I don't have enough time at the moment to explain this as well as I should. Do a quick search on autonegotiation in the archives and I have some recent postings that explain it better. here's the short version:
The only connection method specific in the FastEthernet standard is AUTO. If you do anything other than AUTO you are out of spec and the behavior is not consistent. When you manually set your speed and duplex settings some devices disable autonegotiation (NWAY) entirely. Others still participate in NWAY but they only offer the configured settings. The problems occur when you connect two devices that use different methods. If you do, you're guaranteed to get a duplex mismatch. The side that completely disables NWAY will stick to your manual settings, 100/Full, for example. The other side, if it still participates in NWAY, will still expect an NWAY-capable device to be attached. Since it doesn't detect NWAY, it falls back to half duplex, and there's your mismatch. Cisco's newer switches--like the 6500s and 2950s--disable NWAY entirely if you manually set the speed and duplex. About 98% of the NICs in our environment use the other method, which almost guarantees failure if we don't use AUTO. If you're going to manually set your settings with newer switches, 100/Full is the absolute worst possible setting. If you want high speed with manual settings the best setting is 100/Half. That way, if connect two devices that behave differently, you'll still be okay when the NWAY-capable NIC falls back to half duplex. Cisco's older switches, like the 2900XL series, still participated in NWAY even if you used manual settings. So, if you have a 2924XL with manual settings that needs to be replaced and you replace it with a 2950-24 with the identical config, I wish you luck for you are about to learn all of this the same way I did. I used to be a radical anti-auto person until I got our 6513, 2948Gs, 2980Gs, and a bunch of 2950s. I've since changed my mind and I'm now a very pro-AUTO person. The real killer here is that most NICs will continue to report their manual settings regardless of their operational settings. If you manually set a PC NIC to 100/Full, many times it will continue to report full duplex even if it has fallen back to using half duplex. Perhaps later today I'll have more time and I can get into some more details. Regards, John >>> Troy Leliard 5/28/03 4:52:30 AM >>> I have seen this too, and like Ian I would normally go with 100/Full manually configured on botht he Cat and the end device (obviously assuming both devices support this settings). In real life, I have often found that setting the cat to Auto will often lead to duplex / speed mismatches (especially with Sun kit) The only time I have made use of Auto is when I am not 100% sure if the end device support 100MB, some of our legacy printers are 10MB half duplex, and indeed a number of the 2511's are only 10MB too.] ian williams wrote: > > This has come up in the ccie written. > If I understand this subject correctly AUTO , sends out packets > to try and > match the 2 devices up with regards to speed and duplex. > If your getting connection problems this would be a speed > issue. If its some > sort of packet loss/error then this could be a duplex problem. > I have always configured the CAT port manually so there isnt > any problems. > > Why would you choice AUTO? > > > > ----- Original Message ----- > From: "John Neiberger" > To: > Sent: Tuesday, May 27, 2003 5:35 PM > Subject: Re: port/duplex configs [7:69582] > > > > >>>> ian williams 5/27/03 10:29:21 AM >>> > > >I have always configured ports on CAT switch to 100/full > manually instead > > of > > >AUTO. > > >What is recommended when asked this question for the CCIE > written. Should > > >both the end > > >device ( NIC ) and switch both be configured to 100/FULL? > > > > I can't imagine why such a question would be asked on any > exam since the > > correct answer is that you configure whatever is necessary to > establish a > > connection with the end device. In my opinion, you should > always use AUTO > > unless this causes problems, in which case you hard-set your > devices to > > 100/HALF, not 100/FULL. If you'd like the rationale for that > I refer you > to > > the archives for my previous rantings on this subject. > > > > I'd fall over in shock if you were to be asked a question > like this on > your > > exam, but as long as you understand the issues involved you > should be > > adequately prepared for whatever question of this type that > they throw at > > you. > > > > Regards, > > John Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=69690&t=69582 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
