10-19-2023 06:03 PM
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?
10-19-2023 10:47 PM - edited 10-19-2023 10:49 PM
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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10-19-2023 10:51 PM - edited 10-19-2023 10:53 PM
For pvst there are two bpdu type send
One with tag frame
Other with untag frame
That why you detect error in stp.
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