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New Member

Duplex mismatch problem

I have this interesting duplex mismatch problem and hope that someone can help me. I started to see these errros ever since I moved these hosts from CAt OS (4006) to Cat IOS (6500)

For example this error below says fa1/23 is NOT FULL DUPLEX but if you see the config below the port fas1/23 is hard coded to 100/full.

10:25:08.790: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet1/23 (not full duplex), with PCFE40PA FastEthernet0/1 (full duplex)

interface FastEthernet1/23


switchport access vlan 912

switchport mode access

no ip address

speed 100

duplex full

But here is the interesting part, when I do show int fas1/23, it says that is set to half-duplex and that's where I'm lost

FastEthernet1/23 is up, line protocol is up (connected)

Hardware is C6k 100Mb 802.3, address is 0005.5f28.c6ee (bia 0005.5f28.c6ee)

Description: xxxxxx

MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Half-duplex, 100Mb/s

I'm on Cisco 6506 with WS-SUP32P-GE runing 122-18.ZY2.bin. Line card in question is WS-X6348-RJ-45.


Re: Duplex mismatch problem

The server (or whatever device) is probably set to auto-negotiate and is setting the NIC to 100/half. This is very common on HP/Broadcom. Hard set the server(s) and you should b good to go.

Hope that helps.

New Member

Re: Duplex mismatch problem

What happens if you set both sides to auto/auto ? are you able to negotiate 100/full?

New Member

Re: Duplex mismatch problem

Other side is a Cisco 2621XM which is also set to 100/full. There is no command to change the port duplex to auto, isn't strange? Please see below

exe-6506-24(config)#int fa1/23


Cisco-6506(config-if)#duplex ?

full Force full duplex operation

half Force half-duplex operation


Re: Duplex mismatch problem

I *think* if you set the speed to auto, it automagically set the duplex to auto.