1. VLANs (Virtual LANs) CS 158B Elaine Lim Allison Nham

2. 2 LANs vs. VLANs… In a traditional LAN...  Users are grouped physically based on the hub they are plugged into  Routers segment the LAN and provide broadcast boundaries In VLANs...  Group users logically by function, department or application  Configuration is done through special software

3. 3 A sample VLAN network Source: Cisco IOS Switching Services Configuration Guide

4. 4 How VLANs work When a switch receives data from a workstation, it tags the data with a VLAN identifier that indicates which VLAN the data originally came from A packet can only travel from one broadcast domain to another if both domains have the same identifier To set up VLANs, we need VLAN-aware switching devices that must comply with IEEE 802.1Q standards  Intelligent switches (operate at the MAC layer) or  Routers (operate at the network layer of the OSI reference model)

5. 5 Static VLANs Defined  Static VLANs are when ports on a switch are administratively assigned to a VLAN Benefits  Secure & easy to configure and monitor  Works well in networks where moves are controlled Source: Cisco IOS Switching Services Configuration Guide

6. 6 Dynamic VLANs Defined  Switch ports can automatically determine a user’s VLAN assignment based on: MAC address Protocol type Benefits  Less administration when users are added or moved  Centralized notification of unauthorized user Source: Cisco IOS Switching Services Configuration Guide

7. 7 Frame Processing Switches make filtering and forwarding decisions based on data in the frame There are two techniques used  Frame Filtering - examines particular information about each frame (MAC address or Layer 3 protocol type)  Frame Tagging - places a unique identifier in the header of each frame as it is forwarded throughout the network backbone

8. 8 Frame Tagging A preferred way to implement VLANs Uniquely assigns a VLAN ID to each frame before it is forwarded across the backbone Is removed by the switch after frame exits the backbone Source: Cisco IOS Switching Services Configuration Guide

9. 9 Traffic Between VLANs Switches do not forward frames between different VLANs. A router does this!!! Trunking – a method that supports multiple VLANs that have members on more than one switch  Two popular trunking protocols: oCisco Inter-Switch Link (ISL) oIEEE 802.1Q Source: How stuff works

10. 10 Inter-Switched Link (ISL) Cisco created ISL before the IEEE standardized a trunking protocol ISL is Cisco proprietary; it can only be used between two Cisco switches ISL encapsulates each frame in an ISL header & trailer

11. 11 IEEE 802.1Q 802.1Q is an open standard that can be used with multi- vendor switches 802.1Q does not encapsulate – it adds an extra 4-bytes header to the middle of the original Ethernet header 802.1Q forces a recalculation of FCS

12. 12 Disadvantage & Advantages Disadvantage :  VLANs require significant overhead Advantages:  More Security  Ease of administration  Broadcast control  Reduction in network traffic