This problem is addressed by Cisco with the use of Switched Port Analyzer (SPAN). SPAN allows the switch to copy all the packets that are sent to nodes connected to the switch ports and direct them to another port. In essence, the designated switch port becomes a mirror of the monitored port or ports.

Cable Problems

When a cable problem surfaces, it usually appears as an intermittent problem; however, it can cause an immediate failure. Intermittent errors are hard to troubleshoot, and you must keep in mind that almost any connectivity issue can be cable related. You’ll need to replace cables with a cable you know works in order to see if the change resolves your connectivity issue.

Multimeters, time domain reflectometers (TDRs), cable analyzers, and breakout boxes can be used to test for cable problems, but they may not always find the trouble. Just because cables were installed by a certified cable installer doesn’t mean that they aren’t improperly made, won’t break, or don’t have connector failures. Cables are moving parts, and any moving parts are subject to wear and tear as well as failures.

Cross−Over Cables

When I am teaching classes, hardened Microsoft Certified Systems Engineers (MCSEs)s frequently have to ask me what a rollover or crossover cable is. This doesn’t apply to all MCSE’s of course, since I am one myself. I have always been on the networking side of technical support, so it is hard for me to believe that long−time network administrators don’t know the differences between a straight−through and a cross−over cable or when to use them.

A connection to a network node from a switch or hub uses a straight−through cable. A crossover cable is used to connect two network devices: a hub to a hub, a switch to a switch, a switch to a router, and so on. Many times in my classroom or on the job, I have come across someone who couldn’t get a connection between switches because they were using a straight−through cable and should have been using a crossover cable. See Chapter 2 for more information on cables and pinouts.

Switch Troubleshooting Tools

Catalyst switches provide quite a few diagnostic and administrative tools to assist administrators. These tools are located in the IOS command−line interface located on each Cisco device and in the CiscoWorks for Switched Internetworks (CWSI) external software for the end workstation.

Let’s take a look at some of the components of CWSI. Then we’ll examine the troubleshooting commands you can use on the Cisco IOS that are found on Cisco’s line of enterprise switches.

CiscoWorks for Switched Internetworks

CWSI (also called Campus Manager) can run as a standalone application on Windows NT or Unix, including Solaris, HP−Unix, and AIX. This application includes a number of components that not only aid in troubleshooting but also help you during installation and monitoring. These components are as follows:

∙CiscoView—A graphical user interface (GUI) application that provides a virtual look at the switch’s chassis, configuration, and performance monitoring. This component provides very little in the way of troubleshooting functionality. A screen capture of CiscoView is shown in Figure 16.3.

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Figure 16.3: A screen capture from CiscoView.

∙User Tracking—Used in the creation and management of dynamic VLANs. Cisco switches permit VLAN assignments based on dynamic VLAN assignments. This means the Media Access Control (MAC) address is used to assign the port to a specific VLAN. User Tracking defines these dynamic VLANs and maintains the whereabouts of workstations throughout the network.

∙VlanDirector—Another GUI−based application. It is a very powerful tool to aid in the creation of multiple VLANs on a switch. This tool helps the administrator add users and assign ports, and it makes managing VLANs easy.

∙TrafficDirector—A GUI component that is a great way to create usage baselines and to troubleshoot switched environments. This tool allows you to view the switched network as well as trunked and switched ports. A screen capture of TrafficDirector appears in Figure 16.4.

Figure 16.4: A screen capture from TrafficDirector.

∙AtmDirector—Used in Asynchronous Transfer Mode (ATM) networks. This tool can be used to configure, administer, and troubleshoot ATM switched networks.

IOS Software Troubleshooting Commands

Some command−line interface commands are available to aid you in troubleshooting a switch. You can also use a few GUI applications to simplify some of the functions involved in maintaining and configuring the Cisco IOS. These applications are helpful and will be discussed later in this chapter.

Let’s look at some the commands that you can use to view the switch configuration and perform diagnostics in order to troubleshoot switch problems and configuration issues. We’ll describe the following commands; then, in the Immediate Solutions section of this chapter, I’ll give an example of the output you should see when using them:

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∙show cam

∙show cdp neighbors

∙show config

∙show flash

∙show interface

∙show log

∙show mac

∙show port

∙show spantree

∙show system

∙show test

∙show version

∙show vtp domain controller

show cam

The show cam command displays a switch’s transparent bridging table (also known as the Content Addressable Memory [CAM] table). This is a table of the Layer 2 MAC addresses attached to each port that the switch has learned in order to make forwarding decisions.

Duplicate MAC Addresses

Occasionally, because of production mistakes, network devices are configured with identical MAC addresses on their interfaces. There are also some dual−homed Unix workstations that use manually assigned MAC addresses. This is a situation that leaves the door wide open for a duplicate MAC address in the network. This in turn can prevent communication in the local network. When the devices with the same MAC address are on the same broadcast domain, duplicate MAC addresses can become a substantial problem in your network.

By using the show cam command, you can view the list of known MAC addresses for interfaces attached to each switch port. If you are in a network where the administration of network devices is divided within the organization, I recommend that a central process be created to review and document assigned MAC addresses and thereby avoid this duplication problem.

Note On the Cisco CLI−based IOS, use the show mac command.

show cdp neighbors

Cisco Discovery Protocol (CDP) is a Cisco proprietary protocol used to discover neighboring Cisco devices. The show cdp neighbors command displays the hardware, IOS version, and active interfaces. This information is passed between Cisco devices by CDP packets on physical media that support SNAP.

CDP packets are multicast packets that are advertised by the Cisco router or switches but not forwarded. This protocol is available on the Cisco IOS version 10.3 and later. You can use the show cdp neighbors command on both the CLI and Set/Clear command−based IOS.

show config

The show config command displays an incredible amount of troubleshooting information. With this command, you can obtain the entire configuration of switches and modules (except the Route Switch Module [RSM]) including passwords, system information, protocol settings, interface settings, and the system log settings.

Note On the CLI−based IOS, use the show running−config command.

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show flash

Cisco switches operate with software that is very similar to the Cisco IOS on routers. This software is stored and may be upgraded in flash stored on the Supervisor module. The show flash command reports the space required for the installed software and the version of the code, including the file names, software version numbers, and file sizes. Unfortunately, there is no comparable command on the 1900EN series switches.

show interface

You can use the show interface command to get the IP configuration, interface flags, interface state, VLAN information from the Supervisor Console 0 (SCO) interface, and broadcast address. This command can be used on both the Set/Clear and CLI−based IOS.

show log

Using the show log command, you can look at significant events, including reboots of all the modules, traps, logged events, boot history, nonvolatile RAM (NVRAM) logs, module logs, and power supply failures. This command cannot be used on the 1900EN series switches.

show mac

The output from the show mac command is quite long. This command displays numerous counters that are maintained during normal operation. These counters include information on the traffic for each port, number of incoming frames, number of frame discards, total number of frames sent, and maximum transmission unit (MTU) violations.

Note On the Cisco CLI−based IOS, use the show usage utilization command.

show port

Using the show port command, you can receive specific information about selected ports or all the ports on a specified module. This data includes the VLANs the port belongs to, port configuration information, port status, port speed, port duplex, port media type, security information, source MAC address of the last packet received, broadcast threshold, number of collisions, link error rate (LER), link error monitor (LEM), last report cleared time, and whether port trap is enabled.

The use of spanning trees is a necessity in today’s networks in order to provide for redundant links and at the same time avoid a data looping problem. The show spantree command can be used to display the STP configuration crucial to the successful running of a switched network. The output from this command can tell you whether STP is enabled or disabled, the bridge or port priorities, the root bridge priorities, the path cost to the root, BPDU information, the bridge MAC address, timer information, the port states, and information on the fast−start configuration of each port. This command is similar on the Set/Clear IOS and the CLI−based IOS.

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