Spanning-tree "sanity" check ....

[thomasthomsen]

So ... I encountered an issue today, and that made me wonder a couple of things:

1 : On what VLAN does a Meraki switch send its BPDUs ?

Documentation seems to say "Vlan1" - But if you change the Management VLAN in the switch settings, does the Spanning-tree VLAN change (For BPDUs) ?

2: If I set VLAN fx. 20 on the switch management page, but manually configure a switch to fx. VLAN21 does that switch then send BPDUs using VLAN21 ?

3: Or does it always just send the BPDU's on the native VLAN on a trunk port (regardless of the native vlan).

Fx. on a switch two different trunk ports, one trunk port uses vlan 10 as native, and one uses vlan 20 as native vlan. Will I then see BPDU's on vlan 10 on one trunk and vlan 20 on the other trunk port (or lets say .. untagged on both ports) ?

Im really confused.

From packet captures I made Im actually leaning towards "3".

Am I mistaken ? Or what is going on ? 🙂

Thanks

Thomas

[Ryan_Miles · Accepted solution]

#3

[alemabrahao]

You can also check this.

Spanning Tree Protocol (STP) Overview - Cisco Meraki Documentation

[thomasthomsen]

Ok .. So I guess the documentation should read ... from what my additional packet captures tells me : "Untagged VLAN" instead of VLAN1.

And re-reading the documentation it does not actually say VLAN 1 - but it mentions VLAN 1 for interoperability with non-Meraki switches. - So I read that wrong. (My mistake 🙂 )

So I always see BPDU's on the untagged VLAN, fine.

Then, for fun, I tried to create a trunk port with no native vlan ... and the BPDUs on that port are send untagged .... hmmmm .. ... "this is fine ?".

But I think I now have a somewhat better understanding.

[thomasthomsen]

Hmmm just another thought.

What happens if there is another switch that runs a Per Vlan STP method .. and that switch sends a BPDU to the Meraki switch on an tagged VLAN ... what does the Meraki switch do with this packet ? Forward it our all ports with the VLAN on it (tagged or no) or just discard that packet. If its discarding it, my issue is explained I think (still trying to figure every scenario out).

[alemabrahao]

I'm not sure but If a Meraki switch receives a BPDU on a tagged VLAN from another switch running PVST, it would handle it based on its STP settings. If STP is enabled, the switch would participate in the Spanning Tree process, if STP is disabled, the switch would not participate in any STP processing, including handling BPDUs.

[alemabrahao]

https://documentation.meraki.com/MS/Deployment_Guides/Advanced_MS_Setup_Guide#PVST.2FPVST.2B

[thomasthomsen]

Yeah I have read that "Protocol interoperability" a few times ...

... but I just wonder , because I have never tried, and thinks it is "bad", what actually happens when / if you configure a non-Meraki switch to do RPVST or MST (more then one instance).

And you make a little "ring" between lets say 3 switches, one Meraki that thinks it should be root (with a RB priority), and two other switches (just default RB priority), every port in the ring with trunk ports (with more then one vlan).

It will moste likely be "bad" but my brain is trying to figure out why, if the Meraki switch in theory listens to the BPDU it receives on other VLANs too.

[MasG]

Since Meraki switches don't run PVST and they only run STP/RSTP which mean these two protocols are not VLAN aware you could face some issue if you run into a scenario like the one you described above. Therefore to prevent issues we recommend to convert the STP topology into MST single instance as described in the interoperability section of the kb linked below.

https://documentation.meraki.com/MS/Deployment_Guides/Advanced_MS_Setup_Guide#PVST.2FPVST.2B

[thomasthomsen]

Yeah and I think this is what confused me.

Also ... its almost impossible to work around from other equipment.

I cant wait until someday where Meraki also supports MST (with more then one instance) or another per vlan STP version where you can control things a bit more.