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New Member

802.1q SPT

Does SPT use VLAN1 when using 802.1q in a single SPT configuration? Also, can you created multiple SPT's with 802.1q as with ISL using version 6.3.1?


New Member

Re: 802.1q SPT

You can use spanning tree per vlan. it is called per vlan spanning tree (PVST+).

For more info read this URL

New Member

Re: 802.1q SPT

From memory, ISL is required with PVST+. I think dot1q only supports CST.

New Member

Re: 802.1q SPT

Cisco supports ISL with a separate STP instance per vlan. That´s PVST.

For 802.1q, Common STP is the Cisco implementation. That´s only one tree although trunks carry n vlans.

STP+ stands for the "merge area" between ISL and dot1q trees, and here we shoud consider that the "boundary" bridge will manipulate BPDUs that arrived in different Layer 2 format and even consider that ISL can see a BPDU for each VLAN but dot1q will only understand BPDUs on VLAN1.

In addition, cat6000 allows Multiple Instance STP. You can create a STP instance, and assign its corresponding VLANS, allowing faster troubleshooting, easier configuration, and reducing redundant BPDU Traffic.

I hope that gave you a broader perspective.



Re: 802.1q SPT


Cisco is implementing PVST even on 802.1q trunks. I know this is pretty confusing but that's the life. See

MISTP is the way how to decrease the number of STP processes running on the CPU, but it is not implemented on all Catalyst product lines.

I'm afraid there is no way how to force Catalyst to use single tree for all VLANs. If you know the way, please let me know.

Best regards,


New Member

Re: 802.1q SPT


Very interesting.

There´s something about 802.1d and STP in 802.1q that really changed my perspective in the URL you´ve posted.

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