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New Member

Common Spanning Tree

I have to connect two Catalyst 4507R as a redundant core of a network with 3COM 3300 as desktop switches.

We are having multiple VLANs

3COM switches onlu support one instance of Spanning Tree across multiple VLANs.

I think it is possible t configure my catalyst to use only one instance of Spanning Tree (CST ?), but it is not so clear for me how to do it exactly.

In the examples i've seen its not clear how do it

Thanks in advance


Re: Common Spanning Tree


AFAIK, there is no easy way to force Cisco switches to use only one instance of Spanning Tree (CST).

You might think about MST (see for detils) but it's linked with 802.1s in Cisco implemetation.

Another possibility might be MISTP (

But I'm afraid neither of these possibilities would provide you with the features you'd want.

So I'd recommend using PVST+ in the Cisco part of your network and just considering that 3Com CST will behave like your VLAN1 extension.

See for clear explanation.



New Member

Re: Common Spanning Tree

If your concern is compatibility between the Cisco and the 3Com with multiple vlans, then there is no need to configure the switch to do this.

PVST+ sends out an untagged frame on the native VLAN which will be used to communicate with any non-Cisco switch in a network, which provides compatibility with non-Cisco devices. The PVST+ frames are 802.1Q tagged proprietary multicasts which will be unrecognized by any non-Cisco switch.

The result is that the Cisco switch forms a MST with non-Cisco devices, but that it still provides the ability to manipulate PVST parameters between the Cisco switches.

Refer to BPDU formats section under the Spanning Tree Protocol section of the following link.

Now if you want to eliminate the extra BPDU's then you can configure the switch to run MST which will be backwards compatible with IEEE 802.1D running on the 3Coms.