HP ProCurve 6120G/XG Configuration Manual page 581

If A Network Condition Prevents Traceroute from Reaching the
Destination. Common reasons for Traceroute failing to reach a destination
include:
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Executing traceroute where the route becomes blocked or otherwise fails
results in an output marked by timeouts for all probes beyond the last detected
hop. For example with a maximum hop count of 7 (maxttl = 7), where the route
becomes blocked or otherwise fails, the output appears similar to this:
At hop 3, the first and
third probes timed out
but the second probe
reached the router.
All further probes
within the maxttl
timed-out without
finding a router or the
destination IP
address.
Figure C-16. Example of Traceroute Failing to Reach the Destination Address
Timeouts (indicated by one asterisk per probe, per hop; refer to Figure
C-15, above.)
Unreachable hosts
Unreachable networks
Interference from firewalls
Hosts configured to avoid responding
An asterisk indicates a timeout
without finding the next hop.
Troubleshooting
Diagnostic Tools
C-63