3 switch STP - configuration of vlans

[joel75941]

Hello all,

This may sound silly but I'm not sure if I understand the terminology correctly, and I would love to be able to confirm this. 

In a 3 switch topology, where 1 <-> 2, 2-<>3, 3<->1, my understanding is that the root bridge is created per vlan.  So, if I decide that in vlan 1, the root bridge will be switch #1, and in vlan 2, the root bridge will be switch #2:

- in Vlan1, the root ports are touching switch #1.

- in Vlan2, the root ports are touching switch #2. 

- in both situations, the remaining designated ports and blocked ports will be in different spots depending on the priority of the remaining 2 switches.

Is this correct?  Does that mean the term root "port", designated "port", blocked "port", etc. isn't actually referring to the physical port itself, but the placement of the port in the vlan?  Because it sounds like you can definitely run into situations where the physical port can have more than one role depending on the circumstances/number of vlans.

And if this is true, why is it even an option?  When would it ever be beneficial to set them differently across separate vlans?  I appreciate any feedback, thanks.

[Joseph W. Doherty · Accepted solution]

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Posting

What you describe is true if your using per VLAN STP (BTW, something that Cisco supports, but brand X might not).

There are a couple of reasons you might want different roots.  One reason is, it allows you to load balance your links.  For example, in your example of three switches interconnected in a loop, STP must block one of the inter-switch links.  But if you had 3 VLANs, you could block a different inter-switch link per VLAN, allowing each inter-switch link to carry 2 of the 3 VLANs traffic.

Another reason you might desire different VLAN roots, the best gateway for your VLANs might in different locations or the VLAN ports, where used, differs much.  Often you want to minimize the number of switch hops between VLAN ports.

[joel75941]

Thank you for your response, this makes sense.  Also I didn't know PVST wasn't universally supported. 

[Julio Cesar Brochero Bravo]

Hi, there are many reason to implement  per-vlan STP and choose different Root Switch, one of them is to balance the traficc,

let me do some clarifications,

- all the ports in the root switch are Designated. the root ports only exist in the other switch that are not root in each vlan.

-you are right when you mention that the other ports: Designated, root, block depending of every vlan

[joel75941]

Ah thank you for correcting me on the root port locations.  I've updated the original post to reflect this in case someone else comes across it.