VTP Domain Name

[vsurresh]

Hello experts,

Any input from you guys would be greatly appreciated.

We currently have around 18-20 switches and all of them are configured as VTP servers.

My colleague usually creates new VLANs on a switch with the highest 'configuration revision number' (SW10) and the VLAN information gets propagated to all switches. 

When I issue #show vtp status, I couldn't find any domain name on any of the switches!?! which is strange. Is it a normal behavior?

SW10#show vtp status
VTP Version : 2
Configuration Revision : 25
Maximum VLANs supported locally : 1005
Number of existing VLANs : 18
VTP Operating Mode : Server
VTP Domain Name :
VTP Pruning Mode : Disabled
VTP V2 Mode : Disabled
VTP Traps Generation : Disabled

I heard from other colleagues few times the VLANs got deleted and bring down the whole network.

Thanks in advance. 

[chrihussey]

Chances are no one ever gave the vtp domain a name on the switches and it is the default config. So they are all essentially in the same domain with an empty / null name field.

Probably the best thing to do would be to transition to transparent mode, give the devices a VTP name (identical or unique to each) and then configure or delete VLANs as needed on each switch. There is more administration involved, but this is clearly a safer way to go.

Hope this helps

[vsurresh]

Thanks for your prompt response.

Currently, I am not going to add 'domain name' and get someone to do it later.

So  I can create a new VLAN on the switch with higher Configuration Revision number and it will be propagated to all switches? 

Thanks

[Reza Sharifi]

Hi

You need to have VTP domain name:

from the VTP config guide:

A VTP domain is made up of one or more interconnected network devices that share the same VTP domain name. A network device can be configured to be in only one VTP domain. You make global VLAN configuration changes for the domain using either the command-line interface (CLI) or Simple Network Management Protocol (SNMP).

By default, the Catalyst 4500 series switch is in VTP server mode and is in the no-management domain state until the switch receives an advertisement for a domain over a trunk link or you configure a management domain. You cannot create or modify VLANs on a VTP server until the management domain name is specified or learned.

If the switch receives a VTP advertisement over a trunk link, it inherits the management domain name and the VTP configuration revision number. The switch ignores advertisements with a different management domain name or an earlier configuration revision number.

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/25ewa/configuration/guide/vtp.html

HTH

[vsurresh]

Hi Reza,

Thanks for your brief explanation. 

The switches we have is 3560. Just one last question, can a VTP server pass VLAN information to another VTP server? Because all of the switches are running as VTP server with 'no domain name'.

Thanks in advance :)

[Reza Sharifi · Accepted solution]

Hi,

The servers will not pass the vlan info to each other if you don't have a domain name configured at least on one of the switches. 

HTH

[vsurresh]

Hi,

Thanks make sense now.

What happens if I change other switches to Client mode.

(Core switch revision number is higher than others)

Once I change other switches from server to client, would it have any impact on the current vlans?

Thanks

[Reza Sharifi]

Hi,

I think as long as the server revision number is higher, it should be fine. But I would be very careful when dealing with VTP, as it can bring down your entire network and cause a big outage for your organization. If having downtime during business hours is not an option, I would schedule a maintenance window for off-hours, make the changes and test. I personally would never use VTP, as the risk is too high, in case of any configuration mistake. 

HTH

[vsurresh]

Hi Reza,

Thanks for your valuable responses. As you suggested, the VLAN information has not been shared between servers and I had to manually created on each device. 

Thanks