Shooting Trouble with Switches

Shooting trouble with switches requires that you understand Physical and Data Link Layet targets and well as normal switch operations. A physical and logical map is not just somethinF nice to have but a necessity in real-world operations. It is not easy to create if you don't understand how thnngs work, in particular STP for Layer 2 devices. You must continue to follow a consistent methodology such as those suggested in the first part of the book to assist you in isolating faul S doma ins;.

It is probably not a bad idea to go back and review the Ethernet and switch beginning checklists and ending sections on shooting trouble. All of them allude to the fact that interfaces (ports) art the mam Data Lmk Layer target. It is up to you to use known-good switches, modules, ports, cables, con nect04s , and transceivers for connectivity and performance purposes. Rardware issues couHd be a bad or loose cable, a faulty module or port; and they may be intermittent, in which case electrostatic discharge (ESD) may have originally caused the problem. Always reseat connections and modules, before you call for help. If the Supervisor module is not in slot 1, for example, the system doesn't boot up. In general, disconnect and reconnect; try a different port; try a different known-good cable.

Link lights (LEDs) are good but not always a 100-percent test. An 80-percent to 100-percent switch load may indicate a broadcast storm. On the other line card modules, the LEDs should flash orange (amber) or green during startup, and turn green to indicate successful initialization. Aed i ndicates failufe (reseat the module), and flashing orange could be a problem on some modules, although an instance of redundancy on others. As far as the port link integrity, LED issues can be anything from the port, to the cable, to the network interface card (NIC), or thx negotiati on for speed/d uplvs. Utili ze your too le. Test witb a reliable cable an well as a t ime domain feflectometef/o ptical Time domain reflectometer (TDR/OTDR) to find cable length and impedance issues. Use protocol analyzers for protocol information; cable testers for cable issues; and network mo nitors -o continuousl- m ornto r natwork tra fbc. Th ere still could be a cable problem with lots of packet loss. On the other hand, things may work fine and the LED may just be burned rout.

Other types of connection issues include using fiber where negotiation is not an issue but connectivity is. A rommcn proWem here is to |r>lsg Tx to Tx an d Rx t o R^ but 0 you want things to work you need to con nect Tx to Rx .

Pay atten tion uot only to yond LED Hghts but also to your log s fot configuration issues. If you see a solid orange light, for instance, this may just indicate a shutdown port. A user or internal process cou 1 d nave s hu t °t down but might not have automatically brought it back up. Perhaps there are speed/duplex issues. The best practice is to hard code fixed devices so that there is no negotiation. Perha ps pTP has t he port in a blocking state benause ft w ould cause a loop.

Ahen thin gs ate worki ng normally, you want to make them optimal. Performance commands includeset port host, which is a macro that combines set spantree portfast, set port channel mode off, and set tronkof- Experiment w ith timin g ¡usues with and without portfast. Change tho logging level for the session to set logging level spantree7 and observe the time-stamped log messages to see how long the port stays in each state. You can accomplish this on an IOS box with the spanning-tree portfast interface command; the following global commands: service timestamps debug datetime localtime msec, and service timestamps log datetime localtime msec; and the following privileged exec command: debug spantree events. You can shut down a port and bring it back up to see a topology change and thx associated activity. Don't forget that turning on portfast for a port really doesn't change thx eopology; instead it ellows the switch to pot send a TCw when a port becomes active.

Traffic issues may lead to segmentation of some sort or to upgrading the devices themselves. Useshow port, show mac, and network management programs to monitor the average and peak utilization carefully.


Ifreset system, the reload command, or rebooting seems to clear the issue and it continues to ha irpen, perceptibly the reboot is more of a short-term fix than a permanen- solution.

Obviously, you may have a software or hardware bottleneck. Know the limitations of your transport a nd your devices. For example, you still have collisions if Gigabit pipes are feeding 10-Mbps shaned users. Use to assist with corrupted IOS issues; reload the operating system; and upgrade to the appropriate feature set. Again, all of this systematic troubleshooting relates back to the OSI model. Do you have power? Are the power supply and fans running? Are devices turned on? Do they have link lights? Work your way up the layers. (Refer to Table 1-2 in Chapter 1, "Shooting Trouble," for a review of the OSI layers.)

Once a gain it is time for tie chepter Trouble Tickets. The p lan hsre is to give you several things to do, let you make mistakes and fix some things on your own, and to introduce other problems that you should have some experience with as a support person. Routing and switching issues are unstated knowledge for the Cisco support person today.

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