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| 3.6 |  | Implementing Physical Layer Solutions |
| 3.6.4 |  | Solving common problems – Cisco IOS commands |

Interface configuration is a common source of physical layer problems. If an interface configuration does not match the corresponding configuration of a port on an attached device then the interface status will be either:

- interface is down, line protocol is down
- interface is up, line protocol is down

Commonly on serial interfaces lack of physical connectivity could occur because the interface at the clocking end has not been set with a clock rate, or it has been set incorrectly. Also the no shutdown command must be applied to the interface; otherwise it will remain in an administratively down state, which means that it has effectively been turned off.

On serial connections using Frame Relay or ISDN equivalent encapsulation, protocols must be set at both sides of the link. HDLC has many implementations, with different vendors creating different HDLC versions that most often are proprietary and not inter-compatible. Cisco has developed its own implementation of HDLC. Amongst Cisco devices this is fine, however if a Cisco device is connected via a serial link to another vendors equipment it is usual to implement the IETF HDLC on both devices. This version of HDLC is often used between dissimilar equipment because it is a common second choice when a proprietary version of HDLC will not suffice.