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Chapter 7
Virtual LAN and VLAN Trunking Protocol
- All devices reside in the same LAN are in the same broadcast domain. All devices connected to a
switch are normally reside in the same broadcast domain. However, a technology called VLAN
allows a switch to create multiple broadcast domains.
- A Virtual LAN (VLAN) is a broadcast domain created by one or more switches. A switch
creates VLANs by assigning its interfaces to different VLANs.
- Below are some benefits of implementing VLANs:
i) Allows logical grouping of users or devices based on their functions or departments
instead of their physical locations.
ii) Reduces network overhead by limiting the size of each broadcast domain.
iii) Offers enhanced network security by keeping sensitive devices on a separate VLAN.
- VLAN trunking is used when a VLAN span across multiple switches. When a switch receives a
frame from another switch, it uses the frame tag created by other switch to identify the VLAN
membership of the frame and forwards it out to ports associated for the corresponding VLAN.
VLAN 2 VLAN 3
0/1
0/2 0/3
Figure 7-1: Network Setup for VLAN Trunking
- When SW1 receives a broadcast from a device in VLAN 2, it will add a header to the frame and
forward to SW2. SW2 will know which interfaces it should forward to all other members of
VLAN 2. The VLAN identifier will be removed when the frame is forwarded out an access link.
- Where is VLAN 1? VLAN 1 is the administrative VLAN which is recommended for
management purposes only; even though it still can be used for workgroup access purpose.
- Cisco Catalyst switches support 2 trunking protocols for inter-switch VLAN communication:
ISL – Inter-Switch Link IEEE 802.1Q
Cisco-proprietary. Industry standard. Standardized by IEEE.
Encapsulates the entire original frame with a
new header and trailer (CRC), increasing the
network overhead.
Supports multiple spanning trees (one STP
instance per VLAN) with PVST+.
0/2 0/3
0/4
0/1
0/4
SW1 SW2
0/12 Trunk
0/12
VLAN ID Ethernet Frame
Does not have the concept of native VLAN. Uses a native VLAN.
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VLAN 2 VLAN 3
Does not encapsulate the original frame. Adds
an extra 4-byte tag to the original Ethernet
header and a recalculated FCS to the trailer.
Supports multiple spanning trees (one STP
instance per VLAN) with PVST+ and 802.1S
Multiple Spanning Tree Protocol (MSTP).
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com