Duplex Mismatch

I’ve had this happen several times, I connect a 3725 to a 3640 using slot 0 (f0/0) and bam, duplex mismatch… no matter how many times I configure “duplex full” on each interface, one of them goes back to half-duplex. 

 

I believe the problem stems from slot 0 on the 3725 defaulting to GT96100-FE which automatically has the f0/0 & f0/1 set to half-duplex.

The 3725 is set to half-duplex by default…



In this scenario (see diagram) I’m getting a duplex mismatch between R1 (3640) and R2

(3725). R1 is in full and R2… half.

Solution 1: Native Ostrich

Pretend it’s not happening:

R_n(config)# no cdp log mismatch duplex

 

 

 

Solution 2: Fix it!

Shut down the conflicting interfaces (on each router) and set the speed to 100, duplex full and then bring them back up (no shut):

 

Now you can run show int f0/0 and see that the interfaces are in full-duplex and problem solved!

Note: This fixes the problem; trying to do it while the interfaces are up or by using duplex auto or just changing the duplex to full usually does not work (well, it doesn’t work for me… one of the interfaces will go back to stupid mode and you’ll keep getting the duplex mismatch log.)

Gëzuar!

Update 10/16/2012:
Note: "A 10-Mbps Ethernet link (fixed speed) defaults to half duplex, whereas a 100-Mbps Fast Ethernet (dual speed 10/100) link defaults to full duplex. Multispeed links default to autonegotiate the duplex mode."
-CCNP Switch 642-813 Official Certification Guide, David Hucaby