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New Member

Common Spanning Tree

I got a question about CST. I understand the PVST works because the

BPDUs are sent encapsulated on each VLAN so seperate VLANs can create their own STP topology. However, with CST, the BDPUs are going over VLAN 1 only.

So what happens when I use ports that are not VLAN 1? I know trunks have to

carry VLAN 1 but uplink ports may not be in VLAN 1. So if I use 2 ports

that are not VLAN 1, then how is CST going over VLAN 1 going to pick up

BPDUs on those potential loop connections?


Re: Common Spanning Tree

CST is defined by destination address in BPDU frames which is 01-80-c2-00-00-00 (IEEE 802.1d). The reason is compatibility with non-Cisco devices.

Cisco PVST+ approach is:

Each VLAN BPDUs are sent to 01-00-0c-cc-cc-cd plus VLAN1 BPDUs are copied to 01-80-c2-00-00-00 on trunks.

BPDUs are sent to 01-80-c2-00-00-00 on access ports (no matter to which VLAN the port belongs - this is not described in Cisco documentation).

So if you put your access ports to VLAN2 they are a part of VLAN2 STP topology (together with the possible non-Cisco switches connected to these ports). You can even connect new Cisco switches via ports assigned to VLAN3 to these ports and the new switches will become a part of one common VLAN2-VLAN3 STP topology. (You will receive a VLAN mismatch errors via CDP but STP will be still working.)

I know this is pretty confusing but it's necessary to understand that Per-VLAN-Spanning-Tree is meant LOCALLY on a particular switch and port-to-VLAN assignment is also local, so there is possible following spanning tree topology creating one (common) STP tree, e.g.:

sw1, port1,VLAN5-----sw2,port1,VLAN2-----sw3,port1,VLAN3------sw4,port1,VLAN4

no trunks, all other ports in VLAN1

There will be only two SPT processes running on each switch (one for VLAN1, second for VLANx). CST is created by VLAN5,2,3,4 in this case.

So the conclusion:

CST doesn't have to be created by VLAN1, it's just a default.



New Member

Re: Common Spanning Tree

Thanks for the explanation

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