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TAKUMI SUGATA

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Even Without Typing a Command — Understanding What show vlan Actually Shows

#network #vlan #comptia #begginer

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.
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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
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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
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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
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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
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Kinds of the command

There are three main command on this.

1. show vlan 

Show All VLAN information in detail
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2. show vlan brief 

Show VLAN overview simply
　→ The most used command
　→ Frequently appears in the exam
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3. show vlan id [Number] 

Just show specific VLAN

例：
show vlan id 10
　→ Show only VLAN 10 information
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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
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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
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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"
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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
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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
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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
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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
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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
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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.

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