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| 4.1 |  | Characteristics of Data Link Layer Problems |
| 4.1.5 |  | Encapsulation errors |

While framing errors result in communication problems because an interface has trouble recognizing the bit positions, an encapsulation error occurs because the bits placed in a particular field by the sender are not what the receiver expects to see. This condition occurs when the encapsulation at one end of a WAN link is different from the encapsulation in use at the far end. If the protocol is one that expects to see appropriately formatted keepalives or correctly structured negotiation frames, such as PPP, then it is likely that the interface will enter the following state:

> Interface is up, line protocol is down

However, if the Layer 2 protocol is a broadcast medium such as Ethernet, then keepalives are not exchanged between DTE devices and the interface will remain in the state:

> Interface is up, line protocol is up

This will be so, even if incompatible Ethernet encapsulation types are being used. Fortunately, in an IP environment it is rare to change the Ethernet encapsulation type to anything other than the default ARPA. Ethernet encapsulation type mismatches are most likely to occur in an IPX environment where the default Ethernet encapsulation has changed over the years with different versions of Novell’s Netware server operating system.

In the case of Ethernet, there are no tell tale signs that Ethernet encapsulation mismatches exist as the interface simply ignores encapsulation types for which it is not configured.

The following guidelines should be adopted when troubleshooting Ethernet problems:

- In an IP environment, treat any Ethernet encapsulation other than ARPA as very suspicious.
- In an IPX environment, check the encapsulations in use on the servers as this is a common misconfiguration on the router.