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New Member

Unknown duplex, 1000Mb/s,

Has anyone seen the following output showing that the duplex is UNKNOWN, the far end says that the duplex is full. Is this an issue? What can cause this?

I tried to set the duplex but there is only a speed command.

CORE#show int g5/1

GigabitEthernet5/1 is up, line protocol is up (connected)

Hardware is Gigabit Ethernet Port, address is 0002.b934.e50a (bia 0002.b934.e5


Description: Link to AC-01(G0/11)

MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

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

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Unknown duplex, 1000Mb/s, link type is auto, media type is SX

output flow-control is unsupported, input flow-control is unsupported, 1000Mb/


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

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

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

Last clearing of "show interface" counters never

Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

14092269 packets input, 1374524334 bytes, 0 no buffer

Received 9782884 broadcasts (0 multicast)

0 runts, 0 giants, 0 throttles

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

0 input packets with dribble condition detected

24841600 packets output, 302064542 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out


Cisco Employee

Re: Unknown duplex, 1000Mb/s,

Which platform and software version. We had few bugs for this in IOS based CAT4k and 3550's as well. It is cosmetic only and later sofware releases have a fix.

New Member

Re: Unknown duplex, 1000Mb/s,

Thanks for the reply,

The software is as follows:


IOS (tm) Catalyst 4000 L3 Switch Software (cat4000-IS-M), Version 12.1(14)E1, EARLY DEPLOYMENT RELEASE SOFTWARE (fc1)

The 4500 is connected to a 3550:

IOS (tm) C3550 Software (C3550-I5Q3L2-M), Version 12.1(13)EA1a, RELEASE SOFTWARE (fc1)

Do you feel that it is in F-duplex?

I have also configured etherchannel on the gig-ports and was wondering if this could affect the grouping if it is not in F-duplex mode? I am useing dot1.q for the trunks is this a problem?

Thanks for you advise.

New Member

Re: Unknown duplex, 1000Mb/s,


If it is not causing a performance problem, than it is due to bug CSCdz89143. For the $k, the latest release 12.1(19)E1 should have the fix.


Cisco Employee

Re: Unknown duplex, 1000Mb/s,

Gig links are always full-duplex. Upgrade to 19E or 19EW1 and this issue should be fixed. Also, on a side note, the IOS you are running on the 4507 is deferred

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