PVST with do1q encapsulation?????

Unanswered Question
Feb 1st, 2007


our switch has been configured PVST with dot1q encapsulation on trunk port....

Since PVST is incampatable with dot1q encapsulation, how this switch has been working from long time?

Please need your valuable inputs...


I have this problem too.
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azmath.hk Thu, 02/01/2007 - 06:51


It does not talk about PVST with dot1q...

My question is why PVST is working with dot1q trunk?

azmath.hk Thu, 02/01/2007 - 07:14

I am using PVST not PVST+...

My question is why PVST is working with dot1q?

In our switch network someone by mistakenly configured PVST on a switch and on its trunk port is configured as a dot1q encapsulation...how come its layer 2 connectivity is up when PVST is incompatable with dot1q encapsulation



azmath.hk Thu, 02/01/2007 - 08:18

We have 3 vlans running on this switch..By the way switch model is 2950....I am confuse plase help me out..

Francois Tallet Thu, 02/01/2007 - 09:11

On switches, PVST was enhanced to PVST+ as soon as Cisco supported 802.1Q. So you cannot be running PVST on dot1Q trunk, that is necessarily PVST+.

When you say that PVST is not compatible with 802.1Q, is it just a theoric question or do you have a particular problem you want to share with the group?



azmath.hk Thu, 02/01/2007 - 09:34

I have attached show spanning-tree and show runn output....please advice

here is the show version

System returned to ROM by power-on

System image file is "flash:/c2950-i6q4l2-mz.121-14.EA1a.bin"

cisco WS-C2950G-24-EI (RC32300) processor (revision H0) with 20710K bytes of mem


Processor board ID FOC0803W1T9

Last reset from system-reset

Running Enhanced Image

24 FastEthernet/IEEE 802.3 interface(s)

2 Gigabit Ethernet/IEEE 802.3 interface(s)

32K bytes of flash-simulated non-volatile configuration memory.

Base ethernet MAC Address: 00:0F:23:96:8A:40

Motherboard assembly number: 73-7280-05

Power supply part number: 34-0965-01

Motherboard serial number: FOC08032979

Power supply serial number: DAB08020HAX

Model revision number: H0

Motherboard revision number: A0

Model number: WS-C2950G-24-EI

System serial number: FOC0803W1T9

Configuration register is 0xF

Well, this is strange....... Any Cisco folks out there care to step in? I first thought that PVST instances where just local to this switch, like a stand-alone. I can't see redundant patch blocking on this switch. If in fact this switch is not redundant-pathed to a core/dist switch then you might've got lucky in that this switch is forwarding all vlans out all ports and that upstream PVST+ switches are taking care of loop detection/prevention external to this switch.

Maybe try a "debug spanning-tree events" session would help show if anything outside of this switch is responding to BPDUs?

Also send the output of "show spanning-tree summary"


After seeing a couple of other threads and a few vague references I feel that PVST really means PVST+ on all recent platform IOSs.

If you do a

switch>conf t

switch>spanning mode ?

Command you'll see that PVST+ doesn't exist, it only references mast, pvst and rapid-pvst

This is a cosmetic anomaly and can be ignored.

azmath.hk Thu, 02/01/2007 - 12:22

Than you very much for your help...

Can you please tell me that in this switch. Is it possible to have PVST as a default configuration? One more thing Is it possible to run both CST and PVST in switch network? Please advivce..



azmath.hk Thu, 02/01/2007 - 18:52

the above links are very helpful....

Could you please tell me is it possible to have PVST as a default configuration in a switch especially in our case...our guys are saying that its a default STP configuration as no one has configured PVST its beeen there since we have introduced this switch in a network....Please advice...


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