Anycast Coverage Calculator
Check per-location load on an anycast prefix and whether the survivors absorb the traffic when a location withdraws.
About Anycast Coverage Calculator
Anycast advertises one prefix from every location and lets BGP send each user to the nearest, so load divides by location count — but only on average, because BGP optimises AS path length rather than balance. When a location withdraws its route its entire catchment lands on the next nearest sites, which is why the failure case is total ÷ (locations − 1) with the skew applied on top. Anycast failure is not graceful: one withdrawal redistributes a whole catchment to specific neighbours, not to the fleet evenly, so provisioning to the average utilisation is how a single PoP loss cascades into a regional outage.
Formula
Anycast Coverage
load per location = total ÷ locations; after a withdrawal, load per survivor = total ÷ (locations − failed), and the busiest catchment carries that plus the skew.
- catchment skew
- How much busier the largest catchment is than the mean
- survivors
- Locations still advertising the prefix after the failure
- locations you can lose
- Fleet size minus the locations needed to carry the load
Worked example
With the default values below, Anycast Coverage Calculator returns Survivor Utilisation After the Failure of 20.3%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Total request rateaccepts 0–100,000 thousand/s
- 240 thousand/s
- Locations advertising the prefixaccepts 1–500 locations
- 60 locations
- Capacity per locationaccepts 0.1–100,000 thousand/s
- 20 thousand/s
- Locations lost in the failure caseaccepts 0–100 locations
- 1 locations
- Extra load on the busiest catchmentaccepts 0–500 %
- 60 %
Results
- Survivor Utilisation After the Failure
- 20.3%
- Average Load per Location
- 4.00 thousand/s
- Average Load per Surviving Location
- 4.07 thousand/s
- Utilisation in Normal Operation
- 20.0%
- Busiest Catchment Load
- 6.51 thousand/s
- Spare Capacity in the Fleet
- 960.00 thousand/s
- Locations You Can Lose
- 48
- Resilience Verdict
- Resilient — survivors absorb the failure with headroom
Step by step
- 1Values used
- 2Anycast Coverage
- 3Survivor Utilisation After the Failure= 20.3
- 4Average Load per Location= 4.00 thousand/s
- 5Average Load per Surviving Location= 4.07 thousand/s
- 6Utilisation in Normal Operation= 20.0
- 7Busiest Catchment Load= 6.51 thousand/s
- 8Spare Capacity in the Fleet= 960.00 thousand/s
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Anycast Coverage calculated?
load per location = total ÷ locations; after a withdrawal, load per survivor = total ÷ (locations − failed), and the busiest catchment carries that plus the skew. Anycast advertises one prefix from every location and lets BGP send each user to the nearest, so load divides by location count — but only on average, because BGP optimises AS path length rather than balance. When a location withdraws its route its entire catchment lands on the next nearest sites, which is why the failure case is total ÷ (locations − 1) with the skew applied on top.
Why does Anycast Coverage matter?
Anycast failure is not graceful: one withdrawal redistributes a whole catchment to specific neighbours, not to the fleet evenly, so provisioning to the average utilisation is how a single PoP loss cascades into a regional outage.
What values do I need to enter?
This calculator takes 5 inputs: Total request rate, Locations advertising the prefix, Capacity per location, Locations lost in the failure case, Extra load on the busiest catchment. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.