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VLAN Configuration in Cisco Switches

VLAN Configuration in Cisco Switches

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In the previous article, we discussed normal and extended-range VLANs. Normal-range VLAN configuration is stored in the vlan.dat file, which is located in the switch’s flash memory. Flash memory does not need the copy running-config startup-config or write command.

But, other details are usually configured on a Cisco switch simultaneously when VLANs are created; it is best to save running configuration changes to the startup configuration. The figure below illustrates the Cisco IOS command syntax used to add a VLAN to a Cisco switch and give it a name. Naming each VLAN is the best practice in VLAN configuration.

Vlan <vlan ID>

Name <Name of the VLAN> 

The figure below illustrates the topology where Sw-1 has already been configured with VLANs 10 and 20. We can check by using the Syntax Checker show vlan brief command in user exec mode to display the contents of the vlan.dat file. Now, we should create some VLANs, including VLAN-10 and VLAN-20, on Sw-2.

vlan configuration

So, we can create VLANs individually, using a series of VLAN IDs and a range. The IDs can be entered separated by commas, and hyphens can separate the range of VLAN IDs, for example, “vlan vlan-id”. Use the following command to create VLANs 10, 20, 30, 40, and 50-60.

Sw-2(config)# vlan 10, 20, 30, 40

Sw-2(config)# vlan 50-60

Assigning Ports to VLAN

Once the VLAN configuration is done, the next step is assigning ports to the VLAN. Port in access mod can belong to only one VLAN at a time. Only in one case, when the access port is connected to an IP phone, two VLANs are associated with this port, one for voice and one for data. Following is the syntax for defining a port as an access port and assigning it to a VLAN.

WP Table Builder

Access to the command switch port mode is optional, although it is strongly recommended for security. This command permanently changes the switch interface to access mode. We can use the interface range command to execute the command for multiple interfaces.

In the figure below, VLAN 10 is assigned to port Fa0/1 on switch1. As a result, PC4 became a member of VLAN 10. VLAN-20 is assigned ports fa 0/2 and fa0/3, so PC5 and PC6 are part of VLAN 20. We can use the Syntax Checker “show vlan brief” command to display the contents of the vlan.dat file.

If VLAN does not exist, the switchport access vlan command forces the creation of a VLAN on the switch. For example, VLAN 100 does not exist in the output of the show vlan brief command. If the switchport accesses vlan 30 commands entered on an interface without any prior configuration, then it will display the following:

% Access VLAN does not exist. Creating VLAN 100

Now we can show the vlan.dat file using show vlan brief command.

Changing VLAN Port Membership

There are several ways to change the VLAN port association. The table below shows the syntax for changing a switch port membership to VLAN 1 with the no switchport access vlan interface configuration mode command.

VLAN 10 has assigned interface Fa0/1. The no switchport access vlan command is entered for interface Fa0/1 in interface configuration mode, now check the output in the show vlan brief command that instantly follows as shown in Figure below.

The show vlan brief command displays the VLAN association and type for all switch ports. The show vlan brief command shows one line for every VLAN. The output for every VLAN includes the name, status, and switch ports.

WP Table Builder

VLAN 10 is still active, but there are no ports in it. The command show interfaces f0/1 switchport verify that the access VLAN for Fa0/1 interface has been reset to VLAN 1. First, removing a port from a VLAN is not required to change its membership. Now, we can assign these ports to any VLAN again.

Verifying VLAN Information  

After a VLAN configuration or some changes in the VLAN configuration, we must validate the configuration using the Cisco IOS show commands. The command syntax is the following:-

  • Show vlan [ brief | id vlan-id | name vlan-name | summary]
    • Brief – it displays one line for every VLAN with the name of VLAN, status, and ports.
    • Id vlan-id – This parameter displays the information about a specific VLAN with VLAN ID number.
    • name vlan-name – Display the information of a specific VLAN with the name of VLAN.
  • Show interfaces [interface-id | vlan vlan-name | switchport]
    • Interface-id—This is a valid interface ID that includes a module, port numbers, and port channel. The port channel range is 1 to 6.
    • Id vlan-id—This parameter displays information about a specific VLAN with a VLAN ID number. The range of VLAN ID is 1 to 4096.
    • switchport—This parameter displays a switch port’s operational and administrative status, including whether it is blocked and what port protection settings are available.

Now, we will use the show VLAN command with a different parameter. The “show vlan name IT” command output is difficult to understand. The figure below illustrates the output of this command. The output parameter will be discussed in the coming articles.

The recommended commands for VLAN verification are “show vlan brief” and “show vlan summary.” These commands display the count of all configured VLANs. We can also use the “show interfaces” command with different parameters, such as “show interfaces fastEthernet 0/1 switchport.”

This will display all information about the switchport FastEthernet 0/1. If we enter the command “show interfaces switchport,” It will display all existing switchport information. We can also use “show interfaces vlan <vlan ID>” command to display the vlan interface information.  This command will show the IP address information of this VLAN.

Deleting VLAN

We can delete VLANs with the “no vlan vlan-id” command in global configuration mode. For example, “ no vlan 10 “ in global configuration mode will delete VLAN 10 from switch Sw-2 database. The ports of VLAN 10 are not the members of any VLAN now. You can verify it with “show vlan brief” command that VLAN 10 is no longer present in the vlan.dat file after using the no vlan 10 commands.

Best practice before deleting a VLAN is reassigning all member ports to a different VLAN because any port that is not moved to an active VLAN is unable to communicate with other hosts after the deleting VLAN and until it is assigned to an active VLAN.

We can delete the entire vlan.dat file using the “delete flash:vlan.dat” command in privileged EXEC, which is abbreviated “delete vlan.dat.” We can also delete a vlan.dat file if it is stored in its default location.

After executing this command and restarting the switch, the formerly configured VLANs are no longer present. This places the switch into its factory default condition for VLAN configurations.

Conclusion

Configuring VLANs on Cisco switches is an essential aspect of network management. Proper naming, port assignment, and verification of VLAN configurations are crucial steps. Regularly using show commands helps ensure the correct setup of VLANs, and caution should be exercised when deleting VLANs to avoid network disruptions. Following best practices and utilizing recommended commands contribute to a secure and well-organized network infrastructure.

FAQs

  • Why is naming each VLAN considered a best practice in VLAN configuration?
    Naming each VLAN provides clarity and documentation for network administrators, making it easier to identify the purpose or function of a specific VLAN.
  • Can VLANs be created on a Cisco switch individually and in a range?
    Yes, VLANs can be created on a Cisco switch individually by specifying VLAN IDs or using hyphens to set them in a range. For example, the command vlan 10, 20, 30, 40 creates VLANs 10, 20, 30, and 40, while vlan 50-60 creates VLANs 50 through 60.
  • What is the purpose of the ‘switchport mode access’ command in VLAN configuration?
    The switchport mode access command is optional but recommended for security. It permanently sets the switch interface to access mode, ensuring the port can only belong to one VLAN. This command helps prevent unauthorized VLAN hopping.
  • How can I assign ports to a VLAN after VLAN configuration?
    Ports can be assigned to a VLAN by entering the interface configuration mode and using the switchport access vlan [VLAN ID] command. This associates the specified VLAN with the port.
  • What happens if a VLAN does not exist when assigning it to a port?
    If the VLAN does not exist, the switchport access vlan [VLAN ID] command forces the creation of the VLAN on the switch. For example, if VLAN 100 does not exist, entering the command will create VLAN 100.
  • How can I verify VLAN information on a Cisco switch?
    VLAN information can be verified using the show vlan command with various parameters, such as brief, id [VLAN ID], name [VLAN Name], or summary. The show interfaces command can also display information about switch ports and VLAN assignments.
  • What is the recommended command for VLAN verification?
    The recommended commands for VLAN verification are shown in the vlan brief and vlan summary. These commands provide a concise overview of all configured VLANs.
  • How can I delete a VLAN on a Cisco switch?
    To delete a VLAN, use the no vlan [VLAN ID] command in global configuration mode. Before deleting a VLAN, it’s advisable to reassign member ports to a different VLAN to ensure uninterrupted communication.

Is it possible to delete the entire vlan.dat file on a Cisco switch?
The delete flash:vlan.dat command in privileged EXEC mode can delete the entire vlan.dat file. This action, followed by a switch restart, restores the switch to its factory default condition for VLAN configurations.

7 Important Show Commands in Cisco IOs

7 Important Show Commands in Cisco IOs

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The Show commands display important information about the device’s configuration and operation in Cisco IOs Command Line Interface (CLI) mode. Network administrators can use different show commands to check device status, interfaces, processes, and configuration files to confirm the device’s operational status.

The network administrator can also view the router’s process and function and verify it using show commands. The important show commands of Cisco IOS are the following:-

  1. show version

2. show running-config as well as show startup-config

3. show arp

4. show interfaces

5. show protocols and also show ip protocols

6. show ip route and show ip interface brief also

7. show cdp neighbours

Show Commands Details

show version

The show version command on the Cisco router displays hardware information. The command output gives insight into the router’s capabilities. The abbreviation of this command is sh ver. The output of the show version command consists of the following unique information:-

  • IOS version
  • Image filename
  • System uptime
  • Type of processor and also show the amount of RAM
  • Number of ports on the switch
  • Amount of flash memory
  • Current configuration register

The figure below illustrates the output of the show version command on the Cisco router.

Show running-config

We can use this command to display the configuration information running on the terminal. We can also use this command with the show startup-config command to compare the information in running memory to the information stored in NVRAM or a place specified by the CONFIG_FILE. The abbreviation of this command is sh ru

show arp

This command displays the router’s ARP table. The ARP table contains the resolved IPv4 address to MAC address mappings. If a host can’t ping? Ensure you’re getting an arp entry of the host that you want to ping using the “show arp command“. The abbreviated command is Sh ar

show interfaces

The show interface command displays all types of interfaces with configuration and statistics. Its output provides a lot of information. The abbreviation of this command is sh int. The figure below illustrates the output of the show interface command.

show protocols

This command displays the status of the configured layer three protocols on all Cisco router interfaces. This command can be abbreviated as sh prot

show ip route

The show ip route command is used to show the router’s routing table. The routing table lists all networks the router can communicate, their metrics, and how to communicate with them. This command can be abbreviated with sh ip ro. The command also has parameters after route (to), like sh ip ro rip for all RIP routes. The figure below illustrates the output of the show ip route command

show ip interface brief

The show ip interface brief is one of Cisco devices’ most frequently used commands. This command provides an abbreviated output compared to the show ip interface command. It outlines the key information for all the network interfaces on a router. The figure below illustrates the output of this command.

The output displays all router interfaces, including the IP address assigned interfaces, if any, and their operational status. The show ip interface brief command can also be used to verify the status of the switch interfaces.

Show CDP neighbours

We can use this command to display information about currently connected Cisco devices. Cisco Discovery Protocol (CDP) is a proprietary protocol that will only detect Cisco products. To show detailed information about neighbouring devices discovered using CDP, use the show cdp neighbours command in privileged EXEC mode. The command can also be abbreviated with sh cd ne.

FAQs

Q1: What are Cisco IOS show commands? 

A1: Cisco IOS show commands display a Cisco device’s current status and configuration. They provide valuable information for troubleshooting and monitoring network performance.

Q2: Why are show commands important for network administrators? 

A2: Show commands are important for network administrators because they help diagnose problems, verify configurations, and ensure the network runs smoothly. They also provide insights into various aspects of the network, such as interfaces, routing tables, and device status.

Q3: Can you list some common Cisco IOS show commands? 

A3: Some common Cisco IOS show commands include:

  • show ip interface brief
  • show version
  • show running-config
  • show ip route
  • show interfaces
  • show vlan
  • show cdp neighbors

Q4: How do I use the show ip interface brief Command? 

A4: The show ip interface brief command provides a summary of the interfaces on a Cisco device, including their IP addresses and status. It is useful for quickly checking the status of all interfaces.

Q5: What information does the show version command provide? 

A5: The show version command displays information about the Cisco IOS software version, device uptime, system image file, and hardware details. It is useful for verifying the software and hardware configuration of the device.

Q6: How can I view the current configuration of my Cisco device? 

A6: You can view the current configuration of your Cisco device using the show running-config command. This command displays the active configuration that is currently running on the device.

Q7: What does the show ip route command do? 

A7: The show ip route command displays the routing table of the device, showing all the routes known to the router and how they were learned. It is useful for understanding the routing decisions made by the device.

Q8: How can I check the status of a specific interface? 

A8: You can check the status of a specific interface using the show interfaces command followed by the interface identifier (e.g., show interfaces GigabitEthernet0/1). This command provides detailed information about the interface, including its status, errors, and statistics.

Q9: What is the purpose of the show vlan command? 

A9: The show vlan command displays information about VLANs configured on the device, including VLAN IDs, names, and associated ports. It is useful for managing and troubleshooting VLAN configurations.

Q10: How do I use the show cdp neighbors command? 

A10: The show cdp neighbors command displays information about directly connected Cisco devices discovered using the Cisco Discovery Protocol (CDP). It provides details such as device IDs, interfaces, and platform information.