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

Some Layer 2 problems can stop the exchange of frames across a link, while others only cause network performance to degrade.

For example, consider this Layer 2 problem. A leased line WAN link is misconfigured such that the router at one end of the link is 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 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.

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.


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.