Your CCNA studies are going to include quite a bit of information
about switches, and for good reason. if you don't understand
basic switching theory, you can't configure and troubleshoot
Cisco switches, either on the CCNA exam or in the real world.
That goes double for trunking!
Trunking is simply enabling two or more switches to communicate
and send frames to each other for transmission to remote hosts.
There are two major trunking protocols that we need to know
the details of for exam success and real-world success, but
before we get to the protocols, let's discuss the cables we
need.
Connecting two Cisco switches requires a crossover cable.
As you know, 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. For instance, 2950 switches dynamically
trunk once you connect them with the right cable. If you use
the wrong cable, you'll be there a while!
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 is 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!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNP and CCNA tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, just visit the website!
You can also get FREE CCNA and CCNP exam questions every day!
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