Bandung, Telkom University - Your CCNA survey contains quite a bit of information on switches, and for good reason. Without an understanding of basic switching theory, you will not be able to configure and troubleshoot Cisco switches in CCNA exams or in real-world environments. Double for trunking.
Trunking simply means that two or more switches can communicate with each other and send frames to be sent to a remote host. There are two important trunking protocols that we need to know more about for exam and real-world success, but before we get into the protocols, let's talk about the cables involved.
A crossover cable is required to connect two Cisco switches. As you know, an Ethernet cable has 8 wires. In a crossover cable, four cables are "crossed" from one of his pins to another. With many newer Cisco switches, creating a trunk is as simple as connecting the switches with a crossover cable. For example, a 2950 switch dynamically switches trunks as soon as the correct cable is connected. Using the wrong cable will cause problems for some time.
Today's Cisco switches use two different trunking protocols: ISL, commonly referred to as "dot1q", and his IEEE 802.1Q. There are three main differences between the two. First, ISL is a Cisco proprietary trunking protocol and dot1q is the industry standard. (Those unfamiliar with Cisco testing should be familiar with the terms "Cisco proprietary" and "industry standard.")
If you are working in a multi-vendor environment, ISL may not be a good choice. I have. Also, ISL is Cisco's proprietary trunking protocol, but some Cisco switches only run his dot1q.
Also, ISL encapsulates the entire frame, which increases network overhead. Dot1q may or may not just put a header on the frame. Compared to ISL, dot1q has much less overhead. This brings us to his third big difference: how the protocol works in the native VLAN.
The native VLAN is just the default he VLAN that the switchports are placed in unless explicitly put in a different VLAN. On Cisco switches, the native VLAN is VLAN 1. (This can be changed.) When dot1q is running, frames sent over the main line don't even have headers. The remote switch assumes each headerless frame is destined for the native VLAN.
