Error#2
Trunk Mode Mismatches
A switch port Trunk link is configured manually with the switchport mode trunk command. Trunk ports use DTP advertisements to negotiate the state of the link with the remote switch port. When a port on a trunk link is configured with a trunk mode that is incompatible with the other trunk port, a trunk link fails to form between the two switches.
In this situation, the user computer PC5 (student) cannot connect to the internal web server. Again, using the topology diagram below shows a correct configuration. Let try and locate and solve the problem.
The first thing you do here is to verify the status of the trunk ports on switch SW1 using the show interfaces trunk command.
SW1#show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa0/1 on 802.1q trunking 99
Port Vlans allowed on trunk
Fa0/1 10, 99
Port Vlans allowed and active in management domain
Fa0/1 10, 99
Port Vlans in spanning tree forwarding state and not pruned
Fa0/1 10, 99
Name: Fa0/3
Switchport: Enabled
Administrative Mode: dynamic auto
From the details above, it shows that interface Fa0/3 on switch SW1 is not configured as trunk port. On close examination you will discover that the Fa0/3 interface port is set to dynamic auto mode.
Lets look at the trunks on switch SW3 using the show interface trunk command:
SW3#show interfaces trunk
SW3#show interface fa0/3 switchport
Name: Fa0/3
Switchport: Enabled
Administrative Mode: dynamic auto
The above detail also reveals that there are no active trunk ports. Further check reveals that the Fa0/3 interface is also in dynamic auto mode; this explains why the trunk is down.
Solution:
To activate the trunk link, you need to reconfigure the trunk mode of the Fast Ethernet Fa0/3 ports on switches SW1 and SW3.
Configuration example on SW1:
SW1#config t
SW1(config)#interface fa0/3
SW1(config-if)switchport mode trunk
SW1(config-if)end
And use the show interfaces fa0/3 switchport to verify configuration on SW1:
SW1#show interfaces fa0/3 switchport
Name: Fa0/3
Switchport: Enabled
Administrative Mode: trunk
Reconfigure switchport fa0/3 as trunk link on Switch 3
SW3#config t
SW3(config)#interface fa0/3
SW3(config-if)switchport mode trunk
SW3(config-if)end
Use the show interfaces fa0/3 switchport to verify configuration on
SW3#show interfaces fa0/3 switchport
Name: Fa0/3
Switchport: Enabled
Administrative Mode: trunk
…..
Use the show interfaces trunk on Switch 3 to verify connectivity
SW3#show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa0/3 On 802.1q trunking 99
Port Vlans allowed on trunk
Fa0/1 10, 99
Port Vlans allowed and active in management domain
Fa0/3 10, 99
Port Vlans in spanning tree forwarding state and not pruned
Fa0/3 10, 99
S3#
The above detail shows that interface Fa0/3 has been reconfigured as a trunk.
Now ping from the PC5 command line to very connectivity:
PC5> 192.168.10.30
Pinging 192.168.10.30 with 32 bytes of data
…
Reply from 192.168.10.30: Reply from 192.168.10.30: bytes-32 times-147ms TT-128
Reply from 192.168.10.30: Reply from 192.168.10.30: bytes-32 times-147ms TT-128
Reply from 192.168.10.30: Reply from 192.168.10.30: bytes-32 times-147ms TT-128
The output from PC5 above indicates that connectivity has been regained to the WEB/TFTP server found at IP address 192.168.10.30.
