Ways to create/simulate interface errors?

Unanswered Question
May 21st, 2007

I am creating an in-house training session on using the IOS command "show interface". As part of the material, I would like to be able to actually show some error conditions when bringing up a show interface. This has proven harder than I first thought, after a few obvious error conditions such as duplex mismatch, I am somewhat at a standstill as to creating an interface error condition that I can simulate repeatedly. I am looking for ways to increase interface port counters and I need to be able to do this several times for multiple sessions. Any "tips" or tricks on how to simulate interface error conditions would be appreciated.

I have this problem too.
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scottmac Mon, 05/21/2007 - 20:01

A fairly common mistake in do-it-yourself cabling (which produces some interesting, fairly predictable errors) is a "split pair on pins 3&6"

Many folks (including some "old-timer" networking people) make a cable based on "if it looks the same on both ends, it'll work fine" .... which is untrue.

Consider a cable with the four pair in any color order (but matching on both ends)

like:

Pins--1&2---3&4---5&6---7&8

Ethernet (10BASE-T and 100BASE-Tx) uses pins 1&2, 3&6 ... if you notice, with PPPP, pin 3 is in one color pair, pin 6 is in another (a "split pair") ... the effect is that the cable is no longer acting as "twisted pair" and the crosstalk spec goes out the window.

If you attempt full duplex, the split pair gets and gives a lot of interference with the other pair, and errors will greatly reduce the throughput of the cable (and should increment your counters).

The standards use the fllowing color pair order (EIA/TIA568a):

(pin1)Wht-Green, Green, Wht-Orange, Blue, Wht-Blue, Orange, Wht-Brown, (pin8)Brown

EIA/TIA 568b (swap the orange and green pairs):

(pin1)Wht-Orange, Orange, Wht-Green, Blue, Wht-Blue, Green, Wht-Brown, (pin8)Brown

Note that now the pair is formed by 3&6, the cable (assuming the rest of the spec is followed) should perform properly.

For switch connections, the hallmark symptom of a split-pair is that the interface will operate slowly on full duplex ... typically the complaint is that it "doesn't work at 100, but works fine at 10Meg" (100 is full duplex by default, 10Meg is half duplex by default) or that the switch always falls back to 10meg and shuts down the port (err disable) when forced to 100 and / or full duplex.

So .. .make youself a crappy cable, split either or both pair and try to run it at full duplex.

While you're at it, crush, twist, kink, stretch and otherwise abuse a variety of cables. According to research sponsored ny Anixter, 70% of all network errors are cabling related.

Cabling is one of the most important network components, and also the most overlooked for proper design and in troubleshooting. It is the foundation of the network ... you can put the most expensive active components on the marlet in your net ... if the cabling infrastructre is not correct and up to spec, it'll perform like crap (if at all).

Always a good place to start education: Layer 1 and go up from there.

Good Luck

Scott

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