VTP Concept Notes

VTP: Concept Notes

This post is part of a series of revision notes I put together and used for studying CCNP Switch concepts. I am now writing them up on this website so that they might be useful to others.

They formed a part of my notes intended to explain concepts in simple terms. I hope they come in use, but as a reminder, please do not use these notes as any indication of what is sufficient for CCNP level understanding.

Let’s get started.

Definition

  • VTP stands for VLAN Trunking Protocol

What It Is

  • VTP is a way of propagating VLAN information throughout the network so we don’t have to manually configure each VLAN multiple times on different switches
  • VLAN Trunking Protocol (VTP) is a Cisco proprietary protocol
  • It propagates VLANs to the entire LAN devices
  • To do this, VTP carries VLAN information to all the switches in a VTP domain
  • VTP advertisements can be sent over 802.1Q, and ISL trunks

How it Works

Domain:

  • All devices must be in the same Domain to accept updates from each other
  • By default, the domain is set to NULL
  • Whenever a switch is plugged into the network and has a custom Domain name set, then this will put all other Switches with a “NULL” Domain Name into its Domain
  • If this new Switch has a higher revision number (incremented when changes are made), then it will propagate its own VLANs to all the other Switches…can cause chaos!

Revision Numbers:

  • The Revision Number essentially indicates how many times configuration changes have been made to VLANs on a Swith
  • This value is important because it determines which updates are to be used
  • Higher is better

Modes:

  • Server
    • The default VTP mode
    • You can make, modify, and delete VLANs
    • You can also configure other parameters within VTP, such as the version or VTP pruning
    • VTP servers advertise their VLAN database to other Switches within the same VTP Domain
  • Client
    • Behaviour-wise, same as Server mode, however you cannot make, change, or delete VLANs
  • Transparent
    • The device does not participate in VTP
    • A Switch in Transparent mode does not advertise its VLAN configuration
    • A Switch in Transparent mode does not synchronise its VLAN configuration based on received advertisements
    • However, Switches in Transparent mode DO forward received VTP advertisements
  • Off
    • Configurable only on CatOS switches
    • The Switch behaves the same as in VTP transparent mode, however VTP advertisements are not forwarded

VTP Pruning:

  • Although VTP is useful, it can mean that some VLAN information being forwarded is irrelevant
  • For example, if a Switch on a certain floor of a building has no ports in a certain VLAN, what is the use of propagating that VLAN to that Switch?
  • There is no use, and it just results in unnecessary bandwidth usage
  • VTP Pruning is a feature that we can use in order to stop unnecessary traffic from being spread to Switches who have no need for it
  • However some VLANs are not eligible to be pruned, these are:
    • VLAN 1
    • VLANs 1002-1005
    • VLANs 1005-4094

Manual Pruning:

  • Although VTP Pruning is a useful feature, it is automatic
  • Being something that works automatically is not necessarily a bad thing, but it means we have less control over the network
  • The alternative to VTP Pruning is Manual Pruning
  • Manual Pruning is when we explicitly allow or deny specific VLANs across trunk links

VTP Versions:

  • Version 1
    • The original VTP protocol
  • Version 2
    • V2 introduced support for Token Ring VLANs
  • Version 3
    • All the features of previous Versions plus the below
    • Extended VLAN range support (1001-4094)
    • Private VLAN support
    • The VTP Authentication password can be optionally prevented from being displayed in plain text in the running configuration

For all versions, VLANs 1002 to 1005 are reserved VLANs

VTP Primary Server:

  • This is a feature/role introduced in VTP Version 3
  • Before V3, if a Switch with a higher Revision Number was added to the network, it had the capability to wipe the entire network’s VLAN configuration from all Switches
  • This is clearly undesirable behaviour and is why the Primary Server feature was developed
  • When a Switch is configured as the Primary Server, only it can make new VLANs
  • Even if another Switch is configured as a VTP server, it wont be able to make new VLANs, this is reserved for the Primary Server only
  • This eliminates the previously mentioned problem and provides some VTP stability and reliability

Configuration

Setting the VTP Domain:

Adding a password for the VTP Domain:

Configuring the VTP Mode on a Switch:

To enable VTP Pruning:

To Manually Prune a Trunk link:

Changing the VTP Version

Confgure a Switch as a VTP Primary Server (Remember, this is VTP Version 3 only!)

Verification

We can use the below show command to check VTP on a Switch:

Revision Flashcard

Last but not least, I have put together a digital flashcard based on my notes.

Please be aware that neither this post nor this flashcard is to be taken as all the information you need to know for VTP, it just contains the basics and is intended to be used as a supplement to your own notes and studies.

I hope it helps.

VTP

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