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Virtual vs. Physical LANs: Device Functionalities


  1. Overview
  2. Differences Between Physical and Virtual LANs
  3. Complexities of Virtual LANs
  4. Summary

Article Description

If you have a good grasp of the basics of local access networks (LANs), it's time to step up your game. Sean Wilkins, co-author of CCNA Routing and Switching ICND2 200-101 Official Cert Guide, explores the concepts behind virtual LANs, how they differ from physical LANs, and why VLANs make sense for modern organizations.

Complexities of Virtual LANs

As in many other technologies, virtual LANs include a caveat; however, the VLAN caveat is also an advantage: While a VLAN allows hosts to be virtually separated at Layer 2, it doesn’t provide a mechanism for communicating between VLANs. To return to our earlier telephone system analogy, sometimes phones in an isolated part of an organization must communicate with other, non-isolated phones. Typically with phone systems this requirement means dialing a specific prefix to call “outside” that isolated part of the phone network; for example, we’re told “Dial 9 to get an outside line.”

With networks that use VLANs, the only way to communicate with other devices in different VLANs is to bring in a Layer 3 device (a router or Layer 3 switch); this device is configured to allow communications between the devices in different VLANs. This device’s Layer 3 (and above) features might be configured to limit the amount and types of traffic allowed between devices. This type of connection can be implemented in a few different ways: using a router that has an interface connected per VLAN on the switch, using a router along with switch trunking capabilities (IEEE 802.1Q), or using a Layer 3-capable switch for multi-layer switching (MLS). We’ll address these connection types in more detail in a future article.