Which IOS command would you use to display the current operating config?

VLANs are typically configured via the CatOS or IOS command-line interpreter (CLI), like any other feature. However, some IOS models, such as the 2950 and 3550 switches, have a configurable VLAN database with its own configuration mode and commands. This can be a challenge for the uninitiated, especially because the configuration for this database is completely separate from the configuration for the rest of the switch. Even a write erase followed by a reload will not clear the VLAN database on these switches. Configuring through the VLAN database is a throwback to older models that offered no other way to manage VLANs. Luckily, all newer switches (including those with a VLAN database) offer the option of configuring the VLANs through the normal IOS CLI. Switches like the 6500, when running in native IOS mode, only support IOS commands for switch configuration. The Nexus line does not have a configurable VLAN database.

Cisco recommends that you configure the VLAN Trunking Protocol (VTP) as a first step when configuring VLANs. This idea has merit, as trunks will not negotiate without a VTP domain. However, setting a VTP domain is not required to make VLANs function on a single switch. Configuring VTP is covered later (see Chapter 5, Trunking and Chapter 6, VLAN Trunking Protocol).

For CatOS, create a VLAN with the set vlan command:

There are a lot of options when creating a VLAN, but for the bare minimum, this is all you need. To show the status of the VLANs, execute the show vlan command:

Notice that VLAN 10 has the name you assigned; VLAN 20’s name, which you did not assign, defaulted to VLAN0020. The output shows which ports are assigned to VLAN 20 and that most of the ports still reside in VLAN 1 (because VLAN 1 is the default VLAN, all ports reside there by default).

There are no ports in VLAN 10 yet, so add some, again using the set vlan command:

You’ve now added ports 6/1, 6/3, and 6/4 to VLAN 10. Another show vlan will reflect these changes:

The output indicates that VLAN 1 was modified as well. This is because the ports had to be removed from VLAN 1 to be added to VLAN 10.

This method is included for the sake of completeness. Older switches that require this method of configuration are no doubt still deployed. IOS switches that support the VLAN database, such as the 3750, actually display this message when you enter VLAN database configuration mode:

If you have an IOS switch with active VLANs, but no reference is made to them in the running configuration, it’s possible they were configured in the VLAN database. Another possibility is that they were learned via VTP (we will cover this in Chapter 6). On 3750s, when the switch is in VTP server mode, even when you configure VLANs in CLI, they do not appear in the running configuration.

Since you’re more likely to see the VLAN database in older switches, I’ll continue with examples from a 2950, though they all behave pretty similarly. If you find any switch configured using the VLAN database, my advice is to convert it to an IOS configuration.

To configure VLANs in the VLAN database, you must enter VLAN database configuration mode with the command vlan database. Requesting help (?) lists the commands available in this mode:

To create a VLAN, give the vlan command followed by the VLAN number and name:

You can show the VLANs configured from within VLAN database mode with the command show. You have the option of displaying the current database (show current), the differences between the current and proposed database (show changes), or the proposed database as it will look after you apply the changes using the apply command or exit VLAN database configuration mode. The default behavior of the show command is show proposed:

Nothing else is required to create a simple VLAN. The database will be saved upon exit:

Now, when you execute the show vlan command in IOS, you’ll see the VLAN you’ve created:

Adding ports to the VLAN is accomplished in IOS interface configuration mode, and is covered in the next section.

Adding VLANs in IOS is relatively straightforward when all of the defaults are acceptable, which is usually the case. Here I’ll revert to a 3750, since you’re likely to encounter modern switches using this method.

First, enter configuration mode. From there, issue the vlan command with the identifier for the VLAN you’re adding or changing. Next, specify a name for the VLAN with the name subcommand (as with CatOS, a default name of VLANxxxx is used if you do not supply one):

Exit configuration mode and then issue the show vlan command to see the VLANs present:

You assign ports to VLANs in IOS in interface configuration mode. Each interface must be configured individually with the switchport access command (this is in contrast to the CatOS switches, which allow you to add all the ports at once with the set vlan command):

Modern versions of IOS allow you to apply commands to multiple interfaces with the interface range command. Using this command, you can accomplish the same result as before while saving some precious keystrokes:

Now, when you execute the show vlan command, you’ll see that the ports have been assigned to the proper VLAN:

NX-OS uses a command interface similar to IOS. NX-OS behaves a little bit differently, especially concerning the configuration of interfaces. The methods used for configuring VLANs are very similar to IOS. First we create the VLAN with the vlan vlan-# command:

Once you’ve created the VLAN, enter VLAN configuration mode and name the VLAN with the name vlan-name command:

One of the cool features of NX-OS is that you no longer need the do command to run show commands from configuration mode. This behavior is similar to the PIX and ASA configuration mode, and is a most welcome change. Here, I’ve executed the show vlan command from within VLAN configuration mode:

Another new feature in NX-OS is the capability to configure a range of interfaces without using the interface-range command. Simply enter the range you want to configure as if you were using the interface-range command in IOS:

This automatically puts us into interface range configuration mode. Now we assign the ports to a VLAN the same way we would in IOS—using the switchport access vlan vlan# command:

Now there’s a message you don’t see on a Catalyst switch by default. The Nexus 7000 switch behaves differently than a catalyst. By default, all switch ports are router ports! To perform switch port commands on a Nexus port, you must first put them into switchport mode with the switchport command:

Now that we have placed the ports into switchport mode, we can assign them to a VLAN without further interruption:

The show vlan command now shows our ports assigned to VLAN 10. Notice once more how I’ve executed a show command from within configuration mode. I love this feature!