NB: Accidently posted reply before I completed it.
We need to be very careful about the topology being discussed.
Indeed!
The first question is who owns the fiber, and that may impact how the implementation works. If the fiber is owned by the organization which will use the connection it is easy.
OP doesn't say who owns the dark fiber, but usually "dark fiber", as a term, at least to me, implies another party owns the fiber. The OP's later replies appear to confirm that assumption, i.e. another party owns the dark fiber, but if there is no other party involved, fully agree it's "easy", as it's not much different from activating any other fiber (although the transceivers tend to be more expensive, due to often much longer fiber run distances, also such transceivers often support monitoring stats the lower distance transceivers don't; the possibility of additional transceiver information might be something to consider that you've not encountered before).
. . . configure IP routing between the sites.
Based on OP, referencing extending VLANs, Rick's description of routing between sites may not apply, but it might still be considered.
Rick continues describing the probable complexity of a service provider, even a single one, providing complexity between sites, concluding with:
But more likely the fiber provider will have the fiber from each customer site to provider, and provider will forward traffic from one site to the other. This is not so easy to implement, needing more complex routing, and presenting the possibility that traffic from outside might be injected into the network, for things like firewalls become necessary.
Perhaps Rick's and my experience differ much, but whatever the complexity the service provider faces to provide a "private" link between your sites is, generally, invisible to the customer and further it's very, very unlikely another customer can access your network in any way which, usually, negates a need for FWs on such connections.
Do you have life jackets in your auto? There are cases where people drown when their auto enters a body of water, but it's very unlikely to happen to you.
Likewise, on a "private" service link, you can have FWs, but, it's probably not worth the expense.
That said, if you have a hub, with multiple private links, having all those links run through the same FW, especially if it's also being used for Internet access, may make sense, but what about the remote site(s) end? Now the expense may start to become noticeable.
BTW, if you connect sites via Internet, that can be quite different, mostly logically, but possibly physically too.
@robertkwild in a later reply, believe you mentioned both sites are physically within London. In that case, possibly a dark fiber connection might be possible, but more likely a MAN service provider can provide a logical/private p2p link, which to you, might be configured much similar to a dark fiber link.
Years ago, the company I was working for, had inter site p2p links that the physical hand off was usually LX gig. One site, though, had SX on one end and LX on the other. How can that be? Further, even the sites using LX, were they in LX range?
The answer was, they connected to an optical network, but that network was invisible to my equipment.
Rarely, we also sometimes had DWDM transceivers that connected to the optical network. Even those, though, were plug and use, i.e. interface configured the same regardless of transceiver type.
So, @robertkwild often you configure your equipment very much like you would use fiber, internally, between two of your devices. Practically, the two major differences, you get some kind of monthly bill for the private link and for some problems, you have to involve the service provider. (For example, a dark fiber link drops due to a physical backhoe cut. Provider may not know that, immediately. Further, it would be the provider who arranges and insures repair.
