Switching - Changed VTP domain name, yet VLAN from the previous domain sticks?

[David Anderson]

Hey fellow geeks, you were so helpful with my other VTP question I found another here.

I was working within a lab environment on Switch A and Switch B connected by a single trunk port, and configured VLAN 12, both in Server mode running version 2 VTP. I entered only the domain name "CCNP" on Switch A, and Switch B dynamically learned it and its VLAN, and changed its domain name to CCNP as well.

So I then manually configured the switch with a domain name of "ccnp" to see what the reaction would be on Switch B, and configured in the output of "sh vtp status" the Domain name changed, but it still has the VLAN 12 lingering on it.

I have tried doing first a wr, reload, then delete vlan.dat / reload, yet the VLAN remains.

Does anyone know what causes this behavior, that even though there are only two switches, both running version 2 but using different Domain Names, why Switch B is having Switch A's VLAN stuck in its config?

Any input appreciated, thanks!

[Peter Paluch · Accepted solution]

Hi David,

I was working within a lab environment on Switch A and Switch B connected by a single trunk port, and configured VLAN 12, both in Server mode running version 2 VTP. I entered only the domain name "CCNP" on Switch A, and Switch B dynamically learned it and its VLAN, and changed its domain name to CCNP as well.

This is expected. A switch whose domain name is empty (sometimes shown as NULL) will adopt the first non-null domain name it hears from its neighbor. Once you configured Switch A with a domain name, it sent out a series of VTP messages indicating its new name, and Switch B - considering itself unconfigured yet - adopted the non-null domain name happily along with the VLAN database from Switch A.

So I then manually configured the switch with a domain name of "ccnp" to see what the reaction would be on Switch B, and configured in the output of "sh vtp status" the Domain name changed, but it still has the VLAN 12 lingering on it.

This is also expected. You have changed the domain name, but you did not touch the VLAN database contents, so whatever the switch B had learned before the domain name change, it retained it afterwards. Changing a domain name is not going to erase the existing VLANs on a switch, it is just going to make the switch stop sharing and synchronizing its VLAN database with the neighbors whose domain name is different.

I have tried doing first a wr, reload, then delete vlan.dat / reload, yet the VLAN remains.

There are multiple possible reasons why the VLAN 12 was recreated:

1. If you actually used the VLAN 12 on a switchport in a switchport access vlan command, then on Catalyst switches, the VLAN would be automatically created after the reload if it did not exist.
2. It is also very likely that once Switch B reloaded with an empty VTP/VLAN configuration, it simply synchronized to Switch A through VTP again, adopting the domain name and synchronizing its VLAN database

You would actually need to do write erase, delete vlan.dat, and reload with Switch B disconnected from Switch A to make sure that it does not sync up to it again.

Feel welcome to ask further!

Best regards,
Peter

[David Anderson]

Wow that was conclusive, thank you so much for that input!

[Peter Paluch]

David,

You are very much welcome!

Best regards,
Peter