| Your CCNA studies are going to include quite a bit of | | | | should get used to the phrases "Cisco-proprietary" and |
| information about switches, and for good reason. if you | | | | "industry standard".) If you're working in a multivendor |
| don't understand basic switching theory, you can't | | | | environment, ISL may not be a good choice. And even |
| configure and troubleshoot Cisco switches, either on | | | | though ISL is Cisco's own trunking protocol, some |
| the CCNA exam or in the real world. That goes double | | | | Cisco switches run only dot1q. |
| for trunking! | | | | ISL also encapsulates the entire frame, increasing the |
| Trunking is simply enabling two or more switches to | | | | network overhead. Dot1q only places a header on the |
| communicate and send frames to each other for | | | | frame, and in some circumstances, doesn't even do |
| transmission to remote hosts. There are two major | | | | that. There is much less overhead with dot1q as |
| trunking protocols that we need to know the details of | | | | compared to ISL. That leads to the third major |
| for exam success and real-world success, but before | | | | difference, the way the protocols work with the native |
| we get to the protocols, let's discuss the cables we | | | | vlan. |
| need. | | | | The native vlan is simply the default vlan that switch |
| Connecting two Cisco switches requires a crossover | | | | ports are placed into if they are not expressly placed |
| cable. As you know, there are eight wires inside an | | | | into another vlan. On Cisco switches, the native vlan is |
| ethernet cable. In a crossover cable, four of the cables | | | | vlan 1. (This can be changed.) If dot1q is running, frames |
| "cross over" from one pin to another. For many newer | | | | that are going to be sent across the trunk line don't |
| Cisco switches, all you need to do to create a trunk is | | | | even have a header placed on them; the remote |
| connect the switches with a crossover cable. For | | | | switch will assume that any frame that has no header |
| instance, 2950 switches dynamically trunk once you | | | | is destined for the native vlan. |
| connect them with the right cable. If you use the | | | | The problem with ISL is that is doesn't understand |
| wrong cable, you'll be there a while! | | | | what a native vlan is. Every single frame will be |
| There are two different trunking protocols in use on | | | | encapsulated, regardless of the vlan it's destined for. |
| today's Cisco switches, ISL and IEEE 802.1Q, generally | | | | Switching theory is a big part of your CCNA studies, |
| referred to as "dot1q". There are three main | | | | and it can seem overwhelming at first. Just break your |
| differences between the two. First, ISL is a | | | | studies down into smaller, more manageable parts, and |
| Cisco-proprietary trunking protocol, where dot1q is the | | | | soon you'll see the magic letters "CCNA" behind your |
| industry standard. (Those of you new to Cisco testing | | | | name! |