Amy Arnold

Network Computing Blogger


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Thursday, July 25, 2013
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Troubleshooting Wires in Wireless Networks

When troubleshooting wireless networks, it's good to remember just how many actual wires are involved, especially when dealing with wireless network bridges. Here are two examples from the kingdom of WLAN that required this stalwart engineer to do battle in the physical realm.

Quest one: Reports have come in that evil forces are at work at the edge of your kingdom--communications to the Dark Forest have been completely cut off. The queen, more commonly known as upper management, has tasked you to identify the source of the disturbance and restore the networking devices to working condition.

You promptly log into the root bridge and see no sign of the alleged commotion; all interfaces are in an up/up state. Feeling smug, you log into the bridge on the other side of the connection. This is where a troll leaps out, beats you and steals your sack lunch. Or something like that. What you find is this:

Sample 1
(click image for larger view)

Notice that while the BVI and the radios are up/up, FastEthernet0 and its subinterfaces are taunting you in an up/down state. In the wired world, an up/down state typically indicates a successful physical connection but something astray in the protocol configuration.

In this particular case, however, your "show run" looks pristine, and communications to the Dark Forest have been working for quite some time. So you hop on your flying unicorn (in this case a Saturn L300) and leave the castle to get a closer look.

You find this:

Nibbled Wire

Clearly, goblins have been munching on your bridge's cables. You replace the damaged lines with the enchanted Goblins-Be-Gone brand. As a reward for restoring communications, the elves in the Dark Forest give you a stick-of-whacking that adds +5 to your troubleshooting intelligence but a -5 to your disposition.

Feeling confident in your recent victory, you take on quest two. Damsels in distress beckon to you from the tall tower across the forest, but you can barely make out what they are saying due to the jitter of the connection.

[Wires are also a factor to consider in upgrading to 802.11ac. Find out what you need to know in The 802.11ac Paradox ]

You are, however, able to log into the root bridge and see that all interfaces are in an up/up state. You run the magical incantation "dot11 dot11radio 0 linktest" and upon viewing the results, you suspect that an evil witch has cast a dastardly spell on your RF waves.

Interpreting numbers for RSSI and SNR often feels like black magic in itself. However, because you baseline your network like the awesome engineer you are, you happen to know that a good link test on your Cisco 1300-series bridge should look something like this:

Sample Bridge
(click image for larger view)

You’re confident that you are not in the happy signal zone with a retry rate of over 200%. Your connection appears to be fluctuating between POOR and HOPELESS DESPAIR, so once again you take a long ride out into the darkness (carefully tracking your mileage for reimbursement purposes, of course).

This time you are greeted with:

Downed Antenna

You conclude a hungry ogre tried to use your precious bridge antenna as a dinner fork. With no fear of life or limb (but of course following all OSHA regulations for a safe workplace), you bravely scale the 15-ft tower and install a suitable replacement antenna. The villagers rejoice and give you a basket of fruit and several large chickens. A gift card would’ve been nice, and easier to transport, but you can’t be too picky.

And that brings an end to our wireless quests for today—and likely my writing career. The next time you troubleshoot a WLAN, remember wires matter more, not less.

Do you have a wire-related troubleshooting tale to share, or perhaps some tips or best practices? If so, the comment box awaits you.

Amy Arnold, CCNP/DP/Voice, currently works as an engineer in the public sector with a focus on all things networking. You can follow her on Twitter at @amyengineer.


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