These days, I just hard-code for job security.
Posted by qualityofservice on August 12, 2008
Ultimately, it’s a lot easier to keep one’s job if you can convince your manager that it takes highly skilled, motivated, and incredibly rare individuals to log into a switch and change a port default or two. I suspect this is also the reason why I read about cable runs taking nine-and-a-half weeks. The whole stack of cards comes tumbling down the minute one of us admits that it should really only take 2-3 minutes, given the proper cable length and a path with minimal obstruction between distribution frames.
But I digress: http://etherealmind.com/2008/07/15/ethernet-autonegotiation-works-why-how-standard-should-be-set/
Some interesting notes there. I’ve actually been a fan of set-and-forget over the years, but never could explain why. It just seemed to fix a lot of errors, and ultimately that was what was important. Having had the chance to work with quite a few makes and models over the last two years or so, I’m getting more comfortable with the idea of auto-negotiation.
After all, auto-neg IS a standard, and the Gigabit Ethernet specification is pretty clear on the matter of its importance. If it doesn’t work between two devices, it’s ultimately not a design problem (as I preferred to think of it), but rather a driver incompatibility problem (one of your vendors is making garbage NIC’s). These days, my thinking is more along the lines of “if you have to disable a standard to make it work, you’ve got bigger problems than a few collisions.”
You can indeed force Gigabit Ethernet on a Cisco switch to “speed 1000,” but you will be unable to set the duplex; it will default to full and stay there. You’d think this was a good thing until you see the GigE spec excerpted in the article, where it states that GigE uses auto-neg to detect which end of the link will provide clocking. : )
Ex:
lab-sw-3(config)#int gi0/24
lab-sw-3(config-if)#speed 1000
lab-sw-3(config-if)#duplex full ! accepts this command without complaint; it defaulted to “full” in the first place
lab-sw-3(config-if)#duplex half
Gigabit port is restricted to full duplex ! gets upset if you try to set it to “half”
lab-sw-3(config-if)#duplex auto
Gigabit port is restricted to full duplex ! equally upset if you try to set it to “auto”
lab-sw-3(config-if)#
Little food for thought!
