VTP (VLAN TRUNKING PROTOCOL)

VTP (VLAN Trunkning Protocol)




VTP (VLAN Trunking Protocol) – Table of Contents

  1. Introduction to VTP (VLAN Trunking Protocol)

  2. Why VTP Is Required in Enterprise Networks

  3. What Is VTP and How It Works

  4. VTP Properties and Requirements

  5. Benefits and Use Cases of VTP

  6. VTP Modes Explained

    • VTP Server Mode

    • VTP Client Mode

    • VTP Transparent Mode

  7. VTP Configuration Revision Number

  8. VTP Pruning

    • VTP Pruning Disabled vs Enabled

    • How VTP Pruning Optimizes Bandwidth

  9. VTP Versions Overview

    • VTP Version 1

    • VTP Version 2

    • VTP Version 3

  10. Difference Between VTP Versions (VTP v1 vs v2 vs v3)

  11. VTP Best Practices and Limitations

  12. Common VTP Interview Questions for Network Engineer.



Now that we have discussed the basic part of networking, its time to move on to another topic of switching. After switching we have routing, firewall and VPN to discuss and after VPN the preparation would be complete. You can choose according to your skillset, on what skillset you want to interview for. 


Moving on with further topics of switching, we have VTP, STP, Etherchannel, BPDU's and various other topics to discuss but we would be discussing limited topics here as per the interview 90 percent questions would be asked from those topics. If I found time I'll write about others topics of switching later.  




• Introduction:-

Lets consider a situation: You got 100 switches to configure for Vlans. So now you have two options-

  1. First is to configure each switch manually but that’s not           convenient as its prone to errors and Time consuming.

  2. Second would be Through VTP.

 

 

• What is VTP:-

 

VTP is a protocol which helps simplify the configuration of Vlans when it comes to configuring Vlans on multiple switches overcoming the issues we face configuring each device manually.

 

Through VTP we can configure one switch with the Vlans and the same would be replicated to all the switches using VTP depending upon the VTP modes we have configured them with.

 

• Properties:-

 

1.           It’s a cisco proprietary protocol.

2.           Used to share Vlan configurations with multiple switches and to maintain consistency throughout the network.

3.           VTP manages the creation, deletion and modification of Vlans across the network from the central point of control.

4.           Information will be passed only if the switches connected with the fast ethernet or higher port.

5.           Links connecting the switches should be trunk links.

6.           Switches must be configured with the same VTP domain.

7.           Domain name are case sensitive.

 

• Functions/Benefits:-


1 .  Vlan administration.

2 .  Dynamic filtering of Vlan traffic (VTP Pruning)- Will discuss it moving forward.

 

• VTP Modes:-


1.  VTP Server:- 


- - This is the default mode. 

- - Creation, deletion and modification is possible in this mode. 

- - Switch generates information to other switches for                 synchronization. 

- - Propagates advertisement (That is: forwards advertisement or   information received by any client).

-  - Saves configuration in NVRAM. 

   - Saves configuration in NVRAM.

 

2.  VTP Client:-


- Cannot add, delete or modify its Vlan config. 

- Doesn’t stores its information on NVRAM, So always get it   from server every time it boots up. 

- Propagate VTP advertisements to other switches for                  synchronization. 


3.  VTP Transparent:-


- Can add, delete and modify Vlan config. 

- Doesn’t synchronize Vlan info. 

- Propagates info. 

- Saves config in NVRAM. 


• Revision Number:-


- Higher the revision number latest the information. 

- It always starts from 0. 

- It’s the index used by VTP switches to keep track of most recent         information. 

- The VTP advertisement process always starts with configuration       revision number 0 (Zero). 

- When subsequently changes are made on a VTP server, the                 revision number is increased before the advertisements are sent. 


* *  Server always carries greater revision number than clients.

 

·• VTP Pruning:-


- It makes more efficient use of trunk bandwidth by                 reducing unnecessary flooded traffic. 

- Broadcast and unknown unicast frames on the Vlan are         flooded over a trunk link only if the switch on the                 receiving end of the trunk has ports in that Vlan.

- Preserves bandwidth by configuring it to reduce the             amount of broadcast, multicast and unicast packets.


- Lets take an example for your better understanding:-

 

1. Pruning Disabled:-

 

 



: Looking at the scenario when Pruning is disabled, Vlan 10 is only present on SW1 and SW4. So when PC A (in Vlan 10 on SW1) sends traffic to PC B (in Vlan 10 on SW4), the traffic exit SW1 and reaches SW2 which forwards it to SW4 and SW3 but as Vlan 10 is not present on SW3 it shouldn’t have been forwarded to SW3.

 

Same is the case when SW2 forwards it to SW4, SW4 sends it directly to PC B but along with that it forwards it to SW5 and then SW5 forwards it to SW6 whereas Vlan 10 isn’t present on any of those but still they receive the traffic meant for Vlan 10.

 

** This unnecessary forwarding of traffic impacts the bandwidth and utilization of network which is an issue for the appropriate optimization and efficiency of the network. To resolve this issue we use VTP Pruning.

 

2. Pruning Enabled:-

 

 



 

: Now looking at the scenario where Pruning is Enabled, Here as PC A sends traffic to PC B. The SW1 forwards the traffic to SW2 and now on verifying SW2 forwards the traffic to SW4 as Vlan 10 is not present on SW3.

 

SW4 on receiving the traffic sends it directly to PC B and won’t forwards to the Port leading to SW5 as Vlan 10 traffic is not learned from that port.

 

## How the switch learns if a specific Vlan is present on the connected switch or not is a whole mechanism which I would discuss someday later. For now you can remember this- Downstream switches advertise which VLANs have active ports (i.e., there’s at least one access port in that VLAN) through VTP subset advertisements.

 

• VTP Versions and Differences b/w  them:-

 

Here’s a clean, well‑structured comparison table that clearly explains the differences between 3 Versions of VTP-

 

 

 

Feature / Capability

VTP Version 1

VTP Version 2

VTP Version 3

 

Basic VTP Functionality

 

Supported

 

Supported

 

Supported (enhanced and more secure)

VLAN Support

Supports only standard VLANs (1–1005)

Supports only standard VLANs (1–1005)

Supports both standard and extended VLANs (1–4094)

Private VLANs Support

Not supported

Not supported

Supported

Token Ring VLAN Support

Supported

Improved support

Supported

Transparent Mode Behavior

Does not forward VTP advertisements

Forwards VTP advertisements

Forwards VTPv3 advertisements but only relevant for Version 3

Consistency Checks

Basic consistency checks

Improved checks (e.g., VLAN names)

Most robust consistency validation

Pruning Support

Supported

Supported

Supported

Authentication

Supports MD5 password

Supports MD5 password

Enhanced authentication with stronger mechanisms

Primary/Secondary Server Concept

Not available

Not available

Yes — only Primary Server can modify VLAN database

Protection Against Accidental Updates

Low

Low

High — prevents accidental overwrites by rogue switches

Protocol Interoperability

Works only with VTPv1

Interoperable with v1 and v2

Can coexist but requires v3-capable devices to use v3 features

MST (Multiple Spanning Tree) Integration

No

No

Supported

Database Propagation

VLAN database only

VLAN database only

VLAN, MST, and other database types

Use Cases

Small/basic networks

Medium networks with better checks

Large enterprises needing security, extended VLANs & better control

 



l Interview questions for Network Engineer  asked from Switch Stacking:-


## DISCLAIMER:- The questions asked in the interview are all scenario based and indirect, So you should try to understand the concept instead of cramming. You will be fortunate if you receive a direct question from the interviewer. 



1. What is VTP?

2. Why is VTP used?

3. Modes of VTP?

4. What is Pruning/VTP Pruning?

5. What is Revision Number?

6. Difference between VTP Versions?

 

 





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