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Question for an expert - Layer 2 / 3 Bridging

Posted: Sat Feb 02, 2019 11:20 pm
by millst

I have two CRS328 switches running stable Router OS.
They are connected over a 500m fibre SFP+ which is trunking serveral vlans.

It is an AV scenario with audio and video and control seperated on different VLANS.

Al the standard IP stuff works fine, I can plug devices into one switch and they can be accessed via the other, they are all layer 3 devices.

I have a couple of devices that are specifically layer 2 and cannot have any layer 3 functions present. They cannot have any other traffic on their network.
I have a dedicated VLAN for each of these, so for example, port 24 is on VLAN 24 on both switches.
The problem I'm having is that the devices are spitting the dummy and not getting clock sync, presumably because they don't like a specific feature that is turned on.
I've disabled STP, loop protect, IGMP and every other Layer 3 protocol I can find on that VLAN.
I also tried setting up a dedicated bridge for that port pair to see if that would work but no luck.

The document outlining the requirements is on page 4 of here: ... -ISS_1.pdf

I'm starting to run out of ideas.

Basically what I need is to trunk the two switches togehter but have groups of ports that allow Layer 3 and groups of ports that don't.
Any ideas on the best way to achieve this that isn't going to turn me grey?

Re: Question for an expert - Layer 2 / 3 Bridging

Posted: Mon Feb 04, 2019 4:35 pm
by markmcn
You could try a packet capture on the wire to see what else is going on. Once suggestion is to make sure that Mikrotik neighbour discovery is disabled on the interfaces also.
You might want to look at the port speed & duplex settings also maybe limit the switch to only try and negotiate the speed you want.

Re: Question for an expert - Layer 2 / 3 Bridging

Posted: Mon Feb 04, 2019 5:55 pm
by cdiedrich
Besides the mentioned points this sounds like a QoS problem to me.
Try to capture packets and take a look at dscp values.
If not set up properly on the switch, this can cause undesired behavior - especially when running other dscp-critical protocols like Dante on the same trunk.
Since my experience with MikroTik switches is exactly zero, I can't tell how to configure QoS on those.

Re: Question for an expert - Layer 2 / 3 Bridging

Posted: Mon Feb 04, 2019 10:01 pm
by millst
Thanks all.
I'll try turning off neighborhood discovery.

I've managed to get it a little more stable, when I take everything else out of the switch and just run this traffic over the trunk, its reasonably stable, but as soon as I plug something else into the switch (especially a dante source) it all falls apart.

I had already set the ports to 100Mbps full duplex manually as that is highlighted in the manual as a potential issue.

One thing I am noticing in the switch logs is that when I plug the devices in, I see "excessive broadcasts/multicast, probably a loop" appear in the logs 5 times, and then the device loses clock sync permanently until I unplug it and plug it back in.
I've turned off STP everywhere and I've also checked Broadcast Flooding and Unknown Multicast Flooding but the switch's still seems to be getting upset.
Its basically 100Mbps in each direction of broadcast traffic so its a lot of traffic and probably looks to the switch like a broadcast storm, however its a very important broadcast storm that I don't want to block.

Re: Question for an expert - Layer 2 / 3 Bridging

Posted: Wed Feb 06, 2019 11:29 pm
by millst
turning off neighborhood discovery made a huge difference. the devices are no longer losing clock sync.
still getting some audio glitches but I will try mirroring a port and running wireshark to see if I can spot anything else.