Funny, that is, but the "YMMV" here is the most proper way to point this. I've had experience with so many different problems between devices that I try first to leave it as is (autoneg) but on the first time one of the devices causes a problem I hardcode it. I've seen problems when connecting customers back end with a Telindus 1030 router to a cisco 1841 router where in some cases the auto negotiation went fine, and sometimes it didn't, and some cases hardcoded caused CRC errors, so I had to evaluate every single case, and we're talking about the same both make and models on similar environments, so what would you suggest me to do? What I just did was to see if the auto worked, and then tried the manual, but IF the manual caused problems then I had to make the guys at the E1 circuit carrier make some adaptations in the shaping of the line. Every time I had to connect a new customer all we had to do is plug and pray... See, sometimes is not only up to IEEE 802.3 stuff, but also on how every device handles the stuff.
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of John Neiberger Sent: Friday, August 20, 2010 4:46 PM To: [email protected] Subject: [c-nsp] Why hard-setting speed and duplex on Fast Ethernet is bad Someone in the other thread mentioned that they were surprised to see that so many people were against manually configuring speed and duplex, so I thought I would explain why this is a bad idea most of the time. The Fast Ethernet standard does not mention how devices are supposed to behave when manually configured. It only deals with Nway autonegotiation. The problem is that there are two possible behaviors when settings are manually configured: #1: Participate in Nway, but only offer the configured settings #2: Disable Nway entirely and run at the configured settings Cisco's older switches, like the XL series, used behavior #1, as do most PC/Server NICs that I've run across for that past eight years or so. Beginning around the time the 2950s came out, Cisco decided to switch to behavior #2. If you connect two devices that use behavior #1, you'll be fine. If you connect two devices that use behavior #2, you'll be fine. But what happens if you connect a "#1" device to a "#2" device? You get a duplex mismatch! The device that still participates in Nway is going to expect to see an autonegotiating link partner. When it doesn't detect one, it follows the standard and assumes it is connected to a hub or some device that can't do full duplex and it falls back to half duplex, often without telling you. This creates input errors on the full duplex side and late collisions on the half duplex side. If you have hard-set your speed and duplex on a Cisco switch and you're seeing a lot of input errors, you likely have a duplex mismatch because of this problem. Setting BOTH sides to auto usually resolves this issue. Auto is the only reliable way to go these days unless you know ahead of time which of the two behaviors your devices are choosing. If you don't know, autonegotiation is the most likely way to get a good connection at 100/full. I hope that helps clear some of that up. -John _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/ ************************************************************************************ This footnote confirms that this email message has been scanned by PineApp Mail-SeCure for the presence of malicious code, vandals & computer viruses. ************************************************************************************ ************************************************************************************ This footnote confirms that this email message has been scanned by PineApp Mail-SeCure for the presence of malicious code, vandals & computer viruses. ************************************************************************************ _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
