Content

| 4.1 |  | Characteristics of Data Link Layer Problems |
| 4.1.7 |  | Critical characteristics – no network layer connectivity |

Total failure of Layer 2 always results in a loss of network layer connectivity as the Layer 2 frames are required to encapsulate and transmit packets. Layer 2 failures include:

- Framing errors
- Layer 2 to Layer 3 addressing problems
- Encapsulation mismatches

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 lay with Layer 2 to Layer 3 addressing, or, second, it is in fact a Layer 3 problem.