Configuration problems can arise when user traffic must traverse several switches. The following sections list some common configuration errors. But before you begin troubleshooting, create a plan. Check the implementation plan for any changes recently made, and determine likely problem areas. 

Troubleshooting User Connectivity 

User connectivity can be affected by several things: 

■ Physical connectivity: Make sure the cable, network adapter, and switch port are good. Check the port’s link LED. 

■ Switch configuration: If you see FCS errors or late collisions, suspect a duplex mismatch. Check configured speed on both sides of the link. Make sure the port is enabled and set as an access port. 

■ VLAN configuration: Make sure the hosts are in the correct VLAN. 

■ Allowed VLANs: Make sure that the user VLAN is allowed on all appropriate trunk links. 

Troubleshooting Trunking 

When troubleshooting trunking, make sure that physical layer connectivity is present before moving on to search for configuration problems such as 

■ Are both sides of the link in the correct trunking mode 

■ Is the same trunk encapsulation on both sides ? 

■ If 802.1Q, is the same native VLAN on both sides? Look for CDP messages warning of this error. 

■ Are the same VLANs permitted on both sides ? 

■ Is a link trunking that should not be ? 

References:

Troubleshooting VLANs