Trunking Protocols
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May 14, 2008

Free Cisco CCNA Training: Trunking Protocol

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There are two types of trunking protocols, the Inter-Switch Protocol (ISL) and the IEEE 802.1Q (dot1q). Because of this, trunking makes for a great exam-type topic that will likely be an important part of the Cisco CCNA exam. In this CCNA training video, I’ll explain the differences between the two protocols, what makes one favorable to the other, and how to configure and understand the different settings available (modes, encapsulation, status, etc).

As you’ll learn in the video above, the Inter-Switch Protocol, or ISL, is the Cisco-proprietary trunking protocol. This protocol can only be used when all switches involved are Cisco switches. Using this protocol, the entire frame being sent is encapsulated before being transmitted across the trunk. Even if you are only using Cisco switches, it may not be the best choice considering the overhead involved.

The IEEE 802.1Q protocol, more generally referred to as the “dot1q” protocol, is the industry standard trunking protocol. The dot1q protocol is usually the preferred protocol for two reasons. First of all, dot1q does not encapsulate the entire frame. Instead, a small 4-byte header is added to the Ethernet header indicating which Virtual LAN the frame is intended for. Secondly, if the Virtual LAN is the native VLAN (VLAN 1), dot1q will further reduce overhead by not including a header at all. Instead, the frame will be sent as-is and will be assumed for the native VLAN.

For more information on the different modes of trunking, check out the next video in our CCNA training series, Trunking Modes.

About the Author

(CCIE #12933) has extensive experience in both the practical and theoretical sides of Cisco and Networking technologies. You will benefit greatly from his detailed instruction and passion for helping others, especially since Chris has recently been “down the Cisco road”, passing his CCNA, CCNP and his CCIE! Take advantage of Chris's vast experience, wealth of knowledge and all-around good advice that will help you pass your certification exams and develop the hands-on skills that the market demands.


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