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Community Member

Ethernet duplex mismatch


1601R connected with an ethernet crossover cable to a 2612.

Both devices have ethernet (10Mbit) ports.

I've set the ports up manually on both sides using the command "duplex full".

The speed command is not available as (I guess) these ports work only at 10Mb.

The communication works fine, including extensive pings; however every N seconds I get on screen the error:

"02:07:55: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on Ethernet0 (not full duplex), with 2612-B Ethernet0/0 (full duplex)."

On the 1601 I 've checked the duplex setting (sh int eth 0) and it is successfully set to full-duples.

The speed here is set to "unknown" for some reason.

On the 2612 I've verificated the duplexing setting with a "sh contr eth 0/0" and I can confirm it is set to full.

If I leave the auto settings (removing the dupleging with "no duplex full" no erro messages appear on screen but the communication happens in half-duplex.

Any help/advice?


Community Member

Re: Ethernet duplex mismatch

Sounds like you're running into a bug. As a workaround you can disable CDP v2 on the 1601.

no cdp advertise-v2

What code are you running on the 1600?

Hall of Fame Super Gold

Re: Ethernet duplex mismatch


The error message that you are seeing is generated by CDP. In the newer version of CDP when it is sent out Ethernet interfaces the CDP packet indicates the duplex setting. So I am guessing that you probably see the error message every 60 seconds when the 1601 receives a CDP packet from the 2612. Probably the CDP code on the 1601 is assuming half duplex operation since half duplex is pretty standard on 10M Ethernet.

If the show interface indicates that the 1601 is operating in full duplex, and if there is no collisions detected (and especially if there are no late collisions - since late collisions are a typical sign of duplex mismatch) then the routers are working ok and the error message is cosmetic. You can leave the settings at full duplex, get somewhat better performance, and endure the cosmetic error message, or you can put the interfaces back to half duplex, get no error message, and get 10M half duplex performance.



Community Member

Re: Ethernet duplex mismatch

Here is the output from the show version on the 1601R

1601R#sh ver

Cisco Internetwork Operating System Software

IOS (tm) 1600 Software (C1600-SY-M), Version 12.3(6), RELEASE SOFTWARE (fc3)

Copyright (c) 1986-2004 by cisco Systems, Inc.

Compiled Wed 11-Feb-04 16:43 by kellythw

Image text-base: 0x02005000, data-base: 0x02C2F6B8

ROM: System Bootstrap, Version 12.0(3)T, RELEASE SOFTWARE (fc1)

ROM: 1600 Software (C1600-RBOOT-R), Version 12.0(3)T, RELEASE SOFTWARE (fc1)

1601R uptime is 2 hours, 40 minutes

System returned to ROM by reload

System image file is "flash:c1600-sy-mz.123-6.bin"

cisco 1601 (68360) processor (revision C) with 19711K/4865K bytes of memory.

Processor board ID 14433408, with hardware revision 00000001

Bridging software.

X.25 software, Version 3.0.0.

Basic Rate ISDN software, Version 1.1.

1 Ethernet/IEEE 802.3 interface(s)

1 Serial(sync/async) network interface(s)

1 ISDN Basic Rate interface(s)

System/IO memory with parity disabled

8192K bytes of DRAM onboard 16384K bytes of DRAM on SIMM

System running from RAM

7K bytes of non-volatile configuration memory.

8192K bytes of processor board PCMCIA flash (Read/Write)

Configuration register is 0x2102

Rick, I unserstand your point and I think we are on the good way to the solution now.

I confirm the the error on screan appears every 60 seconds.

What about sort the speed problem on the 1601R.

Setting up the speed as 10 rather than unknown would it sort change anything? Still, I don;t have the "speed" command available in interface mode.

I'm just wondering if this 2 things are correlated

Hall of Fame Super Gold

Re: Ethernet duplex mismatch


I do not have much experience with the 1601 so can not address this from direct experience. But I believe that the 1601 has a simple Ethernet interface which is only capable of 10M speed. It is normal for 10M interfaces to operate in half duplex mode (and many of the 10M interfaces do not allow the duplex to be changed). It seems that your interface on the 1601 does allow the duplex to be set to full.

It seems to me that there are two things at work here: 1) what is the interface really doing; and 2) what does CDP think that the interface is doing. If the interfaces are operating in full duplex on both sides there will not be collisions on either interface. If one interface is operating in full duplex and one interface is operating in half duplex there should be collisions reported on the half duplex interface (and also late collisions should be reported on the half duplex interface).

So let the routers run for a while and pass a good amount of traffic over the interfaces. Then check both interfaces for any collisions. If there are no collisions then full duplex is operating successfully and the message is cosmetic.

I suspect that the show interface on the 1601 showing speed unknown reflects the fact that it is not operating with its default settings. So long as the routers do communicate successfully over the Ethernet then the speed is correct.



Community Member

Re: Ethernet duplex mismatch

Since you're running relatively new code you can use the new feature implemented to workaround the issue.

Under the 2600's interface connected to the 1600, put: 'no cdp log mismatch duplex'

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