Cisco – Duplex mismatch between a Fast Ethernet router and Gigabit Ethernet switch with auto negotiation

ciscogns3routerswitch

i am connecting c3745 router on fast ethernet 0/0 interface with a IOSvL2 switch on gigabit ethernet 0/1 on GNS3.

i have set the negotiation on the switch to auto for all interfaces on the switch. And also set the duplex and speed on the router interfaces to auto.

but i still get a Duplex mismatch message on the switch.

here is my router configuration:

Current configuration : 1098 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R1
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
no ip icmp rate-limit unreachable
ip cef
!
!
!
!
no ip domain lookup
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
ip tcp synwait-time 5
!
!
!
!
!
interface FastEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 duplex auto
 speed auto
!
interface Serial0/0
 no ip address
 shutdown
 clock rate 2000000
!
interface FastEthernet0/1
 no ip address
 shutdown
 duplex auto
 speed auto
!
interface Serial0/1
 no ip address
 shutdown
 clock rate 2000000
!
interface Serial0/2
 no ip address
 shutdown
 clock rate 2000000
!
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
no cdp log mismatch duplex
!
!
!
control-plane
!
!
!
!
!
!
!
!
!
!
line con 0
 exec-timeout 0 0
 privilege level 15
 logging synchronous
line aux 0
 exec-timeout 0 0
 privilege level 15
 logging synchronous
line vty 0 4
 login
!
!
end

and my switch configuration:

Current configuration : 3548 bytes
!
! Last configuration change at 18:20:49 UTC Tue Apr 16 2019
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
service compress-config
!
hostname Switch
!
boot-start-marker
boot-end-marker
!
!
!
no aaa new-model
!
!
!
!
!
!
!
!
ip cef
no ipv6 cef
!
!
!
spanning-tree mode pvst
spanning-tree extend system-id
!
vlan internal allocation policy ascending
!
!
!
!
!
!
!
!
!
!
!
!
!
!
interface GigabitEthernet0/0
 media-type rj45
 negotiation auto
!
interface GigabitEthernet0/1
 media-type rj45
 negotiation auto
!
interface GigabitEthernet0/2
 media-type rj45
 negotiation auto
!
interface GigabitEthernet0/3
 media-type rj45
 negotiation auto
!
interface GigabitEthernet1/0
 media-type rj45
 negotiation auto
!
interface GigabitEthernet1/1
 media-type rj45
 negotiation auto
!
interface GigabitEthernet1/2
 media-type rj45
 negotiation auto
!
interface GigabitEthernet1/3
 media-type rj45
 negotiation auto
!
interface GigabitEthernet2/0
 media-type rj45
 negotiation auto
!
interface GigabitEthernet2/1
 media-type rj45
 negotiation auto
!
interface GigabitEthernet2/2
 media-type rj45
 negotiation auto
!
interface GigabitEthernet2/3
 media-type rj45
 negotiation auto
!
interface GigabitEthernet3/0
 media-type rj45
 negotiation auto
!
interface GigabitEthernet3/1
 media-type rj45
 negotiation auto
!
interface GigabitEthernet3/2
 media-type rj45
 negotiation auto
!
interface GigabitEthernet3/3
 media-type rj45
 negotiation auto
!
ip forward-protocol nd
!
no ip http server
no ip http secure-server
!
!
!
!
!
!
control-plane
!
line con 0
 exec-timeout 0 0
 logging synchronous
line aux 0
line vty 0 4
 login
!
!
end

when i run the show interfaces on the router i get this:

FastEthernet0/0 is up, line protocol is up
  Hardware is Gt96k FE, address is c401.07a8.0000 (bia c401.07a8.0000)
  Internet address is 192.168.1.1/24
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Half-duplex, 10Mb/s, 100BaseTX/FX
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:21, output 00:00:08, 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 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     50 packets input, 8748 bytes
     Received 50 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
     55 packets output, 5963 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface reset

and on the switch when i run show interfaces status i get this: 

Port      Name               Status       Vlan       Duplex  Speed Type
Gi0/0                        connected    1          a-full   auto RJ45
Gi0/1                        connected    1          a-full   auto RJ45
Gi0/2                        connected    1          a-full   auto RJ45
Gi0/3                        connected    1          a-full   auto RJ45
Gi1/0                        connected    1          a-full   auto RJ45
Gi1/1                        connected    1          a-full   auto RJ45
Gi1/2                        connected    1          a-full   auto RJ45
Gi1/3                        connected    1          a-full   auto RJ45
Gi2/0                        connected    1          a-full   auto RJ45
Gi2/1                        connected    1          a-full   auto RJ45
Gi2/2                        connected    1          a-full   auto RJ45
Gi2/3                        connected    1          a-full   auto RJ45
Gi3/0                        connected    1          a-full   auto RJ45
Gi3/1                        connected    1          a-full   auto RJ45
Gi3/2                        connected    1          a-full   auto RJ45
Gi3/3                        connected    1          a-full   auto RJ45

and the message i get is:
*Apr 16 17:36:35.555: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on GigabitEthernet0/1 (not half duplex), with R1 FastEthernet0/0 (half duplex).

please help me.

thanks .

Best Answer

The message is from CDP duplex mismatch detection - it might indicate a duplex mismatch that isn't there, indicating a problem with CDP itself (esp. in GNS3).

Check each side's interface status (show interface status) to see if the duplex modes really differ. If the message is wrong you can shut it up by no cdp log mismatch duplex.

[edit after show interface status and show interface additions]: in addition to the duplex mismatch, the router's link shows 10 Mbit/s half-duplex which might even be a link speed mismatch which isn't normally possible. Unless there's another device in between, autonegotiation has failed altogether which most probably is a glitch in the simulation.

Related Topic