DUPLEX_MISMATCH - both ends are Full

Unanswered Question

2611 Ethernet port connecting a 3650 switch:

duplex mismatch discovered on Ethernet0/0 (not full duplex), with 3560G GigabitEthernet0/5 (full duplex)

now this is the E0/0 interface config:

interface Ethernet0/0

description LAN on VLAN42

ip address



why do I get this error if E0/0 is configured to full?

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
glen.grant Fri, 06/06/2008 - 07:32
User Badges:
  • Purple, 4500 points or more

Some of the early 2600 series even though the full-duplex command was there it did not in fact actually make the connection full duplex so it could really be running half duplex . We used to see that when we ran those early 2600's .

Edison Ortiz Fri, 06/06/2008 - 07:33
User Badges:
  • Super Bronze, 10000 points or more
  • Hall of Fame,

    Founding Member

Post the switchport config portion from the 3560G switch.



Edison Ortiz Fri, 06/06/2008 - 07:39
User Badges:
  • Super Bronze, 10000 points or more
  • Hall of Fame,

    Founding Member

Looks good. It sounds like Glen is on target. Personally, I haven't seen that problem before but I trust Glen :)

Try upgrading the IOS in the 2600 router and see if that helps.



this is the version details:

IOS (tm) C2600 Software (C2600-I-M), Version 12.1(5)T7

yes, it is old...

now there are 2 questions:

1. does it show not-full on E0/0 but work as full? because I care for performance not for the messages

2. if I upgrade, which version should I use (or what is the earliest version that fix this problem)?



Edison Ortiz Fri, 06/06/2008 - 07:59
User Badges:
  • Super Bronze, 10000 points or more
  • Hall of Fame,

    Founding Member

1) It is using half-duplex, not full-duplex. It could be due to the old IOS version.

2) Please post show version in order to make the proper recommendation.

BranchOffices#sh ver

Cisco Internetwork Operating System Software

IOS (tm) C2600 Software (C2600-I-M), Version 12.1(5)T7, RELEASE SOFTWARE (fc1)

TAC Support: http://www.cisco.com/cgi-bin/ibld/view.pl?i=support

Copyright (c) 1986-2001 by cisco Systems, Inc.

Compiled Mon 16-Apr-01 20:38 by ccai

Image text-base: 0x80008088, data-base: 0x809719E0

ROM: System Bootstrap, Version 11.3(2)XA4, RELEASE SOFTWARE (fc1)

ROM: C2600 Software (C2600-I-M), Version 12.1(5)T7, RELEASE SOFTWARE (fc1)

BranchOffices uptime is 35 weeks, 3 days, 12 hours, 32 minutes

System returned to ROM by power-on

System restarted at 22:59:23 EST Mon Oct 1 2007

System image file is "flash:c2600-i-mz.121-5.T7.bin"

cisco 2611 (MPC860) processor (revision 0x203) with 27648K/5120K bytes of memory

Edison Ortiz Fri, 06/06/2008 - 08:08
User Badges:
  • Super Bronze, 10000 points or more
  • Hall of Fame,

    Founding Member

c2600-i-mz.123-26.bin should meet your needs.



Please rate helpful posts

Sushil Kumar Katre Fri, 06/06/2008 - 12:02
User Badges:
  • Gold, 750 points or more


Found a similar issue -


3640 running 12.1(5)T12 code accepts full duplex command for ethernet interface but it doesn't work. Devices connected to the port show 10Mbps. Half Duplex.

12.1(5)T7 is also affected.

I also remebber something like this happening long time back. Can't recall which router it was but there was some etherne interface whcih doesn't support full duplex.

Can you paste the show interface output just to check the hardware.

-> Sushil

BranchOffices#sh int e0/0

Ethernet0/0 is up, line protocol is up

Hardware is AmdP2, address is 0005.3286.7ca0 (bia 0005.3286.7ca0)

Description: LAN on VLAN42

Internet address is

MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,

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

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

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 4w4d

Queueing strategy: fifo

Output queue 0/40, 0 drops; input queue 0/75, 5 drops

5 minute input rate 2000 bits/sec, 4 packets/sec

5 minute output rate 2000 bits/sec, 3 packets/sec

6578472 packets input, 1539704903 bytes, 0 no buffer

Received 1069818 broadcasts, 0 runts, 0 giants, 0 throttles

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

0 input packets with dribble condition detected

5466012 packets output, 657579877 bytes, 0 underruns(0/0/0)

1 output errors, 0 collisions, 0 interface resets

0 babbles, 0 late collision, 0 deferred

1 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

alex.spitzer Mon, 06/09/2008 - 22:24
User Badges:

I am having a similar issue between a 3640 router and a 3560G switch. The only way I can get the link to come up is to code the port on the 3640 as half-duplex. I have two ethernet ports on the router and I am going to try switching ports to see if it is a hardware issue. I would think that the best thing to do would be to code both ports 10/full.


This Discussion