Regardless of the cause of the Layer 2 problem, the Layer 3 symptom will be
the same; a loss of connectivity. This could reveal itself in a number of ways:

- Inability to ping across a failing data link
- Routing protocols reporting the link as being down or inaccessible
- Routing protocols reporting configured neighbors as unreachable
- Console messages
These symptoms are similar to those experienced when Layer 3 fails. It
is necessary for the troubleshooter to distinguish between a Layer 2 and a
Layer 3 failure. To specifically test Layer 2 connectivity use the
show cdp neighbors command. If two neighboring devices can
see each other using the show cdp neighbors command and yet
it is not possible to ping between the devices, there are
two explanations. First, the problem may be with Layer 2 to Layer 3 addressing,
or second, it is in fact a Layer 3 problem.