spanning tree issue between nortel and cisco switch

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Sep 28th, 2010
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Hi all

I have a Nortel Passport running CST, common spanning tree for all vlans, this then plugs into a switch switch that for some reason keeps blocking the port, saying messages like received 802.1q on non dot1q trunk.

Is this because the Nortel spanning tree does not work with the cisco spanning tree per vlan ?


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Overall Rating: 5 (1 ratings)
Peter Paluch Tue, 09/28/2010 - 13:22
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It is hard to tell without knowing more detailed information what is going on. So far, we can only trust that the Catalyst tells you the truth - it is receiving 802.1Q-tagged frames on an access port which is clearly an indication of misconfiguration because an access port should not be receiving 802.1Q-tagged frames (with the exception of packets in voice VLAN and 802.1p priority-tagged frames but let's forget about those for now).

You should investigate very carefully where do these 802.1Q-tagged frames arriving on your access port originate from. They may be PVST+ BPDUs that were simply tunneled across your Nortel switch (they are packed differently from IEEE 802.1D STP), or they may be any other 802.1Q-tagged frames that somehow arrive at your access port.

Please provide us with the complete dump of the messages you're seeing (not just "messages like ABCD" - that's hardly a foundation for solid troubleshooting). Also, a simple topology schematic describing how your switch is connected to the Nortel and how are individual interconnections configured would be very helpful.

Best regards,


carl_townshend Wed, 09/29/2010 - 07:46
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Hi there

Just another question, Should common spanning tree, normal STP, work with PVST+ ? will it just tag the bpdu with the native vlan coming from the port ?

I think it could be this ?


Peter Paluch Wed, 09/29/2010 - 11:12
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Hi Carl,

By definition, STP and PVST+ do not interoperate. This is because

  1. PVST+ BPDUs have different encapsulation (Ethernet SNAP) than STP BPDUs (802.2 LLC) and thus are not recognized by legacy standard IEEE 802.1D implementation. In addition, PVST+ BPDUs are 802.1Q tagged. 802.1D STP BPDUs are always untagged.
  2. PVST+ BPDUs are also addressed to a different multicast MAC address than normal STP BPDUs

For a switch that does not support the PVST+, these PVST+ BPDUs are simply multicast frames that are flooded through all other ports as any other multicast 802.1Q-tagged frames. In essence, PVST+ BPDUs are tunneled through non-PVST region.

I suggest reading these two links to better understand the PVST+ and 802.1D interoperation:

Best regards,



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