User cannot establish a trunk link between a switch and a router due to misconfiguration or physical layer issues

[TCC_2]

Resolution

To resolve this issue, perform these steps:

1. On the switch, issue the show port command to verify these options:   
   • Port status is connected.       
   • No physical layer errors (Align-Err or FCS-Err) are occurring.       
   • Duplex and speed settings are correctly set to match the router side.
      
2. On the router, issue the show interface command to verify these options:   
   • Speed and duplex settings are correctly set to match the switch side.       
   • The physical interface has a subinterface configured for each VLAN which you want to route traffic.       
   • Each subinterface has the correct encapsulation type.       
   • Each subinterface has the correct VLAN number.       
   • Each subinterface has the correct IP address and subnet mask.
      
3. On the router, issue the show version command to ensure the router is running a release of Cisco IOS  Software that supports trunking.   
4. Other misconfigurations which can lead to trunking failures include these options:   
   • Mismatched VLANs       
   • Incorrect trunk mode (on, off, desirable, auto, nonegotiate)       
   • Mismatched or unsupported trunking encapsulation (Inter-Switch Link [ISL], Dot1q)       
   • Native VLAN issues       
   • Port not capable of trunking mode being configured
      

For more information on these issues, refer to System Requirements to Implement Trunking.