Thursday, April 21, 2005

Duplex is important to keep an eye on.

Mismatch duplex settings can cause all sorts of headaches. I just had to clean some up this morning between a router and a switch. Here is a cheat sheet on which settings work or not.

Auto DetectHalf DuplexFull Duplex
Auto DetectOKOK - not optimalNO - see note
Half Duplex

OK - not optimal



Full Duplex

NO - see note



Note: A forced duplex setting such as half or full will stop the port from sending Fast Link Pulse (FLP) or the 802.3u auto-negotiation protocol. FLP notifies the other end the auto-negotiation options of the source sending the FLP. If a port is set to auto and it does not receive FLP, the default behavior is for it to assume the other end of the connection is set to half duplex (last priority or lowest common denominator). You can now see why full/auto won't work to well. Oh, and obviously if the speed of the interfaces don't match things aren't going to work to well either. Here is the table of priority order that 802.3u uses, in case you are interested.

Auto-negotiation Process:
Priority Physical-Layer
1 100BaseT2 full duplex

2 100BaseT2 half duplex

3 100BaseTX full duplex
4 100BaseT4 half duplex*
5 100BaseTX half duplex
6 10BaseT full duplex
7 10BaseT half duplex

Note: * 100BaseT4 supports only half duplex.

- Ed

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