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Auto Auto or Full 100?

A debate is raging in the company that I work for that all of our Cisco switch ports should be set to auto auto. Until recently, I thought the standard recommended by Cisco was to set the ports to 100/Full. What is the current wisdom reguarding this? Thank you.


Re: Auto Auto or Full 100?

Auto/Auto is the lazy way to let the network do it's thing. Actually, it works pretty well. There is that pesky negotiation while the ports/nics are trying to feel up each other.

Most folks I argue with insist that manually setting devices to 100/full is the safest way to avoid an accidental duplex mismatch. If you do, you MUST be sure the nic's are manually set to 100/full also - or you WILL have a duplex mismatch (at least in the Microsoft OS world).

Since Auto/Auto is the default for all the client PC's on the network, I hold my nose and let the devices do their negotiation dance. If you do get a mismatch, the client will still work, just slowly due to collisions. The infamous "have you rebooted?" help desk reply usually fixes the problem by initiating a new autonegotiation.

If I ever get the time, I will set all my boxes for 100/full. However, I don't expect that to ever happen.

Since all my servers are Gig Ethernet, negotiation is not an option. That is, Gig Ethernet only supports 1000/full.

There are best practices, and then there is reality. If auto/auto caused enough problems, it would be worth fixing. It just works too well to bubble to the top 100 things to fix though.


Re: Auto Auto or Full 100?

Auto/auto should be fine for 99% of the connections. Use it on ports where desktop and laptop computers are going to be plugged in.

Periodically check the ports for the actual speed and duplex that was negotiated: if you see anything that's 100 Half, track it down to see why it didn't go to Full. If it's not a hub, could be a driver update is needed, because most everything these days that can do 100 can also do Full Duplex.

If you see anything that's Half Duplex, track it down and figure out why it didn't go to Full. (Some 10BASE-T network interfaces for printers just won't do Full Duplex.)

If you see anything that's 10 instead of 100, track it down and find out why.

Server-to-switch connections should be set manually, in order to be sure that they are always operating at top speed. As the previous post said, be sure to set 100 Full at both the switch and the device. Also, tag or label the patch cable at the switch port end, to identify what device it goes to and which port it must be plugged into on the switch. Because it won't work correctly if someone moves it to an auto/auto port on the switch.

Switch-to-switch connections, even though they will probably autoselect just fine, should be manually set to top speed/duplex, and tagged/labeled as mentioned above.

Router-to-switch connections, double-check that they are capable of Full Duplex, then set them manually too if they can handle it.

Hope this helps.

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