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Review of Virtual LAN Concepts 71

Foundation Topics

Whether you have read Chapter 10, “Virtual LANs and Trunking,” of the CCNA INTRO Exam Certification Guide, or whether you have already passed the INTRO exam, you should have learned the basics of VLANs. Therefore, this chapter just briefly reviews the concepts behind VLANs. You should also already know the basics of VLAN trunking with ISL and IEEE 802.1Q from your study for the INTRO exam. However, because fewer people know about trunking before starting to prepare for CCNA, this chapter describes trunking in detail.

VLAN Trunking Protocol (VTP) provides the only major topic about VLANs that is not on both CCNA exams. VTP is covered in this chapter after trunking. Finally, this chapter closes with a section about VLAN configuration.

Review of Virtual LAN Concepts

VLANs are pretty simple in concept and in practice. The following list hits the high points:

A collision domain is a set of network interface cards (NICs) for which a frame sent by one NIC could result in a collision with a frame sent by any other NIC in the same collision domain.

A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC is received by all other NICs in the same broadcast domain.

A VLAN is essentially a broadcast domain.

VLANs are typically created by configuring a switch to place each port in a particular VLAN.

Layer 2 switches forward frames between devices in the same VLAN; they cannot forward frames between different VLANs.

A Layer 3 switch, multilayer switch, or router can be used to essentially route packets between VLANs.

The set of devices in a VLAN typically also is in the same IP subnet; devices in different VLANs are in different subnets.

Figure 3-1 shows a switch with two VLANs. Fred and Dino can send frames to each other, but neither can send frames to Wilma.