A routing loop is a state in which the packet is processed over and over by a series of routers ( it goes in a circle ). To deal with such packets, IP has TTL ( Time To Live ) field in the header. Max value is 255, but default value can vary depending on OS, however, TTL is decremented each time it passes through a router. When it reaches 0, the packet is discarded.
In opposite to frames, wich does not have such feature, when L2 loop occurs, frames caught in the loop are processed "forever".
In "normal" network which is configured properly, routing loops should not occur.
This problem can arise, for example, by bad redistribution. For an example, how routing loop can occur during redistribution, see the link below:
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