What defines a routing loop?

A routing loop is characterized by a situation where a packet is repeatedly transmitted through the same sequence of routers without making progress toward its final destination. This often occurs when the routing tables in the involved routers contain incorrect or outdated information, causing them to direct packets back and forth between each other. The process continues until the Time to Live (TTL) value of the packet decrements to zero, at which point the packet will be discarded.

In the context of the provided options, focusing on continuously routing through the same routers until the TTL expires accurately encapsulates the essence of a routing loop. The TTL is designed as a safeguard to prevent packets from getting caught in such loops indefinitely. Once the TTL reaches zero, the packet is dropped, which is a consequence of the loop but not a defining characteristic. Similarly, the other options, while they may relate to aspects or effects of routing issues, do not fully capture the specific nature of a routing loop as defined in networking terms.