Content

| 4.1 |  | Characteristics of Data Link Layer Problems |
| 4.1.9 |  | Critical characteristics – console messages |

In some instances a router will recognize that a Layer 2 problem has occurred and will send alert messages to the console. Typically a router does this when it detects a problem with interpreting incoming frames (encapsulation or framing problems) or when keepalives are expected but do not arrive. The most common console message that indicates a Layer 2 problem is a line protocol down message.