MX250 RSTP Questions

[nadalfan1984]

I'm having a hard time understanding the Meraki MX250 BPDU forwarding process. Since Meraki runs RSTP, to my knowledge, any physical loop will be blocked regardless of VLAN configuration (unlike PVST in Cisco). So does the MX250 just forward all BPDUs to all connected LAN interfaces? My assumption is as follows.

Note: All ports are forwarding unless there is an X specified for the link in the diagram

- Agg1 switch is the root switch in the topology so all connected interfaces go designated/forwarding

- Agg1 BPDU is sent out on port 22 to Agg2 & port 24 to MX250-1

- MX250-1 forwards Agg1 BPDU out of all LAN ports

- Agg2 now receives Agg1 BPDU on port 24 from MX250-1 and then blocks the connection since it already has a better path to Agg1 (root).

- I don't understand how all 3 connections from Dist2 to MX250-1 are in the forwarding state? If all BPDUs are being blindly forwarded by the MX250-1 (similar to a hub) then two of the connections would end up going to backup port state.

- All switches are Meraki and RSTP is in use throughout the network

Anyways, I'm very confused! Any help would be much appreciated!

[jdsilva · Accepted solution]

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@nadalfan1984 wrote:

 So does the MX250 just forward all BPDUs to all connected LAN interfaces?

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Yes. That.

Given this topology (red x's are blocked ports):

The BPDUs will flow like this:

[jdsilva]

One note about that example... It's an old diagram I had lying around and shows the MX's directly connected to each other  (heartbeat cable). That link is no longer considered best practice so it should not actually ever be used. 

[nadalfan1984]

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@jdsilva wrote:

One note about that example... It's an old diagram I had lying around and shows the MX's directly connected to each other  (heartbeat cable). That link is no longer considered best practice so it should not actually ever be used. 

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Thanks for the explanation!!!  So just to be clear, only the root BPDUs are forwarded from the MX250?  For example, in my topology only Agg1 (root switch) frames would be forwarded out from the MX250s not BPDUs from the Dist 2 switch?  I'm guessing this is because the MX250 received a superior BPDU from the root switch?  Which is exactly how a switch would behave.  Thanks for all the help!!

[jdsilva]

No, not quite. An MX will forward EVERY BPDU out every port. It doesn't listen for STP, so it doesn't act on it at all. You can literally think of the MX as a dumb hub when it come to STP. 

My explanation above was somewhat simpllified, and the arrows really show it more as if we were dealing with 802.1d. Since we're actually dealing with 802.1w there's actually an exchange of BPDUs on each link between adjacent switches, so the arrows would actually be a real mess if I was to draw everything. The good news here though is that 802.1w is meant to act much like 802.1d with regards to how decisions are made for blocked ports, so I can draw things in that manner and the concepts apply. 

So, if you go with an MX will forward a received BPDU out all ports, and all other rules for root and blocked port decisions apply, you should end up in the right place. 

[nadalfan1984]

What's really interesting is that I'm seeing Dist 2 switch ports 26,27, & 28 in the topology as designated ports and can see them forwarding BPDUs in the wireshark packet captures but when I run a packet capture on all of the LAN interfaces on the MX the only STP traffic is from the root switch (Agg1). 

I've verified that Dist 2 ports 26,27 & 28 are all in the forwarding state and forwarding traffic including BPDUs but it doesn't appear the MX is receiving them or forwarding them even though they are directly connected.  My thought process was that if a BPDU was sent out on port 26 and received by the MX250 it would forward the frame out all ports.  When Dist 2 ports 27 and 28 received that BPDU from the MX250 they would then go to the backup port state since they had now received Dist 2's BPDU on those ports.  Thoughts?  Thanks for all the help!!!  This is truly helping my understand of RSTP and the MX!

[ww]

Are all mx to switch and switch to switch ports  in the same native vlan?

[GIdenJoe]

I agree with ww.

It's true the MX just forwards BPDU's when they arrive on the native VLAN (unless drop untagged is turned on BAD FEATURE).
However if you have a different native VLAN or access port in a different VLAN that BPDU will not be forwarded on.
The MX would however receive BPDU's from the root through Dist 2 but on a different VLAN.