Count-to-Infinity Routing Workbench

Step through stale distance-vector advertisements, routing loops, split horizon, poisoned reverse, and triggered updates

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routing
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networking
Interactive count-to-infinity animation for learning distance-vector routing convergence, stale route advertisements, RIP infinity, split horizon, poisoned reverse, triggered updates, and routing-loop diagnosis.
Interactive Animation Distance Vector Routing Failure

Count-to-Infinity Routing Workbench

Watch a failed destination route spread through a distance-vector network. Step through stale neighbor updates, see metrics climb toward RIP infinity, then compare split horizon, poisoned reverse, and triggered updates.

Round 0Update round
B=2 C=2Destination metric
Stale routeNetwork state
Loop riskDiagnosis
Goal
See why distance-vector routing can temporarily believe a route that no longer exists.
Try
Compare no guard, split horizon, poisoned reverse, and triggered update behavior.
Watch
Use Step to follow the neighbor advertisement and the metric selected by each router.
Check
Decide whether traffic loops, waits for timeout, receives poison, or converges quickly.

Failed Destination and Neighbor Advertisements

Destination network N is no longer reachable from Router A. Routers B and C still have stale distance-vector entries and may believe each other's advertisements.

Net Nfailed destinationdown
Router Alost direct route16
Router Bnext hop C2
Router Cnext hop B2
B advertises N
C advertises N
Start: the failed route is known at A, but stale routes still exist at B and C.

Update Trace

Diagnosis

0 roundsRounds after failure
16Infinity value
periodicUpdate style
lowUpdate overhead

Without loop prevention, B and C can keep advertising stale reachability to each other. Each round adds one hop until the metric reaches infinity.

Bellman-Fordnew metric = neighbor metric + link cost
Loop clueB and C point at each other for the failed destination.
Fix clueSplit horizon suppresses reverse advertisements; poison reverse sends infinity explicitly.
Quick Reference
  • Distance vector: each router advertises distances to destinations to its neighbors, then applies neighbor metric plus link cost.
  • Count to infinity: after a failure, stale neighbors may keep increasing a route metric until it reaches the protocol's infinity value.
  • RIP infinity: RIP treats metric 16 as unreachable, which bounds the count but limits useful paths to 15 hops.
  • Routing loop symptom: the next hops point back toward each other for a destination that is no longer reachable.
How To Debug
  • Find routers whose next hops form a cycle for the failed destination.
  • Check whether route advertisements include routes learned from the neighbor receiving the update.
  • If convergence waits for timeout, split horizon may be omitting bad news rather than actively poisoning it.
  • If update size is high, look for poison reverse entries that advertise many reverse routes as infinity.
Concept Map
  • No guard: stale metric 2 becomes 3, 4, 5, and continues until infinity.
  • Split horizon: do not advertise a route back to the neighbor from which it was learned.
  • Poisoned reverse: advertise the reverse route, but set the metric to infinity.
  • Triggered update: send changed routes quickly instead of waiting for the next periodic timer.
Technical Accuracy Notes
  • This is a teaching model for distance-vector convergence, not a packet-accurate RIP implementation.
  • RIP uses hop-count metrics and metric 16 as infinity. The infinity control is included to show the tradeoff between maximum diameter and failure convergence time.
  • Poisoned reverse quickly breaks two-router loops, but split horizon alone does not solve every multi-router loop.
  • Triggered updates speed convergence, but regular updates and lost updates can still create transient races.
Source Links