The Internet Protocol Suite and the OSI Reference Model
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 ...view middle of the document...
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: