Add up detection, flooding, SPF and FIB installation to get end-to-end routing convergence and the worst term.
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.
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.
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.
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.
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.
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.