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.