Content Overview Troubleshooting Layer 2 problems can be a challenging process. Although data link protocols are usually quite simple, the configuration and operation of these protocols is critical to creating a functional, well-tuned network. This module explores the following Layer 2 protocols and technologies and outlines troubleshooting strategies and common problems for each of them: The interaction of Layer 3 protocols with various Layer 2 protocols is often the source of network problems. Specific examples and trouble shooting techniques for routed and routing protocols in various Layer 2 environments are also examined.
Content 4.1 Characteristics of Data Link Layer Problems 4.1.1 Data link layer problems overview Data link layer problems have common symptoms that assist in identifying Layer 2 issues. Recognizing these symptoms will help the troubleshooter narrow down the number of possible causes of a problem. Before examining the individual protocols and their unique characteristics, it is helpful to know the symptoms that are associated with data link layer problems in general. Lab Activity Lab Exercise: Isolating Physical and Data Link Layer Problems After completing this lab, the student will be able to analyze user feedback and end-system data to decide at which OSI layer to begin isolating problems.
Content 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.
Content 4.1 Characteristics of Data Link Layer Problems 4.1.3 The network is functional, but operating at less than the baseline level While complete data-link layer failures are relatively easy to identify and rectify, it is much more difficult to isolate problems caused by suboptimal Layer 2 operations. The fact that some, or all, frames do actually reach their destination can make it difficult to determine the boundary of the problem and focus on the devices or link causing the problem. There are two distinct types of suboptimal Layer 2 operation that can occur in a network: Although these are very different problems that can be caused by Layer 2 or Layer 1 faults or design issues, they share some similar symptom: It is important to remember that the loss of some frames can be tolerated by many applications, since TCP will retransmit the lost segment. For delay sensitive traffic such as voice, video, or SNA of IBM, suboptimal levels of performance can actually bring the application to a halt. An example of a problem which would cause frames to take an illogical path through the network would be a poorly designed Layer 2 spanning-tree topology. In this case, the network might experience high bandwidth utilization on links that should not have that level of traffic. Problems that cause frames to actually be dropped can be identified through error counter statistics and console error messages that appear on the switch or router. In an Ethernet environment, an extended or continuous ping will also reveal if any frames are being dropped. There are Layer 2 protocols, such as X.25, that have their own retransmission mechanisms which will ensure that dropped frames are retransmitted. In this case, it can be difficult to determine if frames are being dropped. Variations in round trip times for ping replies can indicate a problem, but to be certain that frames are being lost or corrupted, an examination of the error counters on the interface would be required.
Content 4.1 Characteristics of Data Link Layer Problems 4.1.4 Framing errors The data-link layer uses standardized frame formats and physical addressing to establish communications within a broadcast domain. If devices disagree on the frame format communications will fail. Problems with the frame format usually translate to encapsulation incompatibilities between nodes. Actual framing errors occur when a frame does not end on an 8-bit byte boundary for one of the following reasons: All of these problems result in a receiver having difficulty establishing where one frame ends and another frame starts. When the interface is capable of recognizing this condition, the show interfaces command will reveal an incrementing frame error count. These problems are critical as they will cause the communications protocol itself to fail or frames will be incorrectly addressed and fail to reach their destination.