Interoperability Problems
The first version of the autonegotiation specification, IEEE 802.3u, was open to different interpretations. Although most manufacturers implemented this standard in one way, some others, including network giant Cisco, implemented it in a different way. Autonegotiation between devices that implemented it differently failed. This led many network administrators to not depend on autonegotiation and instead manually set the speed and duplex mode of each network interface card. Even Cisco recommended its customers not to use autonegotiation. However, the use of manually set configuration often led to duplex mismatches, in particular when two connected devices are:
- One manually set to half duplex and one manually set to full duplex
- One set to autonegotiation and one manually set to full duplex
- Both sides manually set to full duplex where one side still expects an autonegotiating link partner and the other side has autonegotiation completely disabled (the side that expects an autonegotiating link partner will fall back to half duplex because it does not detect a partner capable of full duplex)
Duplex mismatch problems are difficult to diagnose because the network is apparently working, and simple programs used for network tests such as ping report a valid connection; however, the network is much slower than expected.
The debatable portions of the autonegotiation specifications were eliminated by the 1998 release of 802.3. This was later followed by the release of IEEE 802.3ab in 1999. The new standard specified that gigabit Ethernet over copper wiring requires autonegotiation. Currently, all network equipment manufacturers—including Cisco—recommend to use autonegotiation on all access ports. Cisco also recommends that you check back with them yearly for any potential changes in their recommendation as this has caused much confusion over the years.
In some large installations that have had to deal with negotiation issues, network staff may believe that "autonegotiation doesn't work", and consider turning it off a best-practice. This should be avoided - once autonegotiation is turned off, it will not work by definition, creating a self-enforcing problem.
Read more about this topic: Autonegotiation
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