A routing loop occurs when packets circulate endlessly between routers without reaching their destination. This happens during convergence when routers have inconsistent tables.
Example: Router A thinks the path to goes through Router B. Router B thinks it goes through Router A. Packets bounce forever.
The IP header's TTL (Time to Live) field limits damage. Each router decrements TTL by . When TTL reaches , the packet is discarded. Default TTL: Linux , Windows , Cisco .