Count To Infinity | Sahithyan's Notes

A failure mode of distance vector routing. When a link fails, two routers increment each other’s stale route indefinitely — each believing the other still has a valid path to the destination.

Why It Does Not Occur on Active Links

While $A$ ‘s direct link to $X$ is up, $A$ always advertises the true direct cost. That cost is always less than any indirect path through $B$ (which would be $A$ ‘s cost $+ 1$ at minimum). Bellman-Ford selects the minimum — so $A$ ‘s direct route always wins. $B$ ’s advertisement back to $A$ is never preferred, and no loop forms.

Count-to-infinity requires the direct route to disappear entirely. Only then does a stale indirect advertisement become $A$ ‘s only option.

Mechanism

Consider routers $A$ and $B$ , where $A$ has a direct link to destination $X$ .

Link $A \to X$ fails.
$A$ has no route to $X$ . $B$ still advertises a route to $X$ via $A$ (stale from the previous update cycle).
$A$ adopts $B$ ‘s route: cost = $B$ ‘s distance $+ 1$ .
$B$ sees $A$ advertising a route to $X$ and updates its own: cost = $A$ ‘s new distance $+ 1$ .
Each periodic update increments both distances. The count climbs toward infinity.

Mitigations

Split horizon
A router does not advertise a route back to the neighbor it learned it from. Breaks the two-router loop before it starts.
Route poisoning
When a route fails, the router immediately advertises it with metric = infinity to all neighbors, before the next periodic update propagates stale information.