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

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.

Version history
Revision #:
1 of 1
Last update:
‎06-22-2009 05:28 PM
Updated by:
 
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