C5c PTP Breaks Spanning Tree

It appears that this firmware will break spanning tree. I haven’t done a packet capture to verify, but it acts as if the link doesn’t pass along STP BPDUs. Downgrading to 2.3.3 resolves the issue.

I have upgraded 2 days ago to and my RSTP ring works well for me.

I’ve just verified in the lab that RSTP BPDUs are indeed blocked. I also confirmed with another ISP that they are seeing the same thing. I used the following setup in my lab test:

Switch — C5c_AP – C5c_STA — PC_running_TCPDump

With 2.3.3 I see RSTP BPDUs making it to the PC. With the BPDUs do not make it to the PC running TCPDump.

To break it down further, in my lab outlined above, the following behavior was observed:

AP on and STA on No BPDUs at the PC
AP on and STA on 2.3.3: No BPDUs at the PC
AP on 2.3.3 and STA on BPDUs at the PC
AP on 2.3.3 and STA on 2.3.3: BPDUs at the PC

I can’t sniff the BPDU packets but my ring works. When I reboot one of the C5c the traffic changes. After the reboot the traffic goes through the C5c…
I use Mikrotik 6.38.7 routers and RSTP.

One more bit of info. If the topology of the C5c’s is reversed, as in this:
Switch – C5c_STA – C5c_AP – PC

Then BPDUs will flow through the link on Only when the AP is on the side of the switch do the BPDU’s not flow.

To put it another way, if BPDUs are flowing from STA to AP it will work fine. However, if BPDUs are flowing from AP to STA it will not work.


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Thanks for contacting us and bringing this to our attention. We are working on a fix, and will keep you posted.

We have same problems with RSTP on 2.4.x. We downgraded all C5c’s do 2.3.3 and everything works fine.

I’ve been told the fix will be coming in the next release, 2.4.1.

The fix for this is in 2.4.1. You can also talk to Support now and get 2.4.1 beta 4 which addresses this issue. You should see GA 2.4.1 firmware soon as well, so whichever you want to do.