Routing Convergence Calculator
Add up detection, flooding, SPF and FIB installation to get end-to-end routing convergence and the worst term.
About Routing Convergence Calculator
Convergence is a serial chain, so the total is a sum and not a maximum. In a tuned network detection dominates until the table gets large, at which point programming hundreds of thousands of prefixes into hardware becomes the longest term. Voice and trading traffic care about the total, not the components, and knowing which term dominates stops you from buying BFD when the real cost is FIB programming — or from tuning SPF timers that contribute 5 percent of the outage.
Formulas
Routing Convergence
Convergence = failure detection + flooding across the diameter + SPF wait and computation + FIB installation for every prefix.
- detection
- Hello × multiplier, or roughly 3 × the BFD interval
- flooding
- Per-hop LSA/LSP propagation × hops to the furthest router
- FIB install
- Prefixes × per-prefix hardware programming time, the term that scales with table size
FIB programming
FIB installation time = prefixes × µs per prefix ÷ 1000 milliseconds.
Worked example
With the default values below, Routing Convergence Calculator returns Total Convergence Time of 1,250.0. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Failure detection timeminimum 0 ms
- 900 ms
- LSA/LSP flooding per hopaccepts 0–5,000 ms
- 10 ms
- Hops to the furthest routeraccepts 1–100 hops
- 5 hops
- SPF wait timeraccepts 0–10,000 ms
- 50 ms
- SPF computation timeaccepts 0–10,000 ms
- 150 ms
- Prefixes to installminimum 1 prefixes
- 5,000 prefixes
- FIB install time per prefixaccepts 0.1–1,000 µs
- 20 µs
- Convergence targetminimum 1 ms
- 1,000 ms
Results
- Total Convergence Time
- 1,250.0 ms
- Flooding Time
- 50.0 ms
- SPF Wait + Computation
- 200.0 ms
- FIB Installation Time
- 100.0 ms
- Largest Contributor
- Failure detection — move to BFD or faster hellos
- Meets the Target
- No — 250 ms over the 1000 ms target
Step by step
- 1Values used
- 2Routing Convergence
- 3FIB programming
- 4Total Convergence Time= 1,250.0 ms
- 5Flooding Time= 50.0 ms
- 6SPF Wait + Computation= 200.0 ms
- 7FIB Installation Time= 100.0 ms
- 8Largest Contributor= Failure detection — move to BFD or faster hellos
- 9Meets the Target= No — 250 ms over the 1000 ms target
Assumptions and limits
- The result is derived only from the 8 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, 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 Routing Convergence calculated?
Convergence = failure detection + flooding across the diameter + SPF wait and computation + FIB installation for every prefix. Convergence is a serial chain, so the total is a sum and not a maximum. In a tuned network detection dominates until the table gets large, at which point programming hundreds of thousands of prefixes into hardware becomes the longest term.
Why does Routing Convergence matter?
Voice and trading traffic care about the total, not the components, and knowing which term dominates stops you from buying BFD when the real cost is FIB programming — or from tuning SPF timers that contribute 5 percent of the outage.
What values do I need to enter?
This calculator takes 8 inputs: Failure detection time, LSA/LSP flooding per hop, Hops to the furthest router, SPF wait timer, SPF computation time, Prefixes to install, FIB install time per prefix, Convergence target. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Where does loop-free alternate or fast-reroute fit?
It bypasses most of this chain. LFA or TI-LFA pre-computes a backup and switches on detection alone, giving sub-50 ms restoration while the full convergence above happens in the background. The chain still matters, but it stops being customer-visible.