In what situations, *** are displayed which means a router in the path is not reachable?
Why after *** are displayed, traceroute does not stop, so user sees lines of ***.
*** doesnt mean the router is unreachble, it just means the sender doesnt receive the ICMP ttl expired reply.
Here is the steps when IP A trace IP B
1,sender sends ICMP echo to destination B with ttl=1
2,next hop receive the ICMP packet, and reply ICMP with ttl expired error message.
3,sender receive the ICMP reply or after timeout, sends ICMP echo to destination B with ttl-2
4,2 hops away router recive the ICMP packet, and reply ICMP ttl expired error message.
repeat the above steps until reach the destination, a ICMP destination unreachable error message is sent back to the sender.
On the sender, it will show as '* * *' if it doesn't receive a ICMP reply before timeout. That could because some security policy block the ICMP or just because link congestion.