How does spanning tree know when to type_inc a port with only vlan 1.

[osema-to]

Hello.
We all know that STP may block an interface in BKN* type_inc when it detects that an access port is facing a trunk port.

I created a lab with only 2 switches (G0/0 <-----------> G0/0).

I left the default config of STP and connected the two switches.

I made one interface as trunk with dot1q and the other as access,  DTP is disabled on both ends, native Vlan is still Vlan 1.

```

! config on SW1's G0/0

int g0/0

   switchport trunk encapsulation dot1q

   switchport mode trunk

   switchport nonegociate

! config on SW2's G0/0

int g0/0

  switchport nonegociate

  switchport mode access

```

this gives the known error of:  `*type_inc` 

When i check on wireshark, STP traffic is sent without tagging (no dot1q header is sent in STP packets because STP is only on native Vlan).

So my question is the following:

How does STP knows that we have type inconsistency if both sides send traffic without tagging (no dot1q header) and the stp header doesn't seem to include this information?

[Martin L]

Interesting question; BPDU should give u error message, so STp process plays the role here;  Check your logging buffer; You should see logging message header with % xyz indicating who detected error (after % sign)

   Is this on real gear or CML ?

Regards, ML
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[MHM Cisco World]

For pvst there are two bpdu type send 

One with tag frame 

Other with untag frame 

That why you detect error in stp.

Read this 

https://ine.com/blog/2008-07-17-pvst-explained