Count-To-Infinity Problem

Followed illustration shows an imagined network and denotes the distances from router A to every other router. Until now every thing works fine.



The illustration shows that link (A, B) is broken. Router B observed it, but in his routing table he sees, that router C has a route to A with 2 hops.
The problem is, that router B doesn't know that C has router B as successor in his routing table on the route to A.
That occurs followed count-to-infinity problem. B actualizes his routing table and takes the route to A over router C.
In the next picture, we can see the new distances to A. In C's routing the route to A contains router B as next hop router, so if B has increase his costs to A, C is forced to do so. Router C increases his cost to A about B + 1 = 4.
Now we see the consequence of the distributed Bellman-Ford protocol: Because router B takes the path over C to A, he reactualizes his routing table and so on!
At the end this problem is going to immobilize the whole network.

References

[Luntovskyy2004] All Pictures

"Count-To-Infinity Problem" is mentioned on: Distributed Bellman-Ford | Highly Dynamic Destination-Sequenced Distance-Vector Routing