GNS3 – Speed and Duplex Mismatch

KB ID 0000983 Dtd 10/08/14

Problem

I don’t know why this happens sometimes with GNS3, but occasionally I will get a connection between two devices that constantly complains.

GNS3 speed duplex mismatch

%CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on {interface-name} (not half duplex), with {host-name} {interface-name} (half duplex).

For the uninitiated, a speed/duplex mismatch, usually happens when both ends of the link are set differently, or (more commonly) both ends are set to ‘auto’.

!
interface FastEthernet0/1
ip address 192.168.1.1 255.255.255.0
duplex auto
speed auto
!

Solution

WARNING: DO NOT carry out this procedure on live networking equipment, this is only for use in the GNS3 environment.

If this happens to you, you will sensibly try and set the speed/duplex of both ends of the link correctly, on real networking equipment that would solve the problem like so;

PetesRouter(config)#interface FastEthernet0/1
PetesRouter(config-if)#duplex full
*Aug 6 13:40:39.815: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down
*Aug 6 13:40:41.823: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
*Aug 6 13:40:42.823: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
PetesRouter(config-if)#speed 100
*Aug 6 13:40:47.855: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down
*Aug 6 13:40:49.859: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
*Aug 6 13:40:50.859: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
PetesRouter(config-if)#

But in some cases on GNS3 it does not, (not sure if it’s a bug?)

Is that happening to you, the only way to stop it is to suppress the error. To do this add the ‘no cdp log mismatch duplex’ command to the interface giving you the error.

PetesRouter(config)#interface FastEthernet 0/1
PetesRouter(config-if)#no cdp log mismatch duplex
PetesRouter(config-if)#exit
PetesRouter(config)#exit
*Aug 6 13:45:55.235: %SYS-5-CONFIG_I: Configured from console by console
PetesRouter#write mem
Building configuration...
[OK]
PetesRouter#

Related Articles, References, Credits, or External Links

NA

Author: Migrated

Share This Post On