Switchport No config AUTO Trunking

[vaughnconcepts615]

I've run across a switch port on a core switch at one of our branch offices that has no config on it but is not shutdown. A 3750 is plugged into this port, and has negotiated a trunk, with hosts connected to it.  In other words, it's in production, and low and behold it's also throwing the below VTP message in the log.  I obviously need to fix the configuration on the core switch, but my other question is could this cause a L2 loop?

coreswitch#show run int gi6/1
Building configuration...

Current configuration : 36 bytes
!
interface GigabitEthernet6/1
end

coreswitch#show int gi6/1 trunk

Port Mode Encapsulation Status Native vlan
Gi6/1 auto n-802.1q trunking 1

%SW_VLAN-4-VTP_USER_NOTIFICATION: VTP protocol user notification: Version 1 device detected on Gi6/1 after grace period has ended

[Matthew Leuschner · Accepted solution]

I ran into a situation like this a long time ago where a distribution 6500 was configured with a specific access VLAN and on the other end was a 3750 with no configuration at all (everything in vlan 1).  The switch was working as expected and providing access to the appropriate VLAN listed on the 6500.  This I believe is by design and partly what makes VLAN 1 so insecure.

During an outage period I simply updated the configuration on both ends to properly reflect what it should be (proper trunk and hosts on the access switch in the proper vlans).  I assume the coreswitch connects to this other switch and is just an access switch.  If that is the case you shouldn't have any loops in the network but you do have an unsecure configuration. 

The VTP notification is simply informing you that the remote device is running v1.  I assume your coreswitch is running VTP version 2

[Carlos Villagran · Accepted solution]

Hi!

This notification is to let you know that a connected switch is running VTP version 1. Since versions 1 and 2 are compatible then there isn't a potential issue (no STP loop). However I would recommend matching versions anyways.

Please refer to the following official cisco link about this notification:

http://www.cisco.com/c/en/us/support/docs/lan-switching/vtp/98155-tshoot-vlan.html#topic18

Hope it helps, best regards!

JC