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Router Interface Duplex/Speed Settings

hi experts,

i would like to know which command output to refer when finding out duplex/speed settings on the router's interface. show interface displays full/100 while show run interface displays auto.

ROUTER#sh int f0/1
FastEthernet0/1 is up, line protocol is up
  Hardware is Gt96k FE, address is 0017.95bb.44a3 (bia 0017.95bb.44a3)
  Description: LAN
  Internet address is 192.168.1.1/24
  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
     reliability 255/255, txload 2/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive not set
  Full-duplex, 100Mb/s, 100BaseTX/FX  -->
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:20, 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 469000 bits/sec, 238 packets/sec
  5 minute output rate 917000 bits/sec, 243 packets/sec
     2327169929 packets input, 1519671819 bytes
     Received 3 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
     2356621577 packets output, 3748937506 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


ROUTERW#sh run int f0/1
Building configuration...

Current configuration : 145 bytes
!
interface FastEthernet0/1
description LAN
ip address 192.168.1.1 255.255.255.0
duplex auto  -->
speed auto  -->
no keepalive
end

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: Router Interface Duplex/Speed Settings

John

If you have configured the interface to auto-negotiate then to see the actual speed/duplex the interface is running you must look at the output of the "sh interface " command. The reason being that if the interface is set to auto it could negotiate to different things eg. 10 full duplex/100 half  duplex/100 full duplex etc.. You will only know what the interface is actually running at by looking at the output of "sh interface ..." because if you look at the "sh run" output all it will tell you is that the interface is configured to auto-negotiate.

However if you manually set the speed and duplex under the interface then you only need to look at the output of "sh run" because it will tell you what speed/duplex it is using, although if you are experiencing problems i would still check the output of "sh interface .."

Jon

8 REPLIES

Re: Router Interface Duplex/Speed Settings

Hi John

The Show run command will tell you what you have configured for the interface. The show interface will show the current settings of this interface.

As long us you have hard coded it the duplex to  Full/Half or speed to 10mb/100 mb it will remain same. But when you use the auto option the router will negotiate it its peer and both accept to work on one.  The accepted Duplex/Speed will be shown in the show interface.

To answer your question, If you hard coded then you can rely on sh run output .. but incase you have opted for auto setting then you need to look into both output.

Regards

Siva

Silver

Re: Router Interface Duplex/Speed Settings

Hi,

Just wanted to add a documentation as well, please refer to the "Auto-Negotiation on Catalyst Switches that Run Cisco IOS Software" section on the following link:

http://www.cisco.com/en/US/tech/tk389/tk214/technologies_tech_note09186a0080094781.shtml

Andras

Re: Router Interface Duplex/Speed Settings

hi sivasubramanian,

thanks for your feedback! i'm still quite confused on your last statement. so which one do i really need to refer for the interface duplex/speed setting? because both output displays different settings.

Hall of Fame Super Blue

Re: Router Interface Duplex/Speed Settings

John

If you have configured the interface to auto-negotiate then to see the actual speed/duplex the interface is running you must look at the output of the "sh interface " command. The reason being that if the interface is set to auto it could negotiate to different things eg. 10 full duplex/100 half  duplex/100 full duplex etc.. You will only know what the interface is actually running at by looking at the output of "sh interface ..." because if you look at the "sh run" output all it will tell you is that the interface is configured to auto-negotiate.

However if you manually set the speed and duplex under the interface then you only need to look at the output of "sh run" because it will tell you what speed/duplex it is using, although if you are experiencing problems i would still check the output of "sh interface .."

Jon

Re: Router Interface Duplex/Speed Settings

hi jon,

i got what you mean. thanks for the enlightenment!

Purple

Re: Router Interface Duplex/Speed Settings

In your post  your show runn command for the interface shows you it is set as auto .  Your show interface command shows 100/full so it is telling you it successfully negotiated 100/full .   in a multilayer switch it is easier to see , all you have to use is the "show interface status" command , in this one display it shows you what it negotiated and how it is set in the config, it will show a small  "A" in from of the speed duplex which says it is set as auto for speed and duplex.

Re: Router Interface Duplex/Speed Settings

hi glen,

i've tried this time on our LAN switch and show commands displays different output as well. which one do i refer?

SWITCH#sh interfaces  FastEthernet0/5 status

Port      Name               Status       Vlan       Duplex  Speed Type
Fa0/5     PC                 connected    207          half    100 10/100BaseTX


SWITCH#sh run int fa0/5
Building configuration...

Current configuration : 197 bytes
!
interface FastEthernet0/5
description PC
switchport access vlan 207
switchport mode access
bandwidth 4000
speed 100
duplex full
no cdp enable
end

SWITCH#sh int f0/5
FastEthernet0/5 is up, line protocol is up (connected)
  Hardware is Fast Ethernet, address is 000c.8582.0405 (bia 000c.8582.0405)
  Description: PC
  MTU 1500 bytes, BW 4000 Kbit, DLY 100 usec,
     reliability 250/255, txload 26/255, rxload 4/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Half-duplex, 100Mb/s, media type is 100BaseTX
  input flow-control is unsupported output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:52, output 00:00:01, 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 63000 bits/sec, 70 packets/sec
  5 minute output rate 408000 bits/sec, 64 packets/sec
     3902037950 packets input, 919480826 bytes, 0 no buffer
     Received 20178039 broadcasts (0 multicast)
     0 runts, 0 giants, 0 throttles
     3 input errors, 1 CRC, 2 frame, 0 overrun, 0 ignored
     0 watchdog, 893503 multicast, 0 pause input
     0 input packets with dribble condition detected
     127886993 packets output, 3240061448 bytes, 0 underruns
     13926959 output errors, 1756486 collisions, 1 interface resets
     0 babbles, 13926959 late collision, 3988176 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out

Hall of Fame Super Blue

Re: Router Interface Duplex/Speed Settings

John

That interface is running at 100 half duplex even though you have configured it as 100full. Chances are the other end of the link is set to auto-negotiate. If you hardcore the speed and duplex on one end of the link then you should configure the same on the device at the other end of the link.

Jon

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