Introduction to VTP (VLAN Trunking Protocol)

im using packet tracer and i did the same commands but it’s not doing sync with between sw2 and sw3 and sw1 i even did domain name and even did the command to make sure it might be the problem

the error i got native vlan mismatch domain names are correct what can be the issue ?

Hello Abdul

First of all, you mention that you have a native VLAN mismatch error. That is an error that you should correct by ensuring that the configured native VLAN on both ends of the link is the same. Now having said that, I tried to lab this up in Packet Tracer to replicate your results. I also included a VLAN mismatch on the trunk. I found that switches were able to synchronize their VLANs successfully. The VLAN mismatch will not hinder the operation of VTP. This makes sense because VTP uses VLAN 1 and not the native VLAN for communication (even if VLAN 1 is not allowed on a trunk, it makes an exception).

In order to ensure that VTP will function, you must:

  1. Make sure the correct switches are configured as servers and client
  2. Make sure that the domain is the same
  3. Make sure that the VTP version being used is the same
  4. Make sure that the password being used is correct on all devices

I hope this has been helpful!

Laz

hi i am doing this same lab from 3 days now i came to solution . interfaces were set to auto negotiation mode

i set it to switchport mode trunk and thigs got better now switches are syncing with each other .

One thing is i connected one more new transparent switch with the SW2 hoping that it 2 transparent switches will share and exchange info but . transparent mode don’t update its data base . it will forward it .

Just i want to know that in the real world where we use transparent mode ?

Hello Abdul

Thanks for sharing your solution. Yes, VTP will only share VLAN information across trunks. If the connected ports were set to auto-negotiation mode, then they would default to access ports, and thus VTP would not sync across such connections.

Switches in transparent mode will always forward VTP messages but will never update their databases to the information found within them. This is why you don’t see any changes taking place in the VLAN database of these switches.

Transparent mode is used in real-world scenarios. Most often, if you simply don’t want VTP to function at all, and you want to administrate the VLANs of your network manually, you would essentially disable any VTP operation by making all your switches transparent.

In other scenarios, you may be adding a switch to a network that uses VTP, but the switch is only serving a single VLAN. There is no reason to have that switch be informed of dozens or even hundreds of VLANs on your network when you won’t be using them.

In addition, some switches can only handle a specific number of VLANs. I’ve had a situation where I had a 2960 switch that could only support up to 64 configured VLANs but we had 80 VLANs being shared on the network using VTP. This switch couldn’t learn any new VLANs configured, so we made it transparent, and configured it manually only with the necessary local VLANs needed.

I hope this has been helpful!

Laz

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Hi Rene,
Thanks for your clear course.
What is the maximum revision number of VTP protocol? There should be a maximum limit. What will happen when this limit will be reached?

Another question What will happen if a vtp server reload? It is version will start from zero? And it will be synchronized from other vtp clients?
Many thanks
Nicolas

Hello Nicolas

The VTP revision number is a 32-bit field within the VTP header. That allows for over 4.2 billion revisions. Because the VTP revision number always starts at 0 and is always incremented by 1, it will rarely reach this limit within the lifetime of a particular network. There is no way to set the revision number to a particular value, say close to the upper limit, so in order to reach the maximum, it must be reached with legitimate changes to the network.

Now having said that, of course, there is the possibility of a network reaching that maximum value, either due to a malfunction of the VTP protocol, or an attack where VTP packets are spoofed with false revision numbers. Although I have not found any proof to support this, my suspicion is that the timestamp field found within the VTP header is also used to allow the revision number value to “roll over” from 4.2 billion to 0 and to continue.

By using the timestamp, a revision number of 4,294,967,296 and a timestamp of, say, 10/10/2020 14:57:00 will be a smaller revision number than 0 with a timestamp of 10/10/2020 15:00:00 simply because the timestamp is later.

Such a situation is extremely rare, and this is why I believe that there is no information (at least that I can find) about what happens when this maximum is reached. However, the timestamp resolves any such issues.

As for your second question, if a VTP server is reset, it will indeed reset the VTP revision number. This means that any changes to the VLANs made on that server will send VTP updates to VTP clients, but those clients will ignore these updates since their own revision numbers are larger. For this reason, it is a good idea to:

  1. Have more than one VTP server on a network so that if a VTP server is reset, it will receive VTP updates from the other VTP server and receive the correct VTP revision number.
  2. If there is no second VTP server, then reset the revision number of all clients to zero so that any new VTP messages from the server will not be ignored.

I hope this has been helpful!

Laz

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Extremely clear ! Thanks a lot!!!

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Why should one waste time studying VTP when it’s not include in 200-301?

Hello Thomson

I understand your concern about this. However, on all of Cisco’s exam pages, it says the following:

The following topics are general guidelines for the content likely to be included on the exam. However, other related topics may also appear on any specific delivery of the exam.

VTP is definitely considered a related topic to many switching topics, so it’s always a good idea to have included it in your studying. Beyond any certifications, learning about VTP is helpful since you may need to implement it at some point as well.

I hope this has been helpful!

Laz

Hello Rene/Lazaros,

Thank you both for the great and detailed explanations.
Is VTP turned on by default? How do you disable it if you don’t want it in your network?
Is it the same for all models?

Thanks,
Chris

Hello Chris

Initially, a switch will have the following default parameters for VTP:

  • VTP operating mode: Server
  • VTP Domain Name: Null

In such a case, VTP is active, but switches will not actively create any VTP associations with any other switches simply because the domain has not been configured. When all switches have a domain of NULL, it is as if VTP is not functioning.

However, this can still be a dangerous situation, because if a rogue switch is added which has a VTP domain configured, it will start to send VTP messages, and any switch on the network that still has a NULL domain, will automatically adopt that domain as well.

You can’t disable VTP as a feature, but you can ensure that a switch never participates in any VTP domain that may appear on the network by simply changing the VTP mode to transparent like so:

Switch(config)#vtp mode transparent

This ensures that any VTP messages received will be ignored.

I hope this has been helpful!

Laz

Yes it was very helpful, thank you!

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Hi Rene,
I’m configuring VTP pruning, as this topology
prun
R5 and R6 connect to access port of vlan 100 on sw2 and sw4. I enable vtp pruning on sw1.
When I ping from R5 to an IP address, ex 10.1.2.100 to generate ARP packets, sw1 still forwards ARP traffic to sw3 , although sw3 does not have any accessport in vlan 100.
Pls help to explain. Tks you.

Hello Hai

Indeed, based on your explanation, ARP packets sent by R5 should not reach SW3 due to VTP pruning. However, from the image you have shared, it looks like you’re using GNS3. Others have found similar incorrect behavior when using GNS3, and this may be a similar result. Take a look at this Cisco learning network post:
https://learningnetwork.cisco.com/s/question/0D53i00000Kt1H6CAJ/vtp-pruning-not-working-as-i-expected

I hope this has been helpful!

Laz

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Hi, Rene.

  1. Would this (reset the revision number) be the recommended way to avoid the problem you mention early in the letter (experiment on a switch in the lab, delete your experiments, reattach switch to network)? Or is there another/better way to avoid that?

Hello David

Resetting the revision number is probably the best way to ensure that adding a switch to your topology will not change the VTP configuration of all the switches.

There are a couple of ways to reset the revision number in VTP:

  • change the VTP mode to transparent, and then back to server or client mode
  • change the domain name and then change it back

In any case, it’s always best practice to make a switch VTP transparent before connecting it to the network. Once connected, you can then change it back to whatever you need, ensuring that the revision number is indeed smaller than the rest of the switches.

I hope this has been helpful!

Laz

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Hi,

Great lessons. And I have little suggstion to it. I tried to configure VTP manually, but it didn’t work, untill I found on VTP Cisco page, that VTP requires trunk between switches. I think it’s little confusing to learn VTP before Trunk. since VTP doesn’t work without trunk. So suggestions is to put Trunk Lesson before VTP and mention that VTP requires active trunk, so that students can practise VTP without jumpnig between lessons or trying to understand why VTP doesn’t work. I don’t know, if someone had this problems also, but for me it wasn’t clear. (Yeah VTP stays for VLAN “TRUNKING” Protocol, but again as a newbie I didn’t get it)

Thanks

Hello Shahlar

Thanks so much for this feedback. The VTP lesson is part of various courses and may appear elsewhere in the order of lessons as they appear for each course. In the Switching course, VTP appears after trunking:


In other courses, it may indeed appear before trunking. In all cases, however, it is indeed helpful to indicate in the lesson itself that a trunk link is necessary between switches in order for VTP to operate.

I will let Rene know to take a look and make any necessary modifications to the lesson.

Thanks again!

Laz

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Hi Laz

Okay, got it. Thank you for your answer. Glad to be helpful!

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VTP advertisements are not mentioned