Routing Convergence Calculator
Add up detection, flooding, SPF and FIB installation to get end-to-end routing convergence and the worst term.
Inputs
Total Convergence Time
1,250.0ms
Flooding Time
50.0ms
SPF Wait + Computation
200.0ms
FIB Installation Time
100.0ms
Largest Contributor
Failure detection — move to BFD or faster hellos
Meets the Target
No — 250 ms over the 1000 ms target
Step by step
Values used
Failure detection time = 900 ms; LSA/LSP flooding per hop = 10 ms; Hops to the furthest router = 5 hops; SPF wait timer = 50 ms; SPF computation time = 150 ms; Prefixes to install = 5,000 prefixes; FIB install time per prefix = 20 µs; Convergence target = 1,000 ms
Routing Convergence
Convergence = failure detection + flooding across the diameter + SPF wait and computation + FIB installation for every prefix.
FIB programming
FIB installation time = prefixes × µs per prefix ÷ 1000 milliseconds.
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
How it works
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.
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.