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Network Warrior, 2nd Edition by Gary A. Donahue

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How Autonegotiation Works

First, let’s cover what autonegotiation does not do: when autonegotiation is enabled on a port, it does not automatically determine the configuration of the port on the other side of the Ethernet cable and then match it. This is a common misconception that often leads to problems.

Autonegotiation is a protocol and, as with any protocol, it only works if it’s running on both sides of the link. In other words, if one side of a link is running autonegotiation and the other side of the link is not, autonegotiation cannot determine the speed and duplex configuration of the other side. If autonegotiation is running on the other side of the link, the two devices decide together on the best speed and duplex mode. Each interface advertises the speeds and duplex modes at which it can operate, and the best match is selected (higher speeds and full duplex are preferred).

The confusion exists primarily because autonegotiation always seems to work. This is because of a feature called parallel detection, which kicks in when the autonegotiation process fails to find autonegotiation running on the other end of the link. Parallel detection works by sending the signal being received to the local 10Base-T, 100Base-TX, and 100Base-T4 drivers. If any one of these drivers detects the signal, the interface is set to that speed.

Parallel detection determines only the link speed, not the supported duplex modes. This is an important consideration because the common modes of Ethernet ...

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