Cisco admin 101: Five interface commands to know
Wednesday, August 16, 2006 09:38 AM
When working with routers and switches, you usually don't
have to configure an interface using the Cisco IOS every day, and it's easy to
become a little rusty on the necessary commands. Whether you're a seasoned
admin or a newbie, it's important to review from time to time. Let's look at
five Cisco IOS Interface Configuration Mode commands everyone should know.
To begin, here's what Interface Configuration Mode looks like in the Cisco IOS:
Router(config-if)#
As the name suggests, you use this mode to configure an interface on a router or switch. To get to this mode, you need to specify the interface you want to configure. Here's an example:
Router(config)# interface FastEthernet0/0 Router(config-if)#
Alternatively, you can configure a group of interfaces at the same time by using the interface range command. Here's an example:
Router(config)# interface range FastEthernet0/0 – 10 Router(config-if-range)#
Once you're in Interface Configuration Mode, you can always enter help to view a list of commands. But that's a long list--how do you know which ones are the most important? Let's look at five commands you should definitely know.
no shutdown
By default, a Cisco router or switch shuts down all ports,
meaning the ports aren't operational. However, this is different from a device such
as a Linksys router; by default, all ports of one of these devices are
operational when you turn on the device.
To enable ports for use, you can use the no shutdown command. This reverses the shutdown state and enables the port. Here's an example:
Router(config-if)# no shutdown
In addition to using this command the first time you use the ports, you can also use it whenever you're having trouble with the port to "bounce" it. You would also use it after making extensive configuration changes to the port.
ip address
To put an IP address on a port, use the ip address command. When using this command, you must provide the
IP address and subnet mask. Here's an
example:
Router(config-if)# ip address 1.1.1.1 255.255.255.0
Alternatively, you can make the port broadcast and request a DHCP IP address by using the following command:
Router(config-if)# ip address dhcp
description
While adding a description to a port is optional, it offers
a great benefit. For example, if you come back to a router a year after
configuring it, you may not remember which port goes to the HR LAN and which
port goes to the MRK LAN.
By adding details using the description command, anyone working on the router will know, which saves time and could even prevent outages. Here's an example:
Router(config-if)# description HR LAN
You can add even more information. For example:
Router(config-if)# description Sprint T1 to Internet Circuit ID GH.2343 call 800-555-5555 for support
bandwidth
Like the description
command, the bandwidth command is
also optional. However, keep in mind that you use the bandwidth command only to tell routing protocols the preferred interface--it
doesn't dictate the actual bandwidth of the port. (For more information, see Clarifying
the Cisco IOS bandwidth command.")
I also like to use this command for documentation purposes. For example, I might use it to make sure I know the port speed of a fractional T1 circuit.
Below is an example. Note that the specified bandwidth value is in Kb, so 512 is a 512-Kb circuit.
Router(config-if)# bandwidth 512
ip access-group
It's a common practice to apply an access list to an
interface to control traffic coming in or going out. Using an access list, you
can filter traffic in just about any way you choose. (For more information, seeLearn
additional uses for Cisco IOS access control lists.")
For example, let's say you created an access list called NOHTTPOUT. Here's how you would apply it to the interface:
Router(config-if)# ip access-group NOHTTPOUT out
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