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I've got 4 switches connected in a circle and they all run RSTP. Lets assume the following:

switch A (root bridge) ------- switch D
       |                              |
       |                              | <= that link is the alternate path
       |                              |
switch B --------------------- switch C

Now to avoid a L2-loop, one link between two switches will be in a blocking state, that port would have the alternate role on one side (that link is between switch C and D). Now when I disconnect the link between A and B, the link between C and D immediately becomes "active" and I've got what I wannted - in the last test I had 2 ping losses, which is ok I'd say. BUT: when I re-establish the link between A and B again, my network goes down for approx 30-40 sec and I can't access devices on switch B from switch A (sounds like traditional STP, but RSTP is configured).

So my question now points to the mac-address-tables: Having a fast convergence (without timers) according to RSTP and avoiding loops (and therefore broadcast storms and mac-address-flapping) is one thing, updating the mac-address-table is another thing. I'd say it isn't enough to enable the link between A and B and to put the link between C and D into the discarding state, because switch A and D need to update their mac address table to make sure that the frames are sent into the anti-clockwise-direction instead of sending frames into the direction of switch C when accessing devices connected to port B.

I've googled a lot, but I have never found any hints or topics regarding the RSTP synchronization in combination with the the mac-address-table update. So the real question is: Are mac-address-tables upgraded when there are topology changes in RSTP or is that not part of RSTP?

Thanks!

EDIT: I've found the issue here, it was a bug in the firmware. Everything's working perfectly now. Thanks!

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  • Tangential to the main question but - just because you have configured RSTP, doesn't mean you aren't running in STP compatibility mode.
    – richardb
    Commented Jun 28, 2022 at 10:54
  • correct @richardb, that's why we have Wireshark ;)
    – BlackFlag
    Commented Jun 30, 2022 at 13:28

1 Answer 1

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So the real question is: Are mac-address-tables upgraded when there are topology changes in RSTP?

Yes. MAC address tables are flushed for the port that receives the topology change notice.

BUT: when I re-establish the link between A and B again, my network goes down for approx 30-40 sec and I can't access devices on switch B from switch A (sounds like traditional STP, but RSTP is configured).

That does sound suspiciously like STP. Perhaps you have a misconfiguration somewhere. If you post your switch configurations, we can take a look.

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  • Thanks very much, that is a very helpful answer. But please let me ask a further question: You write "MAC address tables are flushed for the port that receives the topology change notice" - you mean all MAC's from that port or all MACs of that switch holding the port? How is a fast transition of a route achieved when there are no ARP replys after the synchronization? I mean a machine behind B is still not known from A when the MAC tables are flushed and no ARPs are sent...
    – BlackFlag
    Commented Jun 27, 2022 at 13:28
  • Unknown MAC addresses are flooded out all ports.
    – Ron Trunk
    Commented Jun 27, 2022 at 13:33
  • ok, so if we apply that information to the example above, which port/switch would flood all unknown macs and via which protocol? arp or is it sent within the bpdu?
    – BlackFlag
    Commented Jun 27, 2022 at 13:52
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    I think you're confusing differing functions. MAC flooding has nothing to do with RSTP. ARP is a layer 3 protocol and has nothing to do with the switching (bridging) function. (R)STP just determines whether a port is forwarding or not). If a switch receives a unknown destination MAC, for example, from a host, it will flood it.
    – Ron Trunk
    Commented Jun 27, 2022 at 13:57
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    Yes. The TC is sent on all designated and root ports
    – Ron Trunk
    Commented Jun 27, 2022 at 14:19

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