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We have 2 Core Routers (cat4500e-UNIVERSALK9-M) and a bunch of L3 switches that are holding about 50 VLANS. I want to determine how the switches are learning new VLANS. I guess I have to troubleshoot VTP. Can someone lead the way for me?

I've tried the following commands:

show vtp ?

But I haven't understood a lot, so is there a way to determine how VLANS are propagated onto switches?

Thanks in advance

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  • VTP is just a way to implement an easy way to administer your VLANS. A VTP server can be configured with your desired VLANS, then the rest of your network can be set to VTP client, which then will inherit all the VLANS from the server. Normally you would use VTP transparent and add/delete VLANS as needed per switch/router.
    – user36472
    Commented Aug 30, 2017 at 13:54
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin
    Commented Feb 19, 2018 at 18:21

2 Answers 2

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VLAN Trunk Protocol (VTP) reduces administration in a switched network. When you configure a new VLAN on one VTP server, the VLAN is distributed through all switches in the domain. This reduces the need to configure the same VLAN everywhere. VTP is a Cisco-proprietary protocol that is available on most of the Cisco Catalyst series products.

VTP modes:

Server

In VTP server mode, you can create, modify, and delete VLANs and specify other configuration parameters, such as VTP version and VTP pruning, for the entire VTP domain. VTP servers advertise their VLAN configuration to other switches in the same VTP domain and synchronize their VLAN configuration with other switches based on advertisements received over trunk links. VTP server is the default mode.

Client

VTP clients behave the same way as VTP servers, but you cannot create, change, or delete VLANs on a VTP client.

Transparent

VTP transparent switches do not participate in VTP. A VTP transparent switch does not advertise its VLAN configuration and does not synchronize its VLAN configuration based on received advertisements, but transparent switches do forward VTP advertisements that they receive out their trunk ports in VTP Version 2.

VTP can be turned off on the older CatOS firmware.

By default, all switches are configured to be VTP servers. This configuration is suitable for small-scale networks in which the size of the VLAN information is small and the information is easily stored in all switches (in NVRAM). In a large network, the network administrator must make a judgment call at some point, when the NVRAM storage that is necessary is wasteful because it is duplicated on every switch. At this point, the network administrator must choose a few well-equipped switches and keep them as VTP servers. Everything else that participates in VTP can be turned into a client. The number of VTP servers should be chosen in order to provide the degree of redundancy that is desired in the network.

There are some disadvantages to the use of VTP. You must balance the ease of VTP administration against the inherent risk of a large STP domain and the potential instability and risks of STP. The greatest risk is an STP loop through the entire campus. When you use VTP, there are two things to which you must pay close attention:

  • Remember the configuration revision and how to reset it each time that you insert a new switch in your network so that you do not bring down the entire network.
  • Avoid as much as possible to have a VLAN that spans the entire network.

I personally use VTP mode transparent and create/delete VLANS separately on every switch or router in my network. In that way i am in control and know exactly what VLAN is on every device.

Source: https://www.cisco.com/c/en/us/support/docs/lan-switching/vtp/10558-21.html

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I'm assuming this is cisco since you mentioned Vtp. When you do a show vtp you'll probably have one switch that is in mode server. The rest are probably in mode client. If you add a vlan to the one that is a server it should show up on the others. The vtp domain name and vtp password have to match on all switches and the revision number on the client's need to be the same or lower than the one in server mode. The revision number increases with each change. If a client has a higher number it won't take the changes because it thinks it has a newer config. Check these items to troubleshoot.

Show vtp status 

Will show the revision number

To reset the revision number use these commands to change the vtp domain and mode, then change them back. Revision will go to zero. Also, reenter password to make sure it's good.

Vtp mode transparent 

Vtp domain diffrentDomainName

Vtp domain putDomainBackToOriginal

Vtp mode client

Vtp password reenterCorrectPassword 

Now do a show 'show vtp status' revision should be zero. Add or remove a vlan on the switch that is a vtp server and it should show up in less than a min. Usually, in a few seconds.

Also, make sure your up links between switches are programmed as trunk ports. Vtp only goes switch to switch on trunk port, not access ports.

Also make sure they are using the same version. If the server switch is v2 and a client is v1 decide what version you want based on features and switch capability. The command to set version is ...

Vtp version 2

Never add a switch to your network without zeroing the vtp revision and putting it in client or transparent mode. If the switch has the same domain and password, is in server mode and the revision is higher it'll wipe out your vlans. If this does happen you can re-add them and things will come back. The vlan assignment on the ports don't get wiped thankfully!

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