Duplex mismatches affecting physical link speed
When interconnecting two Ethernet peers, we recommended using auto-negotiation, which allows Ethernet peers to exchange link speed and duplex information.
Explanation:
Ethernet peers use electrical signals to determine when a link is established. These electrical pulses make up a 16-bit word defining the speed and duplex for auto-negotiation. If the speed and duplex of one peer must be manually set, it is important to set the other peer with the same settings.For example:
If Peer1 = 100 Full duplex, then Peer2 = 100 Full duplexor
If Peer1 = Auto-negotiation, then Peer2 = Auto-negotiation
Failing to set interconnected Ethernet peers to the the same duplex settings will create a duplex mismatch. Duplex mismatches result in both peers using the same wire pair to transmit data. Collisions will occur when peers transmit electrical signals at the same time. Collisions cause packet loss, frame corruption, and reduced performance and speed of the link. Here are some examples of duplex configuration mismatches and the result:
Peer1 = 100 Full duplex
Peer2 = Auto-negotiation
Peer2 will be able to detect the 100Mbps speed but defaults to half duplex which will cause packet loss due to collisions.
Peer1 = 100 Full duplex
Peer2 = 100 Half duplex
Peer2 uses half duplex so collisions will occur.
Note: The Gigabit Ethernet standard requires auto-negotiation for both sides of a link. Gigabit peers should not be hardcoded to a particular speed or duplex.
For more information about duplex mismatch see:
http://en.wikipedia.org/wiki/Autonegotiation
http://en.wikipedia.org/wiki/Duplex_mismatch
For more information on how to set your Meraki APs to auto-negotiation see: