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Ethernet Autonegotiation

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Autonegotiation: finding the best speed

When you plug an Ethernet cable in, both ends need to agree on speed (10/100/1000 Mbps) and duplex (half/full). Autonegotiation (IEEE 802.3u) does this automatically by exchanging capability advertisements before any data flows.

Before autonegotiation, mismatches were common and caused mysterious problems. Today's networks still suffer from partial autoneg failures when one side is hard-coded while the other negotiates.

Packets in motion — click to inspect:

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