Chapter 9. Troubleshooting Virtual LAN

Terms you'll need to understand:

• Frame Tagging

• VLAN

• VLAN Port

• VLAN Trunk

• IEEE 802.10

• IEEE 802.1Q

• Inter-Switch Link (ISL)

Techniques you'll need to master:

• Differentiating between trunk links and access links

• Using VTP and VTP pruning

• Differentiating between dynamic and static VLAN ports

• Differentiating between internal and external route processing

• Configuring VLANs, trunking, VTP, and VTP pruning on a Catalyst switches

• Troubleshooting switched connections

• Identifying switch troubleshooting commands

• Troubleshooting switch hardware and software

• Troubleshooting VTP, ISL, and Spanning Tree Protocol configurations

• Knowing the diagnostic tools which apply to VLAN configuration problems

• Troubleshooting inter-VLAN routing configurations

• Troubleshooting trunk links and switches and routers

This chapter focuses on Virtual LANs (VLANs), which were created by the different switch manufacturers to provide individual broadcast domains. By assigning each port to an individual VLAN, each VLAN becomes its own broadcast domain. In IP networks, each VLAN is considered its own subnet and must be addressed as such. Each VLAN is commonly referred to as a color .

When a VLAN is created and ports on a single or multiple switches are assigned to that VLAN, only those ports that are members of the same VLAN will receive broadcasts from any other member of their assigned VLAN; these are called VLAN Ports. VLAN trunks are used to provide a way for data frames belonging to more than one VLAN to travel a single physical link between network devices.

The coloring of data traffic across a backbone is done by inserting a header between the source MAC address and the Link Service Access Point (LSAP) of frames leaving a switch. The 4-byte header is called the VLAN ID , or color .