Troubleshooting VLANs and Trunks

Common Errors

On this page(s), we will look at common VLAN and trunking problems, which most times are caused by incorrect configurations. As I said earlier on NAT troubleshooting page, most common mistake in troubleshooting is not the inability to use commands but the laxity in the examination of entries and configuration.  When you are configuring VLANs and trunks on a switched network, these types of configuration errors are most likely to be the causes:

• Native VLAN mismatches
• Trunk mode mismatches
• VLANs and IP Subnets
• Allowed VLANs on trunks

We will look at these likely VLAN and Trunk errors orderly. Start your troubleshooting by examining the trunks for a native VLAN mismatch and then work down the list.

Below are list of common Problems with VLANs and Trunks and their result on the network:

Errors	Consequences
Native VLAN mismatches	Poses a security risk
Trunk mode mismatches	Causes loss of network connectivity
VLANs and IP subnets	Causes loss of network connectivity
Allowed VLANs on Trunks	Causes uninvited traffic or no traffic through over the Trunk

 

                                  

                                   Error#1  Native VLAN Mismatches

                                   Error#2 Trunk Mode Mismatches

                                   Error#3  Incorrect VLANs allowed on Trunks

                                   Error#4  Incorrect VLANs IP Subnet Configurations

                                   VLAN Trunking Protocol (VTP)