Speed - auto - in a Fastethernet interface in a Router

Answered Question
Oct 15th, 2008

Is there auto-negotiation when you connect a router FaX/X to a switch interface? I know how to check if speed and duplex are auto in a Switch... how can you see this in a router (command)?

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Correct Answer by hasmurizal about 8 years 2 months ago

hi, from your question, i believe to use the follwing command to know either auto-nego or not:

"show run int fax/x" to see the configuration of a particular interface. If you do not see Speed 100 and duple-full, then your port is auto-nego.

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Jon Marshall Wed, 10/15/2008 - 06:52

Yes there is auto-negotiation providing the switch port is set up for auto-negotiation. If you wanted to set the interface to a specific speed/duplex on a router interface

int fa0/0

speed 100

duplex full

Jon

fjcardenas-1 Wed, 10/15/2008 - 07:01

My specific question is how can I see (which command) that an interface chose the speed and duplex setting with auto-negotiation? With a switch you use the sh int status command... what command do you use in a router?

Thanks.

Jon Marshall Wed, 10/15/2008 - 07:05

Sorry i wasn't specific enough. If there are not speed/duplex entries under the interface configuration then it is using auto-negotiation. That is how you can tell.

Jon

andrewdennehy Wed, 10/15/2008 - 21:37

Try using "show interface "

You should get an output similar to the following. I have marked the point where the duplex is displayed.

FastEthernet0 is up, line protocol is up

Hardware is xxx, address is 0018.19c2.fc8c (bia 0018.19c2.fc8c)

Internet address is A.B.C.D/24

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

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

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

*********************************

Full-duplex, 100Mb/s, 100BaseTX/FX

*********************************

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 never

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 153000 bits/sec, 69 packets/sec

5 minute output rate 137000 bits/sec, 77 packets/sec

26788744 packets input, 2250783189 bytes

Received 1 broadcasts, 0 runts, 0 giants, 0 throttles

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

0 watchdog

0 input packets with dribble condition detected

27330053 packets output, 2049402608 bytes, 0 underruns

0 output errors, 0 collisions, 3 interface resets

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

Regards,

Andrew

fjcardenas-1 Fri, 10/17/2008 - 05:32

The output of "show interfaces" shows the speed and duplex settings but it doesn't show if it got those settings by auto-negotiation or manually... how to know if it got them with auto-negotiation?.

u0087672js Tue, 11/11/2008 - 10:31

If there is nothing connected to it you can tell by, if it would say auto where it shows duplex and speed. See below is a show interface of a port set to auto.

Correct Answer
hasmurizal Wed, 11/12/2008 - 19:17

hi, from your question, i believe to use the follwing command to know either auto-nego or not:

"show run int fax/x" to see the configuration of a particular interface. If you do not see Speed 100 and duple-full, then your port is auto-nego.

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