Content

| 3.4 |  | Identifying Physical Layer Problems |
| 3.4.7 |  | Configuration script errors |

Many things can be misconfigured on an interface to cause it to go down. This will cause a loss of connectivity with attached network segments. Changing the subnet of an interface to a different one from the directly attached network segment is an obvious way to shut down their connection, but this is not a physical layer problem.

If a LAN segment has multiple links to the network core, modifying the bandwidth parameter in a routing protocol could also cause a router to favor alternate routes over the local link, resulting in suboptimal network performance.

Other misconfigurations which are directly related to the physical layer are:

- Serial links re-configured as asynchronous instead of synchronous
- Incorrect clock rate
- Incorrect clock source
- Interface shutdown

Switchport duplex configuration mismatches can cause collisions or port shutdown to occur. Although the current state of auto-negotiation can often negate this type of problem, at least one of the attached devices needs to be capable of link speed and duplex negotiation.