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I have a Cisco router and a Juniper SRX550 sitting next to each other. The Cisco router is on two networks, and the SRX550 is on 3 networks. Since they're close, I could connect them directly?
If I had the Cisco on one floor, and the SRX550 on floor 25 of a building, what's the best way to connect them? Fiber or site-to-site VPN?
I think site-to-site VPNs are for routers separated by miles.
I know this is a general question, but just wondering what the normal procedures are.
Since they're close, I could connect them directly?
Yes, you could. However, you need to consider how it makes sense.
If both sides share a common subnet and there are no address collisions then you can just connect both routers to that subnet/segment. Set up a routing protocol between the routers and set static routes to the remote networks (connected to the other router) on each side and you're set.
If both sides are using completely different subnets you could create a new, shared subnet for both routers to connect with. Set up the routes as above.
If both sides use one or more subnets with the same address and you need to keep them separate, there's a problem. The reasonable solution is to jump the hoop and renumber the subnets to remove the duplicity. The ugly and cumbersome approach is to use address translation (NAT).
what's the best way to connect them? Fiber or site-to-site VPN?
VPN connections are encrypted, secure tunnels (mostly) over 'hostile' or untrusted networks like the Internet. If the entire link is under your control there's no need for VPN.
Direct Ethernet connection can go many miles as well (there are extended fiber PHYs for 100 km or even more).
What you should use depends on what you require and whether the already deployed cabling does the job
If you deploy new cable you need to consider your requirements for the next (at least) 5 or (better) 10 years. Generally, fiber is more costly and more sensitive but also allows for more bandwidth and reach. Today, you either use OM4, OM5 or go directly to OS2. Whatever kind you deploy, hire a professional to do it.
If they're 25 floors apart, and say vertically above each other next to a vertical conduit, you might be inside the 100 m cabling limits of the various twisted pair standards.
Otherwise fibre suggests itself, for which this distance is easily within the spec of cheaper kinds of fibre.
It all depends on what interfaces you have handy, whether you're geared up for fibre or not, have appropriate conduit, and what performance and reliablity you want.
Site-to-site VPN is the sort of thing you'd use if you can't get a private wire/fibre between the two units. Some years ago I had a client with two rented offices, many floors apart in a Manhattan tower; ethernet links were free, but shared a hub (!) with all the other tenants; a direct cable was expensive from the landlord, so we used an encrypted tunnel.
