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| 4.1 |  | Characteristics of Data Link Layer Problems |
| 4.1.2 |  | No component above the data link layer is functional |

Because each layer of the OSI model encapsulates the layer above it, any failure of a Layer 2 protocol on a link will prevent the valid exchange of Layer 3 through Layer 7 information. Some Layer 2 problems can stop the exchange of frames across a link, while others only cause network performance to degrade.

For example, consider a simple Layer 2 problem. A leased line WAN link is misconfigured such that the router at one end of the link is configured using Frame-Relay as the encapsulation type and the router at the other end is using HDLC as the encapsulation type. Obviously, HDLC frames received by the Frame-Relay interface will be meaningless to the router. Therefore, no information can be exchanged at Layer 2. In a Cisco environment this could be confirmed using the show cdp neighbor command. If no information is received about a neighbor when CDP is enabled, that is a very good indication that Layer 2 frames are not being received. Since Layer 2 is not functioning, a Layer 3 test such as ping would also fail. Layer 4 TCP services would also fail, which might be indicated by the failure of a BGP router to peer with neighboring routers. All applications which require the services of lower layers of the OSI model will be affected.

Although this is a simple concept, it underpins one of the fastest ways to narrow down a problem. Network faults are often first indicated by the failure of Layer 7 applications which require network connectivity. The first instinct of the troubleshooter would be to test connectivity at lower layers to attempt to isolate the failure at a specific OSI model layer or layers.

The network layer ping command is the most convenient method for testing connectivity. If a ping is successful, the lower layers can be eliminated as possible sources of the problem. However, if the pings fail, even intermittently, the next step is to find the boundary of the Layer 3 problem. In other words, how far does the ping work?

After examining the Layer 3 configuration of the router, the Layer 2 operation at the edge of this boundary can be confirmed. In a Cisco environment this is likely to involve the show cdp neighbor command. The troubleshooter could also examine the output of the show interfaces command to determine the status of the line protocol.

In most instances Layer 2 problems are not difficult to troubleshoot because the problems are confined to communications between directly connected devices, and Layer 2 protocols tend to be less complex than Layer 3 protocols. The troubleshooting process is enhanced by the ability to drill down through the OSI model to determine if Layer 2 issues exist.

| Lab Activity Lab Exercise: Correcting Problems at the Physical and Data Link Layers In this lab, the student will use various Cisco commands to correct network problems.