Buying a second circuit from a second carrier feels like redundancy. Often it is not. Most metro fiber shares conduit for the last mile, which means a single cut can take both of your circuits down at once, and you will find out from a backhoe, not from a carrier.
This post is about how that happens, how to check whether your redundancy is real, and what to buy when it is not.
Why two carriers can share one path
Fiber is expensive to put in the ground, so carriers avoid doing it twice. In most commercial districts a handful of conduit systems run under the street, built by a utility, a municipality, or one carrier decades ago, and everyone else leases space in them. When Carrier A and Carrier B both serve your building, there is a good chance their fiber enters through the same conduit, the same building entrance, and the same riser to the same telecom closet.
It gets worse further upstream. Carriers routinely lease dark fiber or wavelengths from each other. Your “Carrier B” circuit may physically ride Carrier A’s glass for a stretch, terminate in the same central office, or cross the same bridge. Two logos on two invoices tells you nothing about how many physical paths you have.
Where the single points of failure actually are
Work outward from your equipment and ask, at each step, whether both circuits depend on the same thing.
The closet. Both circuits land on gear in one room, on one power circuit, behind one door. A tripped breaker or a burst pipe is a total outage.
The riser. Most buildings have one vertical pathway from the basement to the floors. Both circuits use it.
The building entrance. The point where fiber comes through the foundation wall. Many buildings have exactly one. This is the classic backhoe target: the sidewalk vault outside.
The street conduit. Shared, as above, sometimes for many blocks.
The carrier central office or hub. Two carriers may home to the same building a few miles away.
The upstream network. Leased capacity, shared long-haul routes, the same bridge or rail crossing.
True diversity means the two circuits share none of these. Practical diversity means they share as few as the building allows, and you know which ones they share.
Ask for the route, not the carrier name
Any carrier can produce a route map of its path to your building if you ask, usually as a KMZ file you can open in Google Earth and sometimes as a PDF. Ask both carriers for one, overlay them, and look for overlap. If two routes share more than a block, you do not have diversity. You have two bills.
Ask specifically:
Which building entrance does your fiber use, and is there a second one available?
Which central office or hub does this circuit home to?
Is any part of the route on leased fiber, and whose?
Can you provide a route map, and will you commit to the route in the contract?
The last point matters. A carrier that will put “diverse from Carrier A’s entrance and conduit” in writing is offering you something. A carrier that will only say “our network is fully redundant” is describing its own core, which has nothing to do with the last hundred meters to your door.
Diversity has levels, and prices
You do not have to buy the maximum. You have to buy the level that matches what an outage costs you.
Carrier diversity: two providers, no route check. Protects you from one carrier’s billing mistake or maintenance window. Does not protect you from a cut. Cheapest, and the one most people have by accident.
Entrance diversity: two circuits through two building entrances. Protects against the sidewalk vault. Requires a building with two entrances, or a second one being built, which is a construction project.
Route diversity: separate conduit for the full last mile and separate central offices. This is what most people mean when they say “redundant,” and it is the level worth paying for at a site that cannot go down.
Medium diversity: one path on fiber, one path not on fiber at all. See below.
When wireless is the honest answer
For a lot of sites, the most defensible backup is not a second fiber circuit. Licensed fixed wireless or a business-grade 5G circuit on a rooftop has no conduit to share, no building entrance to share, and no sidewalk vault. It is medium diversity by construction: the only thing it has in common with your fiber is the closet, and you can put its radio on a separate power feed.
It is usually cheaper than the second fiber run you were about to sign for, and it installs in days rather than months. Licensed fixed wireless can carry a real SLA at hundreds of megabits or more with a clear line of sight to a carrier’s tower or hub. Business 5G is less deterministic (throughput depends on the cell and the hour), but for a backup whose job is to keep email, phones and card terminals alive during a fiber cut, it is often exactly enough.
What wireless is not: a like-for-like replacement for a gigabit dedicated circuit during a week-long outage. Size the backup to what the business actually needs to function for the duration of a realistic repair, not to the primary circuit’s headline speed.
Make the failover real
A second circuit that nobody has ever cut over to is a theory. The cheapest test is also the most informative: pull the primary fiber patch cable during a maintenance window and see what happens. Things that commonly go wrong the first time:
The firewall or SD-WAN device is configured for failover but has never seen a link-down event, and the timers are set to minutes.
The backup circuit has a different public IP block, and inbound services (VPN, hosted phones, remote access) are pinned to the primary’s addresses.
DNS records point at the primary. Failover works for outbound traffic and nothing else.
The backup was sized for “email and browsing” three years ago and now carries the whole office’s cloud voice.
Fix these on your schedule, not the backhoe’s. Then put the test on the calendar and do it twice a year.
A short checklist
Get route maps from both carriers and overlay them.
Confirm entrance, conduit and central-office diversity in writing, or know which ones you are sharing.
Consider licensed fixed wireless or 5G as the second path; it is diverse by nature.
Put both circuits on separate power in the closet.
Test failover with a pulled cable, then test it again in six months.
Frequently asked questions
Does having two internet carriers mean I have redundancy?
Not necessarily. Two carriers can share the same street conduit, building entrance, riser and closet, so a single cut can take down both circuits. Carrier diversity protects against billing or maintenance issues, not against a physical cut.
How do I check whether my backup circuit is on a diverse path?
Ask both carriers for a route map, usually a KMZ file, overlay them, and look for overlap. Also ask which building entrance and central office each circuit uses, whether any part of the route is leased fiber, and whether the carrier will commit to the route in the contract.
What are the levels of network diversity?
Carrier diversity uses two providers with no route check. Entrance diversity uses two building entrances. Route diversity uses separate conduit for the full last mile and separate central offices. Medium diversity pairs fiber with a path that is not fiber at all, such as wireless.
Is fixed wireless or 5G a good backup for business fiber?
For many sites, yes. Licensed fixed wireless or business 5G shares no conduit, entrance or sidewalk vault with your fiber, and usually installs in days. Size it to what the business needs to keep running during a realistic repair, not to the primary circuit’s headline speed.
How should I test internet failover?
Pull the primary fiber patch cable during a maintenance window and watch what happens. Common first-time failures include slow failover timers, inbound services pinned to the primary’s IP addresses, DNS pointing only at the primary, and an undersized backup. Repeat the test twice a year.
When we quote a backup circuit, we ask every carrier for its entrance and route relative to the primary you already have, and we quote wireless alongside fiber so you can see the honest comparison. Send us the address and the primary carrier’s name and we will do the overlay for you.