Deserae Caldwell
Ip troubleshooting
The Internet Protocol Suite and the OSI Reference Model

Traceroute
The traceroute utility sends out either ICMP echo request (Windows) or UDP (most implementations) messages with gradually increasing IP TTL values to probe the path by which a packet traverses the network. The first packet with the TTL set to 1 will be discarded by the first hop, and the first hop will send back an ICMP TTL When the machine running the traceroute receives the ICMP TTL exceeded message, it can determine the hop via the source IP address. This continues until the destination is reached. The destination will return either an ICMP echo reply (Windows) or a ICMP port unreachable, indicating that the destination had been reached. Traceroute can return useful information about TCP/IP connectivity across your network Example of traceroute Return Codes | Code | Meaning | Possible Cause(s) | nnmsec | This gives, for each node, the round-trip time (in milliseconds) for the specified number of probes. | This is normal. | * | The probe timed out. | A device along the path either did not receive the probe or did not reply with an ICMP "packet life exceeded" message. | A | Administratively prohibited. | A device along the path, such as a firewall or router, may be blocking the probe and possibly other or all traffic; check access lists. | Q | Source quench. | A device along the path may be receiving to much traffic; check input queues. | H | An ICMP unreachable message has been received. | A routing loop may have occurred. |

Ping command
The ping command sends an ICMP Echo Request message to a destination host. Use Ping whenever you want to verify that a host computer can send IP packets to a destination host. You can also use Ping to isolate network hardware problems and incompatible configurations.

Isolate Host Routing Problems
Step 1 Check the host’s ability to send packets inside its own subnet. Either ping the host’s default gateway IP address from the host, or ping the host’s IP address from the default gateway. If the ping fails, do the following: * a. Ensure that the router’s interface used at the default gateway is in an “up and up” state. * b. Check the source host’s IP address and mask setting as compared to the router’s interface used as the default gateway. Ensure that both agree as to the subnet number and mask, and therefore agree to the range of valid addresses in the subnet. * c. If the router uses VLAN trunking, solve any trunk configuration issues, ensuring that the router is configured to support the same VLAN in which the host resides. * d. If the other steps do not lead to a solution, investigate Layer ½ problems with the LAN, as covered in Chapter 3. For example, look for an undefined VLAN.
Step 2 Verify the default gateway setting on the host by pinging one of the default router’s other interface IP addresses. Or, from the default router, use an extended ping of the host’s IP address with a source address from another of the router’s interfaces.
Step 3 Test connectivity to the destination host by using the extended traceroute command on the host’s default gateway, using the router’s interface attached to the source host for the source IP address of the packets. If the command successfully completes: * a. No routing problems exist in the forward route or reverse route directions. * b. If the end-user traffic still does not work (even though the traceroute worked), troubleshoot any ACLs on each interface on each router in the route, in both directions.
Step 4 If the traceroute command in Step 3 does not complete, test the forward route as follows: * a. telnet to the last traced router (the last router listed in the traceroute command). * b. Find that router’s route that matches the destination IP address that was used in the original traceroute command (show ip route, show ip route ip-address). * c. If no matching route is found, investigate why the expected route is missing. Typically it’s either a routing protocol issue or a static route misconfiguration. It could also be related to a missing connected route. * d. If a matching route is found, and the route is a default route, confirm that it will be used based on the setting for the ip classless/no ip classless commands. * e. If a matching route is found, ping the next-hop IP address listed in the route. Or, if the route is a connected route, ping the true destination IP address. * If the ping fails, investigate Layer 2 problems between this router and the IP address that was pinged, and investigate possible ACL problems. * If the ping works, investigate ACL issues. * f. If a matching route is found, and no other problems are found, confirm that the route is not errantly pointing in the wrong direction.
Step 5 If Step 4 does not identify a problem in the forward route, test the reverse route: * a. If the forward route on the last traced router refers to another router as the next-hop router, repeat the substeps of Step 3 from that nexthop router. Analyze the reverse route—the route to reach the source IP address used by the failed traceroute command. * b. If the forward route on the last traced router refers to a connected subnet, check the destination host’s IP settings. In particular, confirm the settings for the IP address, mask, and default gateway.