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Community Member

duplex mismatch

Hi all,

I have an 2960 switch connected to 3Com Hub, I don't know why the switch continuous to prompt

me duplex mismatch

Switch#sh run int FastEthernet0/5

speed 100

duplex half

Switch#sh int fastEthernet 0/5

FastEthernet0/5 is up, line protocol is up (connected)

Hardware is Fast Ethernet, address is 001a.6d95.4505 (bia 001a.6d95.4505)

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, media type is 10/100BaseTX

input flow-control is off, output flow-control is unsupported

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 20000 bits/sec, 16 packets/sec

5 minute output rate 1000 bits/sec, 3 packets/sec

103093 packets input, 14393135 bytes, 0 no buffer

Received 93264 broadcasts (0 multicast)

0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

0 watchdog, 57009 multicast, 0 pause input

0 input packets with dribble condition detected

19782 packets output, 1315325 bytes, 0 underruns

0 output errors, 82 collisions, 1 interface resets

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier, 0 PAUSE output

Switch#sh logging

01:56:28: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet0/5 (not full duplex), with 069070588 6/18 (

full duplex).

01:57:28: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet0/5 (not full duplex), with 069070588 6/18 (

full duplex).

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Blue

Re: duplex mismatch

Jack

It depends on what the 3com port was set to. By the sounds of it it was set to 100 full. So when you tried 100 half duplex you got a mismatch.

Setting it to auto also didn't work because the other end wasn't set to auto.

Jon

Re: duplex mismatch

The message that you see comes from the Cisco Discovery Protocol. CDP contains info about speed/duplex settings of the sending interface. The basic issue is that the 3Com does not understand CDP. However, CDP packets from other IOS devices on the 3Com switch are forwarded (CDP uses multicast) and the receiving switch cannot determine wheter the message comes from a directly attached device or not.

So, in fact this message is probably false.

Normally I turn off cdp on the interfaces that are connected to non-Cisco switches. You should of course verify that the actual speed/duplex settings are correct.

regards,

Leo

9 REPLIES
Community Member

Re: duplex mismatch

As u can not change at 3com hub, change at your 2950 as

duplex auto

or

duplex full

Regards

Re: duplex mismatch

Hi

As mentioend by mahesh try both duplex/speed as auto and check out for the negotiated parameters.

Once you have the negotiated parameters you can manually configured the same under the interface.

regds

Community Member

Re: duplex mismatch

Hi all,

I have set int fas 0/5 to (speed auto, duplex auto ), but switch continue to prompt the duplex mismatch

03:49:28: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet0/5 (not full duplex), with 069070588 6/18 (

full duplex).

03:49:30: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to up

03:50:28: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet0/5 (not full duplex), with 069070588 6/18 (

full duplex).

Hall of Fame Super Blue

Re: duplex mismatch

Hi

Hardcode your fa0/5 interface on the switch to be 100full Duplex.

Jon

Community Member

Re: duplex mismatch

Done, no more prompt on mismatch, but what is the reason ? why the switch auto learn will has such problem ?

Hall of Fame Super Blue

Re: duplex mismatch

Jack

It depends on what the 3com port was set to. By the sounds of it it was set to 100 full. So when you tried 100 half duplex you got a mismatch.

Setting it to auto also didn't work because the other end wasn't set to auto.

Jon

Re: duplex mismatch

Sorry, I disagree. Is the 3COM a switch or a hub?

If the 3COM is a hub as you say, then everything MUST be configured to auto or half-duplex. (In the absence of negotiation, anything configured in auto will revert to half-duplex.) In this case, the reason you are seeing the duplex mismatch is that something on the hub is hard-coded as full-duplex, and is announcing the fact on CDP. The thing to bear in mind is that hubs can ONLY do half-duplex.

If the 3COM is a switch, then the situation is different as each port is a seperate collision domain. In this case you can quite easily have one port doing half-duplex while another does full-duplex. In this case, CDP is misleading because the 3COM will pass it transparently instead of intercepting it as a Cisco switch would do. In this case you can switch off CDP to avoid the spurious messages. Once again, leave your hosts in auto - hard coding them as full-duplex would cause a genuine duplex mismatch.

Kevin Dorrell

Luxembourg

Hall of Fame Super Blue

Re: duplex mismatch

Kevin

Yes you are right to disagree and both you and Leo are correct in your interpretation. I made the assumption that the 3COM was a switch but it still doesn't explain the CDP message.

Thanks for correcting me. Both you and Leo have been rated.

Jon

Re: duplex mismatch

The message that you see comes from the Cisco Discovery Protocol. CDP contains info about speed/duplex settings of the sending interface. The basic issue is that the 3Com does not understand CDP. However, CDP packets from other IOS devices on the 3Com switch are forwarded (CDP uses multicast) and the receiving switch cannot determine wheter the message comes from a directly attached device or not.

So, in fact this message is probably false.

Normally I turn off cdp on the interfaces that are connected to non-Cisco switches. You should of course verify that the actual speed/duplex settings are correct.

regards,

Leo

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