Introduction
While studying for CompTIA Network+, I couldn't totally understand what "show VLAN command" shows. I have never typed command on my computer and seen the output.
I understood what VLAN was and what the command actually shows, everything clicked. So, I decided to write it down.
What is VLAN
At first, before understanding Show VLAN command, we need to know VLAN.
VLAN = Virtual Local Area Network
It is a mechanism that divides a physical network into virtual segments.
Why VLAN is needed
Without VLAN：
→ Everyone is in the same network
→ All data are shared across the entire organization regardless of department
→ Problem for security and performance
With VLAN：
→ Be able to divide network depend on department
→ Only sales staff can access data managed by the sales department
→ Ensure security
Familiar example
Without VLAN：
→ Everyone work in one room
→ Every conversation can be heard to everyone
With VLAN：
→ Each department has its own private room
→ Conversation cannot be overheard by other departments
Configuration diagram
【Without VLAN】
Switch
├── PC（Sales department）
├── PC（Development department） ← Everyone in the same network
└── PC（Management department）
【With VLAN】
Switch
├── VLAN 10（Sales department）── PC・PC
├── VLAN 20（Development department）── PC・PC ← Divide network into each department
└── VLAN 30（Management department）── PC・PC
What is show VLAN command
It is the *command to verify VLAN information on a Cisco switch *.
Caution：
The show vlan command is exclusive to switches running Cisco IOS
Other vendors use other command：
Juniper → show vlans
HP/Aruba → show vlans
Dell → show vlan
Kinds of the command
There are three main command on this.
1. show vlan
Show All VLAN information in detail
2. show vlan brief
Show VLAN overview simply
→ The most used command
→ Frequently appears in the exam
3. show vlan id [Number]
Just show specific VLAN
例：
show vlan id 10
→ Show only VLAN 10 information
Output images
Output example for show vlan brief.
VLAN Name Status Ports
---- -------------------- --------- ------
1 default active Gi0/1, Gi0/2
10 Sales active Gi0/3, Gi0/4
20 Engineering active Gi0/5, Gi0/6
30 Management active Gi0/7
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
Meanings of each row
VLAN：
→ VLAN ID（ Identification number）
→ Available range 1〜4094
Name：
→ VLAN name
Status：
→ active = Work normally
→ act/unsup = Not supported
Ports：
→ Ports assigned to the VLAN
→ Gi = GigabitEthernet
About Default VLAN (VLAN 1)
VLAN 1 is a special case
・All ports belong to VLAN 1 as default
・Cannot be deleted
・Cannot be changed from name "default"
Caution for security
It is dangerous to use VLAN 1 itself
↓
Reason：
Misconfiguration can cause unintended traffic across the network because all ports belong to VLAN 1 as default.
Measure：
It is common to move to other VLAN in production environment
When the command is used
Troubleshooting
Verifying whether VLAN setting are correctly configured .
Example：
PC cannot connect to network
↓
Type "show vlan brief" and confirm the output
↓
Discover problem that this port was not assigned to VLAN
Setting confirmation
After creating a new VLAN, run this command to verify the settings were applied correctly.
Example：
Confirm whether settings success correctly by type this command after new VLANN is created
Adding new devices
Setting after confirming "Which vlan we have to add to"
Example：
We want to add a new PC to the sales department network
↓
Confirming VLAN ID for sales department by typing "show vlan brief"
↓
Assigning the port to the VLAN
Command comparison in each vendor
| Vendor | Command |
|---|---|
| Cisco |
show vlan / show vlan brief
|
| Juniper | show vlans |
| HP / Aruba | show vlans |
| Dell | show vlan |
Summary
VLAN ：
To divide physical network into segment virtually
It can ensure security by network segmentation
Show VLAN Command：
The command to confirm VLAN information on Cisco IOS Switch
Main command：
show vlan → Show detailed information
show vlan brief → Show overview（ Most frequently used）
show vlan id XX → Show specific VLAN information
Use case：
・Troubleshooting
・Setting confirmation
・Adding new devices to VLAN
Caution：
・Command dedicated to Cisco IOS
・All port belong to VLAN 1 as default
・Don't use VLAN 1 in production environment
Conclusion
At first, I had no idea what the show vlan command was actually displaying. Once I understood that VALN is a mechanism for dividing a network into virtual segments, the output - including the meaning of each column - fell into place naturally.