of a different VLAN. For the time being, we’ll concentrate on accessing individual ports, configuring port duplex settings, and finally adding descriptions to port interfaces. Recall that the port speeds on a Cisco 1900 switch cannot be changed – the port speeds are fixed at either 10 or 100 Mbps.

A Cisco 1912 switch includes 15 ports in total. 12 of these ports are 10BaseT, found on the front of the switch. 2 of the ports are usually 100BaseT (although other possibilities exist), also found on the front of the switch, towards the right-hand side. The back of a 1912 provides one last port – an AUI interface, which can be used to connect a transceiver, and ultimately another network.

From global configuration mode, individual interfaces are accessed using the interface command, followed by the interface type and number. The Cisco 1900 series follows the slot/port convention for accessing interfaces. For example, to access the first Ethernet interface on a 1912, the command would be:

Cisco1912#config t
Enter configuration commands, one per line. End with CNTL/Z
Cisco1912(config)#int ethernet 0/1

Don’t be confused by the preceding slot number – on a Cisco 1900 series, the slot number is always 0.

Interface configuration mode is identified by the familiar config-if prompt. To view a list of available commands, use the help function.

Interface configuration commands:
cdp Cdp interface subcommands
description Interface specific description
duplex Configure duplex operation
exit Exit from interface configuration mode
help Description of the interactive help system
no Negate a command or set its defaults
port Perform switch port configuration
shutdown Shutdown the selected interface
spantree Spanning tree subsystem
vlan-membership VLAN membership configuration

Let’s start off by configuring the duplex settings for port e0/1. This is accomplished using the duplex command, followed by a duplex type.

Cisco1912(config-if)#duplex ?
auto Enable auto duplex configuration
full Force full duplex operation
full-flow-control Force full duplex with flow control
half Force half duplex operation

Remember that in order for the switch port to communicate in full duplex, only a single system can be attached. If you are attaching a hub to this switch port, leave it configured for half duplex. In this case, we’ll set the port to full duplex. The auto option will attempt to autonegotiate the duplex of the connection, but will only work if the network card of the connected system is also capable of auto-negotiation.

Cisco1912(config-if)#duplex full

It can be helpful to set a description on a port in order to keep track of its purpose. For example, you may be using this switch port as a trunk connection to another switch, or to connect a server. The description command allows you to assign a description to an interface. One caveat here – you cannot use spaces in the description, so you’ll need to use a special character such as an underscore or dash to combine multiple words. The example below configures a description for port e0/1, which will ultimately connect Fileserver3.

Cisco1912(config-if)#description Fileserver3

To view the configuration of a specific port after making changes, use the show interface command, as shown below.

Cisco1912#show int ?
Ethernet IEEE 802.3
FastEthernet FastEthernet IEEE 802.3

Cisco1912#show int ethernet ?
<0-0> IEEE 802.3
Cisco1912#show int ethernet 0/?
<1-25> IEEE 802.3
Cisco1912#show int e0/1
Ethernet 0/1 is Suspended-no-linkbeat
Hardware is Built-in 10Base-T
Address is 0050.F05F.2504
MTU 1500 bytes, BW 10000 Kbits
802.1d STP State: Forwarding Forward Transitions: 1
Port monitoring: Disabled
Unknown unicast flooding: Enabled
Unregistered multicast flooding: Enabled
Description: Fileserver3
Duplex setting: Full duplex
Back pressure: Disabled


One thing that you may have noticed during the interface configuration process is that even on a Catalyst 1912, there appears to be 25 Ethernet ports available. This is due to the fact that a Catalyst 1924 has 25 configurable Ethernet ports – 24 on the front of the switch, with the 25th being the AUI port on the rear. On a 1912, that rear port is still known as port 25. In the same way, a switch that provides 2 fast Ethernet ports references those as ports 26 and 27 respectively.

To configure FastEthernet ports, use the command int fastethernet followed by the slot and port number. For example, to access the first FastEthernet port, the command would be:

Cisco1912(config)#int fastethernet 0/26

By default, FastEthernet ports are configured to auto duplex, meaning that they will attempt to auto-negotiate duplex parameters with connected devices.

Remember that the configuration of interfaces can be viewed using both the show interface and show run commands.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.