Flow control

Unanswered Question
Sep 5th, 2007


Please brief me what is Output flow-control and input flow contron options from "Sh int fa0/0" output.


I have this problem too.
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Joseph W. Doherty Wed, 09/05/2007 - 08:29

I suspect you're looking at Ethernet full-duplex flow control. This feature allows the receiver to tell the sender to suspend (or pause) its transmission. This allows the receiver to process the received data avoiding drops.

Not all Ethernet duplex devices support this capability. Often you need to tell the device to negotiate the option.

Some Ethernet devices can provide a similar capability when using half duplex. What happens is the receiver makes the wire appear busy to the sender.

cisconoval Wed, 09/05/2007 - 08:55

GigabitEthernet1/1 is up, line protocol is up (connected)

Hardware is Gigabit Ethernet Port, address is xxxx.xxxx.xxxx

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)

Full-duplex, 1000Mb/s, link type is auto, media type is 1000BaseSX

input flow-control is on, output flow-control is on

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

Last input never, 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 2000 bits/sec, 3 packets/sec

38 packets input, 3556 bytes, 0 no buffer

Received 14 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

41369 packets output, 3254019 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

from the above output you can see

"input flow-control is on, output flow-control is on"

Please let me know what is this means and how to turn it off?


Joseph W. Doherty Wed, 09/05/2007 - 09:29

Standards are slightly different for gig Ethernet. Not sure flow control can be disabled on gig.

Why do you want to disable it?

What type of box is this, and what image are you using?

cisconoval Thu, 09/06/2007 - 06:58

Cisco 4006

the switch port is connected to Unix server.

when that flow control is on I couldn't ping the server even from switch itself. engineer had did something after that server was reachable. then I checked I noticied only that flow control was turned off.

The engineer told just turned off the speed negotiation. I dont understand what is that. Can you please shed a light on this for me.


Joseph W. Doherty Thu, 09/06/2007 - 11:24

"the switch port is connected to Unix server. " Using fiber? ("media type is 1000BaseSX " under your original post.)

"Cisco 4006" A 4006 or a 4506? Assuming the latter, you can start here, for flow control and speed commands:


or with the command references for the software image you're using, here:


Then look for the command "flowcontrol", to control flow control settings, and/or "speed" to control speed settings.

"The engineer told just turned off the speed negotiation. I dont understand what is that. Can you please shed a light on this for me."

Speed negotiation is how a multi-speed Ethernet interface, e.g. 10/100 or 10/100/1000, attempts to find the best matching speed with the interface on the other side of the link.


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