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First, the back story. We're doing an addition on a building that is about to disrupt an existing old OM1 fiber run. We are using three pairs from this run: 1 going into the building under renovation, and two that pass on to other locations on the far side of a public park.

To avoid service disruption, and as part of the plan for the newly renovated structure, we are moving the network closet to a different location in the building, and running newer OM3 fiber into the building. It would be nice to also run new OM3 fiber (or even single-mode) for the other runs, but cost estimates for that came back around $60,000. Instead, the plan is to run older OM1 from the new closet location to a nearby in-ground vault box just this side of the park, which already provides access to the old fiber, and re-splice there.

So we will have OM3 running from our main network hub to the closet in the updated building, and then OM1 running from the updated building to the sites across the park.

To make this work, I have a MicroTik CRS309 with 8 SFP+ ports and one RJ45 to place in the relocated network closet. I know if I have to I can use this as switch, where I only use one pair of the new OM3 run. But what I'd really like to do is continue using three pairs from the new run, to three separate logical bridges within the MicroTik, where the MicroTik is acting more as a media converter.

So now finally, for the question. I need to do a basic sanity check. Is the three-bridge option likely to work? Is it likely to cause side effects like a routing loop? What configuration choices on the MicroTik should I watch for? Or will I be better off treating it as a switch?

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    Using OM1 for modern networking is probably a very large mistake. If you want to use 10 Gigabit, you will be very distance limited (33 meters). See the answers to this question.
    – Ron Maupin
    Aug 14 '20 at 17:04
  • For a device to be on-topic here, the manufacturer must offer optional, paid support. Unfortunately, MikroTik does not offer that.
    – Ron Maupin
    Aug 14 '20 at 17:06
  • Let's just say I'm comfortable with 1Gb on these links for the foreseeable future. It's what we have now, and they've never gone higher than 7% capacity. Definitely not putting it in for anything really new, but this is more about maintenance. Aug 14 '20 at 18:43
  • The "SFP+" in your question indicated some need to 10G - if you're fine with 1G for the future, OM1 might be all right.
    – Zac67
    Aug 14 '20 at 18:49
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin
    Dec 17 '20 at 19:46
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As Ron has pointed out, sticking to OM1 and even investing in it severely limits your future options. I wouldn't even consider OM3/4, but go for SMF for everything >20 m. But you'll have to make up your own mind.

"Three seperate, logical bridges" means three VLANs. Just configure each port pair with an untagged VLAN and they'll act as (bridging) media converters. Make sure RSTP or MSTP doesn't interfere with that - RSTP might need to be deactivated on those ports, depending on the partner bridges. MSTP can be configured with a separate instance for each port pair.

Depending on your VLAN setup and the level of network virtualization, it might make sense to drop the "media converter" concept altogether and improve the level of network virtualization instead, but you're not providing near enough detail to really tell.

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