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.
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When you start studying for your CCNA and CCNP exams, many books will present you with a huge list of keystroke shortcuts for use on Cisco routers. While the 640-801, 811, and 821 exams may ask you about one or two of these, you really have to get hands-on experience with these commands to master them. Even better, there are some key combinations that Cisco routers mention, but then don’t tell you what they are! Let’s take a look at a few of the more helpful key combinations, and conclude with the “secret” way to stop a ping or traceroute.

The up arrow on your keyboard is great for repeating the last command you typed. Let’s say you mis-enter an access-list. Instead of typing it from the beginning, just hit your up arrow to repeat it, then fix the problem.

CTRL-A takes the cursor to the beginning of a typed line. If you’ve written an extended ACL, you know that can be a very long command, and one you probably don’t want to retype. If you get a carat indicating there is a problem with the line, use your up arrow to repeat the command. If you see the error is near the beginning, use CTRL-A to move the cursor immediately to the beginning of the line. CTRL-E takes the cursor to the end of a typed line.

To move the cursor through a typed line without erasing characters, you’ve got a couple of options. I personally like to use the left and right arrows, but you can also use CTRL-B to move back and CTRL-F to move forward.

Finally, there’s the combination that Cisco mentions to you when you run ping or traceroute, but they don’t tell you what it is! If you send an extended ping or a traceroute, you could be looking at asterisks for a long time if you don’t know this one. In the following example, a traceroute is obviously failing:

R2#traceroute 10.1.1.1

Type escape sequence to abort.

Tracing the route to 10.1.1.1

1 * * *

2 *

The problem is that you’re going to get 30 rows of those asterisks, which is frustrating and time-consuming at the same time. Note the router console message “Type escape sequence to abort”. That’s helpful – but what is it?

Here it is: Just type CTRL-SHIFT-6 twice, once right after the other. You won’t see anything on the router console, but the traceroute will terminate.
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Passing the CCNA, Intro, and ICND exam is all about knowing and noticing the details. (Which makes perfect sense, since becoming a master networking administrator or engineer is also about noticing the details!) One such detail knows the difference between error detection and error recovery. While the terms are sometimes used interchangeably, they are not the same thing.

Error detection is just that – error detection only. Two common error detection methods are found at the Data Link layer of the OSI model, the FCS (Frame Check Sequence) and CRC (Cyclical Redundancy Check). A mathematical equation is run against the data in the frame, and the result is sent along with the data. The receiver runs the equation again, but this time. If the result is the same, the frame is considered valid; if the result is different, the frame is considered corrupt and is discarded.
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When you’re studying to pass the CCNA exam and earn your certification, you’re introduced to a great many terms that are either totally new to you or seem familiar, but you’re not quite sure what they are. The term “broadcast domain” falls into the latter category for many CCNA candidates.

A broadcast domain is simply the group of end hosts that will receive a broadcast sent out by a given host. For example, if there are ten host devices connected to a switch and one of them sends a broadcast, the other nine devices will receive the broadcast. All of those devices are in the same broadcast domain.

Of course, we probably don’t want every device in a network receiving every single broadcast sent out by any other device in the network! This is why we need to know what devices can create multiple, smaller broadcast domains. Doing so allows us to limit the broadcasts traveling around our network – and you might be surprised how much traffic on some networks consists of unnecessary broadcasts.

Using the OSI model, we find devices such as hubs and repeaters at Layer One. This is the Physical layer, and devices at this layer have no effect on broadcast domains.
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When you’re studying to pass the CCNA exam and earn your certification, you’re introduced to a great many terms that are either totally new to you or seem familiar, but you’re not quite sure what they are. The term “collision domain” falls into the latter category for many CCNA candidates.

What exactly is “colliding” in the first place, and why do we care? It’s the data that is being sent out onto an Ethernet segment that we’re concerned with here. Ethernet uses Carrier Sense Multiple Access / Collision Detection (CSMA/CD) to avoid collisions in the first place. CSMA/CD is a set of rules dictating when hosts on an Ethernet segment can and cannot transmit data. Basically, a host that wants to transmit data will “listen” to the ethernet segment to see if another host is currently transmitting. If no one else is transmitting, the host will go forward with its own transmission.

This is an effective way of avoiding a collision, but it is not foolproof. If two hosts follow this procedure at the exact same time, their transmissions will collide on the Ethernet segment and both transmissions will become unusable. The hosts that sent those two transmissions will then send a jam signal out onto the segment, indicating to all other hosts that they should not send data. The two hosts will each start a random timer, and at the end of that time each host will begin the listening process again.
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To pass the CCNA exam and earn that coveted certification, you’ve got to know Cisco switches inside and out. Among the many important details you’ve got to know are the three methods that Cisco switches use to forward frames, and the differences between the three.

The first switching method is Store-and-Forward. The name is the recipe, because that’s just what the switch does – it stores the entire frame before beginning to forward it. This method allows for the greatest amount of error checking, since the Frame Check Sequence (FCS) can be run before the frame is forwarded. As always, there is a tradeoff, since this error checking process makes this the slowest of the three frame forwarding methods.

The quickest method is Cut-Through, where only the destination MAC address of the frame is examined before the forwarding process begins. This means that the part of the frame is actually being forwarded as it is still being received! The tradeoff here is that the FCS does not run, so there is absolutely no error checking with Cut-Through switching.
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