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Duplex mismatches affecting physical link speed

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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 duplex

or

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:

Duplex negotiation on Meraki APs

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Last modified
21:29, 2 Feb 2015

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ID: 1650

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