The business asks for five nines. What does that really take, and how do you respond?
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Say this first
Five nines is about 5 minutes of downtime a year for the whole service, so I first ask which service, measured how, and whether maintenance counts. Reaching it needs independent redundancy at every layer, sub-second failover, upgrades without outages, strict change control and fast detection, and the network alone cannot deliver it because the application and data center are in the path too. Most services need 99.9 to 99.99 percent; the few that need more get a specific, costed design.
Go deeper
How fast the budget goes
5.26 minutes a year is about 315 seconds. With default timers:
- 802.1D spanning tree reconverges in 30 to 50 seconds.
- OSPF on broadcast or point-to-point links declares a neighbor dead after 40 seconds if it relies on hellos (10 second hello, 40 second dead).
- BGP on Cisco waits 180 seconds (hold time) by default.
- A non-redundant switch reload for an upgrade often takes several minutes on its own.
A handful of failovers on default timers, or one unplanned reload, spends the whole year.
What it takes
- Redundancy with independent fate: separate power feeds and UPS, diverse fiber paths, different carriers, ideally different software trains on critical pairs.
- Sub-second detection and failover: routed links, BFD, ECMP, tuned FHRP where layer 2 remains.
- Hitless maintenance: rolling upgrades across independent pairs, or ISSU where it is proven.
- NSF/SSO on chassis with dual supervisors.
- Change discipline: peer review, pre and post checks, automatic rollback, staged rollouts.
- Monitoring that measures the service from the user's side, so you know the real number.
How I answer the business
"Which service, and what does an hour of it being down cost? Five nines for voice in the contact center may be worth it. Five nines for every printer is not. Here are two options with their availability, cost and what they still do not protect against."
They will ask next
Why can the network team not promise five nines alone?
Availability multiplies along the path. If the application, its database and the data center power are each at 99.99 percent, the service is below that whatever the network does. Five nines is an end-to-end target that every team in the path has to design for.
Traps
- Promising five nines because the hardware data sheet says the box is highly available.
- Ignoring planned maintenance. If upgrades need downtime, they count unless the SLA says otherwise.