Cisco CCNA Certification: Cisco Switching Modes Tutorial

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.

The middle ground between these two extremes is Fragment-Free, so named since fragmented frames will not be forwarded. The switch examines only the first 64 bytes of the frame for errors, since that is the part of the frame that will be damaged in case of a collision. There is error checking, but it is not as thorough as Store-and-Forward.

Keeping these three switching schemes straight is vital to your CCNA exam efforts, and it will help you in working with Cisco switches in the real world as well. Keep studying!

Cisco CCNA / CCNP Home Lab Tutorial: The (Many) Cable Types And Their Purposes

One of the most common questions I get from CCNA and CCNP candidates who are setting up their own home labs is "What cables will I need?" The answer is "It depends." As you know from your exam studies, the physical layout of your lab is what determines the cables you'll need. Let's take a look at the most common home lab cable types and when you will need them.

Straight-through cables have quite a few uses in a CCNA / CCNP home lab. You'll need them to connect a switch port to an AUI port on a router (and you'll need a transceiver for that as well). If you have an ISDN simulator, straight-through cables can be used to connect a router's BRI port to the simulator.

Crossover cables are used to connect switches and allow them to trunk. If at all possible, get two switches in your home lab. This will allow you to gain valuable experience in manipulating root bridge election, working with STP, and creating EtherChannels.

DTE/DCE cables are used to connect two routers via their serial cables. If you are planning on using a frame relay switch in your lab, you'll need several of these. You can also get some great practice in by directly connecting two routers and bringing the connection up (and making sure it stays up!). This is valuable practice for your CCNA exam.

Octal cables are used to connect an access server to each of the other routers and switches in your lab.

Finally, there's that precious blue cable, the rollover cable. Rollover cables (sometimes called "rolled cables") allow you to connect a host device directly to a router or switch's console port. These cables have a way of disappearing around an IT shop, so make sure to take one home - and leave it there!

CCNA Certification Exam Tutorial: Cisco Switching Modes

To pass the CCNA exam and earn this important certification, you’ve got to know switching inside and out. While you’re learning all the basic switching theory, make sure to spend some time with the one of three switching modes Cisco routers can use.

Store-and-Forward is exactly what it sounds like. The entire frame will be stored before it is forwarded. This mode allows for the greatest amount of error checking, since a CRC (Cyclical Redundancy Check) is run against the frame before it is forwarded. If the frame contains an error, it is discarded. If there’s no problem with the frame, the frame is then forwarded to its proper destination.


While store-and-forward does perform error checking, the delay in processing the frame while this error check is run results in higher latency than the other modes you’re about to read about. The latency time can also vary, since not all frames are the same size.

Cut-through switching copies only the destination MAC address into its memory before beginning to forward the frame. Since the frame is being forwarded as soon as the destination MAC is read, there is less latency than store-and-forward. The drawback is that there is no error checking.

There is a middle ground, fragment-free switching. Only part of the frame is copied to memory before it is forwarded, but it’s the first 64 bytes of the frame, not just the destination MAC. (Why? Because if there is a problem with the frame, it’s most likely in the first 64 bytes.) There is a little more error checking than cut-through, but not as much latency as with store-and-forward.

Note that the latency of both cut-through and fragment-free is fixed; these modes always look at the first six or 64 bytes, respectively. Store-and-forward's latency depends on the size of the frame.
Learning the similarities and differences between these modes is an often-overlooked part of CCNA studies. Spend some time studying this important CCNA topic – you’ll be glad you did!