A traceroute whose hop addresses repeat until the TTL runs out is a routing loop, and nothing else. Captured on CML with IOS XE 17.18.2: the symptom signature, why TTL keeps a routing loop from behaving like a Layer 2 loop, the four causes ranked by real frequency, and the Null0 discard route...
Two equal links, one carrying everything. CEF picks a path per flow, not per packet, and when every router in a tier runs the same hash you get polarization. Proven on real route, CEF and exact-route output from a CML lab on IOS XE 17.18.2.
An IPv6 host with only an FE80:: address is waiting on a Router Advertisement that never came. Captured on CML with IOS XE 17.18.2: the five NDP messages, the neighbor cache states, the A/M/O flags in a real RA, and why filtering ICMPv6 unplugs a segment.
The prefix is in the BGP table and show ip route says % Network not in table. The leading status characters are the whole diagnosis. Captured live on IOS XE 17.18.2 in CML, where a missing next-hop-self leaves an iBGP path valid, inaccessible and never installed.
Active is a failure state and Idle is where the FSM gives up. Organised by the state you are looking at right now, with a lab finding that matters: on IOS XE 17.18.2 both "no route to peer" and "TCP 179 blocked" showed up as Idle.
INIT means "I hear you, you do not hear me". One router shows a neighbor in INIT, the other shows an empty table, and that asymmetry is the entire diagnosis. Captured live on IOS XE 17.18.2 in CML with an inbound ACL eating OSPF in one direction.
A neighbor parked in EXSTART means the DBD exchange is failing, and IP MTU mismatch is almost always why. Captured on CML with IOS XE 17.18.2: the asymmetric stuck states, the debug line that names the cause, a recipe to reproduce it, and both fixes proven all the way to FULL.