Not really sure what you’re asking here, not enough detail provided.

Are you looking to extend a LAN/L2 or a WAN/L3 connection between buildings/facilities?
Is this your dark fiber, or is this coming from a service provider?
Makes a difference…

Thanks for the reply.
Say we have a Cisco switch could we use one of the uplink ports and then take a fibre from one of these ports to the other building to extend a network.

Thanks

Still unclear what you are asking. Spare capacity is dependent on your current loading of your own routers and servers. Hypothetically, you simply need more cabling and switches.

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Thanks again. We have a network in building A and we wish to extend this to building B. Building A has an existing switch which has spare ports, therefore could we plug into one of these spare ports with an Ethernet to fibre converter and extend the network to building B?

Thanks

I don't see any issues with extending the network to another building using the spare fiber port. I would make sure it is at least a gigabit fiber port though, depending on what you need from the network. The maximum fiber cable length would be determined by the port and fiber type.
Assuming Cisco switches with SFP ports, this page shows the maximum fiber length for the different SFP modules:
https://www.cisco.com/c/en/us/support/docs/interfa...

Sure, but performance will be limited by how much the existing network is already loaded with existing traffic

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In short, yes. Its done all the time.
If the switches have SFP ports, I would use SFPs instead of fiber converters. If you only have copper ports available, then the fiber converters are your only real option.

Now, the fiber type and converters will dictate overall speed of the link. If the fiber is a 62.5 OM1 fiber, your probably not going to get anything over 100 MBPS. If its OM2 or OM3, then you can probably get GIG speeds. If you have single mode, then your blazing...

Thanks for for all the replies.