How is STP implemented on small business switch series ? One STP instance or per VLAN instance?

[Milos Megis]

Hello,
I would like to ask, how is STP protocol implemented on small business series of switches ?

Because when I type command "show spanning-tree" on catalyst switch, I see priority for example as 32768 + VLAN ID
On SMB switch I see just priority for example 32768 without VLAN.

Is it possible that SMB switch uses one STP instance for all VLANs ?

Thank you in advance for any advice.

[ktonev · Accepted solution]

Hello Milos,

My name is Kristian and I am one of the engineers from the Small Business team.

In general, the SMB switches support the following STP variants:

1. Standard 802.1d Spanning Tree support
2. Fast convergence using 802.1w (Rapid Spanning Tree [RSTP]), enabled by default 8 instances are supported
3. Multiple Spanning Tree instances using 802.1s (MSTP)

You can also refer to the datasheet or administration guide for your specific switch model to see the exact supported versions of STP. Also please note that the SMB devices run on a different architecture than Catalyst switches so the output of some commands may be slightly different.

Hope that helps,

Kris

If you find this content helpful please rate it so other users can benefit as well.

[Milos Megis]

Thanks Kristian.

Today I did some simple topology with 2 SMB switches and RSTP running.

Port 2 was in VLAN 2 on both switches.
Port 3 was in VLAN 3 on both switches.
I connected Port 2 on first switch to Port 2 on second switch.
Also I connected Port 3 on first switch to Port 3 on second switch.

So there should not be any loop if each VLAN has own STP instance

But it was detected as loop and one port was disabled as alternating.

[devils_advocate · Accepted solution]

The SGxxx range doesn't support Per Vlan STP, only  old school STP, RSTP and MSTP as stated by Kris above.

The reason you got a blocked port was because of this, there is a single spanning tree instance for all Vlans by default regardless of whether its STP or RSTP.

You can use MSTP for slightly more granularity but there is no PVST with these switches. 

[Milos Megis]

Thanks.

This is what I thought. But I could not find information on internet.