Autonegotiation

If we configure Full Duplex & 100 Mbps on SW1 but Full Duplex & 1000 Mbps on it's Neighbor Router's port then communication won't happen because Standards won't match.

• Because, '#Speed 100' command forces the port to use 100Mbps Fast Ethernet settings which uses only 2 pairs of wires & 802.3u Standard whereas for '#Speed 1000' command which forces the port to use 1000Mbps Gigabit Ethernet Settings it uses all the 4 pairs of wires & 802.3ab Standard.

• Both the nodes tries to Autonegotiate but if either of the two nodes hears nothing then that node follow the

  IEEE Default Autonegotiation rule:

• "IF AUTONEGOTIATION FAILS SET 10Mbps SETTING & FOR 10/100Mbps SET HALF DUPLEX SETTINGS "

  • SPEED:

    Switch will sense the speed of it's Neighbor node before using IEEE Autonegotiation & set the same speed as the other node's speed.

    But if it can't sense the speed then it uses IEEE default Auto Negotiation Rule that says, "IF AUTONEGOTIATION FAILS SET 10Mbps SETTING & FOR 10/100Mbps SET HALF DUPLEX SETTINGS "

  • DUPLEX:

    10/100 - Half Duplex 1000 - Full Duplex (means Gig ports will always use Full Duplex)

  • Autonegotiation failure leads to Duplex Mismatch

Reference:

CCNA 200-301 OCG, Volume 1, Pg. 140-141, 160 - Wendell Odom.