CCNA Certification: How Switches Trunk -- And Why !
By Chris Bryant, CCIE #12933
Trunking is one of those CCNA and CCNP exam terms that sounds mysterious - until you create one.
Trunks are easy to configure, and in some cases trunks require no additional configuration at all. For Cisco exam success, you had better know the details of how trunks form and why we want trunks to form in the first place.
Trunking is simply enabling two or more switches to communicate and send frames to each other for transmission to remote hosts.
In the following example, we've got hosts in VLAN 10 on two separate switches. Unless these is some kind of communication path between the two switches, these two hosts will be unable to communicate with each other. A trunk makes it possible for these hosts to transmit data to each other.
Connecting two Cisco switches requires a crossover cable. There are eight wires inside an ethernet cable. In a crossover cable, four of the cables "cross over" from one pin to another. For many newer Cisco switches, all you need to do to create a trunk is connect the switches with a crossover cable.
To check the status of a trunk, use the show interface trunk command.
This command output shows us that ports fast 0/11 and 0/12 are trunking, that they are in the default mode of dynamic desirable, they're running IEEE 802.1Q encapsulation, and that all VLANs are allowed to send traffic across the trunk.
Speaking of IEEE 802.1Q ....
There are two different trunking protocols in use on today's Cisco switches, ISL and IEEE 802.1Q, generally referred to as "dot1q".
There are three main differences between the two. First, ISL is a Cisco-proprietary trunking protocol, where dot1q is the industry standard. (Those of you new to Cisco testing should get used to the phrases "Cisco-proprietary" and "industry standard".)
If you're working in a multivendor environment, ISL may not be a good choice. And even though ISL is Cisco's own trunking protocol, some Cisco switches run only dot1q.
ISL also encapsulates the entire frame, increasing the network overhead. Dot1q only places a header on the frame, and in some circumstances, doesn't even do that. There is much less overhead with dot1q as compared to ISL. That leads to the third major difference, the way the protocols work with the native vlan.
The native vlan is simply the default vlan that switch ports are placed into if they are not expressly placed into another vlan. On Cisco switches, the native vlan is vlan 1. (This can be changed.)
If dot1q is running, frames that are going to be sent across the trunk line don't even have a header placed on them; the remote switch will assume that any frame that has no header is destined for the native vlan.
The problem with ISL is that it doesn't understand what a native vlan is. Every single frame will be encapsulated, regardless of the vlan it's destined for.
Switching theory is a big part of your CCNA studies, and it can seem overwhelming at first. Just break your studies down into smaller, more manageable parts, and soon you'll see the magic letters "CCNA" behind your name!
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