Half Duplex/Full Duplex Question

[dcanady55]

I apologize if this is a dumb question in advance.

I'm new to Cisco products.

Have changed out a core switch and the new one is a Cat. 3850

I have a PBX that runs 10/100 only and is set to 100 I noticed on the switch the LED blinked amber once every so often for that port and when I looked at the port is shows it running in half duplex. Even when I manually set the switch to 100 full then go back into the interface it shows half duplex running at 100 mbps. I'm just wondering if this is normal since the port can do 1GB?

Thanks in advance.

Derek

[daniel.dib · Accepted solution]

When a port is hardcoded to 100/full and the other side is auto, then the speed will be detected by analyzing the electrical signal. This is sometimess referred to Fast Link Pulses (FLP). The duplex can't be detected so half duplex mode must be assumed.

I don't see why the port would run in half duplex if you manually set it though. You did set both the speed and duplex? Can you post the output?

Daniel Dib
CCIE #37149

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[dcanady55]

Hi Daniel,

yes, when I first discovered this I went in and changed the port to speed to 100 and duplex to full. When I go back into show interfaces gig...port number it shows half duplex still with 100 as speed. I checked the PBX still and it's setup hardcoded to 100 and full there is an auto feature on it so maybe I should try setting both to auto as I'm sure the switch was auto before I tried hard setting it.

Hardware is Gigabit Ethernet, address is XXXXX

  Description: *** PBX ***

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

     reliability 253/255, txload 1/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)

  Half-duplex, 100Mb/s, media type is 10/100/1000BaseTX

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

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

  Last input 00:00:25, 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 679000 bits/sec, 492 packets/sec

  5 minute output rate 665000 bits/sec, 489 packets/sec

     32729905 packets input, 662507959 bytes, 0 no buffer

     Received 10697 broadcasts (2043 multicasts)

     0 runts, 0 giants, 0 throttles

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

     0 watchdog, 2043 multicast, 0 pause input

     0 input packets with dribble condition detected

     32677583 packets output, 599582320 bytes, 0 underruns

     151782 output errors, 106267 collisions, 1 interface resets

     0 unknown protocol drops

     0 babbles, 99569 late collision, 0 deferred

     0 lost carrier, 0 no carrier, 0 pause output

     0 output buffer failures, 0 output buffers swapped out

[dcanady55]

Never mind I changed the setting on my PBX to auto and now when I check the interface on the switch it's full and 1000.

Thanks,

Derek