Carrier diversity: why two circuits can still fail together

Key takeaways

• Two carriers can share one path: A second provider guards against a cut or local failure only when its circuit shares no trench, building entrance, or central office with the first.
• Four places hide shared risk: The last mile, the building entrance, the central office, and resold access are where separate-looking circuits tend to meet.
• Route documents show the overlap: Route maps, entrance points, central office homing, and the last-mile carrier's name show where two circuits share a path.
• Diversity drifts after install: Carriers move circuits for maintenance and capacity, so routes need a fresh check on a schedule and after any change.
• RFP terms make it enforceable: Contract terms for route maps, reroute notice, and re-audits keep diversity enforceable at every site you buy for.

Two internet circuits from two carriers can share a street trench, a building entrance, or one carrier's last-mile cable, and a single cut can take both offline. Carrier diversity protects a site from a fault inside one provider's network, while leaving those shared paths in place.

Outside providers cause most visible failures. Over nine years of publicly reported outages tracked by Uptime Institute, third-party IT and data center service providers, including cloud, internet, and telecom companies, accounted for about two-thirds.

The route a circuit takes is on paper somewhere, and you can ask for it. With the route map, the entrance point, and the last-mile carrier in writing, you can tell before signing whether two circuits share a path, and check again whenever a carrier moves one.

What carrier diversity means for internet circuits

Carrier diversity means buying internet circuits for one site from two or more providers whose networks don't depend on each other. If one carrier's network has a fault, the other can keep the site online. The definition covers the provider only and leaves open where each cable runs.

Two carriers can still share one street conduit. A site also needs physically diverse circuits, with separate cables on separate routes to each carrier's network.

Carrier diversity vs path diversity vs redundancy

Path diversity means two circuits follow separate physical routes, with no shared point where one failure could take both down. The FCC's 911 reliability rules define physically diverse paths the same way, and add that circuits sharing a common segment, such as one fiber cable, don't count as physically diverse even when they are logically separate. Planners also call this route diversity.

Network diversity vs redundancy is the question to settle before buying a second circuit. Redundancy only means a second path exists, and diversity decides whether that path survives the failure that hits the first one.

Sites where an outage is costly need both kinds of diversity, and the redundancy design for every site sets how far to take them.

Where two circuits quietly share a point of failure

Engineers call links that share one resource a shared risk link group, and an IETF routing standard describes the textbook case: two fibers in one conduit.

That shared resource is a single point of failure. In April 2024, workers installing a light pole in Missouri cut into a fiber line and knocked out 911 service for emergency agencies in Nebraska, Nevada, and South Dakota. Shared risk tends to hide in four places.

Last mile

A 2016 DHS fact sheet found that local access, the local loop between a building and its carrier, typically has the least diversity, compared with metro and national networks. Two carriers can share a trench or pole line. Last mile diversity needs a separate route the whole way, which sometimes means building a dedicated fiber route.

Building entrance

Circuits that arrive by separate streets can still share one conduit, one riser, and one meet-me room. A 2018 DHS guidebook flags a facility with several carriers and only a single entry point as a resiliency gap. Building entrance diversity adds a physically separate entry and keeps the circuits apart through risers and termination points.

Central office

If both carriers connect at the same central office or carrier hotel, one outage there can take both circuits down. CISA warns that a central office interruption can cause full or partial outages across a region. DHS guidance published in 2017 calls for separate cable paths to the central office and separate entries there.

Resold circuits

Off-net, a second carrier orders last-mile access from another carrier and resells it, often as a Type 2 circuit. If that underlying carrier also delivers the first circuit, both may ride one cable. The same 2017 DHS guidance warns that leased lines can leave an organization paying two carriers while all its service runs over one physical path. Ask each carrier, in writing, who owns the last mile, since how off-net circuits reach a site decides whether two brands share one cable.

Real outages that hit more than one carrier

Some failures reach past one carrier because carriers share routes, cable systems, or regions.

• France, July 2024: Overnight sabotage hit fiber belonging to at least SFR, Bouygues, and Free. SFR reported damage affecting six departments, and Free reported damage in Marne and Vaucluse.
• Red Sea, September 2025: Faults in the SMW4 and IMEWE subsea cable systems near Jeddah, Saudi Arabia, disrupted internet access for providers and users across the Middle East and South Asia.

In both cases, damage outside any one carrier's control reached more than one network.

How to verify diversity before you sign

Verification turns a promise of path diversity into a map you can check. Run these steps for every site where both circuits matter.

• Get the route as a file: Ask each provider for the route from your site to its network as a KMZ route map, the Google Earth format that shows the cable path. The 2017 DHS guidance notes that a provider's network diagrams give the most precise route information.
• Overlay the routes: Load both maps together and mark every street, pole line, bridge, structure, or central office the routes share. Two diverse fiber paths touch only at your building and stay apart until each reaches its carrier's network.
• Confirm entrances and risers: Walk the building with the landlord or facilities team, and record where each circuit enters, which riser it climbs, and where it ends.
• Name the underlying carrier: Get the last-mile carrier for every off-net circuit in writing, so you know before signing whether two circuits from two brands ride one cable.
• Write diversity into the contract: Require that each route stays separate from the other named circuit, with written notice before any reroute.

Where the maps show only one way into the building, the fix may mean building a second entrance where none exists.

Keep it diverse after install

Diversity that holds on day one can disappear without notice. Carriers groom circuits onto different fiber or equipment to free capacity or work around maintenance, and a groomed circuit can land in the same conduit as another. Without a notice clause in the contract, the carrier has no duty to tell your team.

Re-audit routes on a set schedule and after any carrier change or nearby construction, comparing fresh route maps with the ones on file. Across many sites and countries, each market adds its own carriers and paperwork. When carrier-neutral internet sourcing runs every market through one buyer, one inventory of routes, entrances, and underlying carriers gives each re-audit a known baseline.

Diversity checklist for your next RFP

An RFP sets diversity terms before award, so every bidder prices and answers the same requirements.

• Route maps: Each bidder supplies a map of the route from the site to its network, in KMZ or a similar format, before award.
• Entrance points: Each bidder names the building entrance, riser, and termination point it plans to use.
• Central office homing: Each bidder names the central office or point of presence where the circuit joins its network.
• Underlying carrier: Every off-net bid names the carrier that owns the last mile.
• Separate upstreams and edge routers: Circuits connect to separate upstream networks and end on separate edge routers, so one device or one upstream can't take both down.
• Reroute notice: The contract requires written notice before any change to the route.
• Re-audit right: The buyer can request current route maps at set intervals.
• A path off the fiber: Where fiber can't be made diverse, the design adds cellular backup on a separate network or satellite backup that avoids the local fiber route.

Make your second carrier a second path

Carrier diversity protects a site from a shared failure only when the routes, entrances, central offices, and last-mile carriers stay separate. A quote rarely shows those details.

Momentum is carrier-neutral. We source circuits from the best of 750+ carriers and service providers and choose each second circuit by where its route runs. When the maps show only one way in, we lay our own fiber, splice into carrier networks, and build networks where none exist, including a second path into a site. We recommend the right route even when it isn't ours, and one team and one contract cover every path.

List the sites where both circuits must stay up, with each circuit's carrier and any route maps you already hold. Then, book a meeting, and we'll plan a second path clear of the first at each site on your list.

FAQs

What is the difference between carrier diversity and path diversity?

Carrier diversity means circuits come from providers whose networks don't depend on each other. Path diversity means the circuits follow separate physical routes, with no shared cable, conduit, or facility. The first protects against a fault inside one provider's network, and the second protects against a cut or damage on one route. A site that must stay online needs both.

Is buying from two ISPs the same as route diversity?

No. Two ISPs can deliver circuits through the same trench, building entrance, or central office, and a second ISP may resell access over the first ISP's last mile. Route diversity exists only when the physical paths from the building to each provider's network stay separate, which route maps and written confirmation can show.

How do I verify that my circuits are physically diverse?

Ask each provider for a route map from your site to its network, ideally as a KMZ file, and overlay the two. Mark any shared street, pole line, or structure. Confirm where each circuit enters the building and which riser it uses. For off-net circuits, get the last-mile carrier in writing.

What is building entrance diversity?

Building entrance diversity means two circuits enter a building at separate points through separate conduits, so one cut or construction accident at an entry point can't reach both. Full entrance diversity also keeps the circuits on separate risers and termination points. A second entrance can need construction and landlord approval.

Does path diversity double the cost?

Where two carriers already reach a building on separate routes, diversity may add little beyond the cost of the second circuit. Costs rise when a new route or entrance needs construction. Where fiber can't be made diverse, a 5G or LEO satellite link can add a separate path into the site without new trench or conduit work.