IEEE 802.1W (PVST+) implementation on Meraki switches

[ToryDav]

Hey all,

I'm looking for some assistance to prove, or disprove, some issues noticed with Spanning-Tree protocol, specifically with Meraki in a multi-vendor switching topology.

It has been said, with some evidence to back it up, that Meraki switches do not support true 802.1w per-VLAN Rapid STP.

And in any case where a Meraki switch connects to a non-Meraki, specifically Cisco Catalyst or another vendor such as Aruba, that the non-Meraki switch should be configured with a single-instance of MSTP.

The idea here is that the Meraki implementation of 802.1w runs a single-instance of STP and therefore isn't compatible with a true per-vlan spanning-tree configuration in a mixed vendor environment where there are multiple VLANs with different switches set as the root for different VLANs.

Looking at the data sheet for the MS390, under supported protocols, this is clearly spelled out as it says "Single instance of Rapid STP".

However, looking at the MS 350 data sheet it simply states "802.1w, RSTP" with no mention of RPVST+

So the data sheet confirms this, this has been observed as an issue in the wild, but I'd like to hear from the community on this topic.

1. I am seeking your experience with the above.
2. I am seeking your recommendations to mitigate this.

Thanks,
Tor

[RaphaelL]

Hi ,

You have to run RSTP / MSTP on your switches from other vendors. 

I'm not sure to understand your issue.  Also : It has been said, with some evidence to back it up, that Meraki switches do not support true 802.1w per-VLAN Rapid STP

802.1W is RSTP which is a single instance and not per-vlan. You seem to be refering to RPVST which is a Cisco propriarity protocol

https://documentation.meraki.com/MS/Port_and_VLAN_Configuration/Configuring_Spanning_Tree_on_Meraki_...

https://documentation.meraki.com/MS/Deployment_Guides/Advanced_MS_Setup_Guide   ( section Spanning Tree (STP, RSTP)) 

[ToryDav]

Hi @RaphaelL,

Sorry for the confusion here, yes, I am talking about RPVST/+ not being supported on MS, even though I mention 802.1w. What you have said here makes sense.

What you are saying is that MST must be configured on the Catalyst switch connecting to MS, even though the MS switch only supports RSTP 802.1w. Because MST is backwards compatible this works, which confirms what I am looking to validate with this post. Agree?

Moreover on Cisco Catalyst, when you enable Rapid STP, there is no way (that I am aware of) to configure RSTP only. 

Switch(config)#spanning-tree mode ?
mst Multiple spanning tree mode
pvst Per-Vlan spanning tree mode
rapid-pvst Per-Vlan rapid spanning tree mode

So RSTP/MST is the only compatible way to configure Meraki/Catalyst Spanning-tree domain.

And if the customer enviroment is already configured for RPVST+ with different root bridges configured for different VLANs, and they introduce Meraki without converting over to MST, they will be in for a world of hurt, agree?

[RaphaelL]

Switch#sh run | i spanning
spanning-tree mode rapid-pvst
spanning-tree extend system-id
spanning-tree vlan 1-4094 priority 4096

Switch#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    4097
             Address     XXXX.XXXX.XXXX

Documentation that I posted earlier : 

Rapid-PVST

This is a Cisco proprietary protocol on Catalyst/Nexus switches that is compatible with spanning tree (802.1D) and RSTP (802.1w). It is important to note however that as Rapid-PVST is a multi-VLAN spanning tree protocol, MS series switches can participate in spanning tree only when a spanning tree instance is running on VLAN 1 of all switches. In addition, VLAN 1 must be allowed on all trunk ports running Rapid-PVST, so that BPDUs are seen by the Meraki switches in the topology.  In this configuration, the MS series switches should never be the STP Root Bridge.

Yes it is a multi-VLAN protocol , but since the MS will only work on a single instance / single VLAN ,  the BPDUs have to flow freely on Vlan1.

So RSTP/MST is the only compatible way to configure Meraki/Catalyst Spanning-tree domain.  Yes.

And if the customer enviroment is already configured for RPVST+ with different root bridges configured for different VLANs, and they introduce Meraki without converting over to MST, they will be in for a world of hurt, agree?  Depends on the topology. The documentation states a case that could hurt you in topology like Catalyst -> MS -> Catalyst. 

For a ''normal'' topology of Catalyst ( root ) <- MS  , I don't see how you could mess up if you configure the priority correctly.

[PhilipDAth]

It looks like others have already said this, but you mention the standard 802.1w.  Meraki complies with the "letter of the law" to this standard.  The standard does not allow for per VLAN instances.

Some vendors, such as Cisco on its Catalyst line, decided to deviate from the standard in this area, and they are in fact the ones that are non-compliant.

As you have mentioned, use MSTP in this mixed vendor case.