2/20/2014

LabSim Anywhere

The general symptoms that indicate a routing problem is the inability to access hosts on a specific network or the inability to access any remote network. The following table lists various problems that are typically caused by routing issues.

Problem

Description
If one or more hosts can only communicate with hosts on the local subnet, the problem is likely with the default gateway configuration.

Can't access hosts outside the local subnet

If a single host is having problems, verify the default gateway setting on that host. If multiple hosts are having problems, verify the default gateway setting, and verify that the DHCP server is configured to deliver the correct default gateway address. If all hosts have the same problem, and if the default gateway setting is correct, verify that the default gateway server is up and configured for routing. If hosts are unable to contact hosts on a specific subnet, but communication with other subnets is working, check the following: Verify that the router connected to the subnet is up. Use the route command on the default gateway of the local subnet and verify that the router has a route to the remote subnet. If necessary, configure a static route or a routing protocol so that the route can be learned automatically. Use traceroute to view the route taken to the destination network. Identify the last router in the path, then troubleshoot routing at that point. Check for routing loops in the path to the destination network. A routing loop is caused by a misconfiguration in the routers along the path such that data is sent back along the same path rather than being forwarded to the destination. Routing loops are indicated by: Routing table entries that appear and then disappear (called route flapping), often at regular intervals (such as every minute). Routing table entries where the next