ICND2 – Chapter 9 – Troubleshooting IP Routing

Chapter 9 was another long chapter that had a LOT of information on Traceroute. It also covered PING. It wrapped up with VLSM and ACL troubleshooting which basically took the same show commands already learned from the previous chapters and gave some example. Good read but not a lot of notes here.

ICMP Unreachable codes and their probable cause

Network Unreachable – No route in the routing table to the destination IP
Host Unreachable – end host is not responding
Can’t fragment – do not fragment bit is set and router need to fragment to send
Protocol Unreachable – Protocol stack not running on host (i.e no ftp server when looking to ftp to host)
Port Unreachable – closed port on host (or non-responding)

PING result symbols

! – Echo reply received (Good)
. – Nothing received (Bad)
U – Host Unreachable
N – Network Unreachable
M – Cannot fragment
? – Unknown Packet Received

Cisco’s traceroute use UDP with a destination port that is unlikely to be used on the destination host.

Microsoft’s tracert uses ICMP Echo Requests and does not use UDP.


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