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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When you begin your CCNA studies, you get hit with a lot of different networking terms right away that you might not be familiar with. What makes it a little more confusing is that a lot of these terms sound a lot alike. Here, we’re going to discuss the differences between broadcasts, multicasts, and unicasts at both the Data Link (Layer 2) and Network (Layer 3) layers of the OSI model.

A broadcast is simply a unit of information that every other device on the segment will receive. A broadcast is indicated by having every bit of the address set to its highest possible value. Since a hexadecimal bit’s highest value is “f”, a hexadecimal broadcast is ff-ff-ff-ff-ff-ff (or FF-FF-FF-FF-FF-FF, as the upper case does not affect hex value). The CCNA exam will demand you be very familiar with hex conversions, so if you’re not comfortable with these conversions, get comfortable with them before taking the exam!

At layer 3, a broadcast is indicated by setting every bit in the 32-bit binary string to “1″, making the dotted decimal value 255.255.255.255. Every host on a segment will receive such a broadcast. (Keep in mind that switches will forward a broadcast, but routers do not.) In contrast to a broadcast, a unicast is a packet or frame with only one destination.
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To be truly prepared for your CCNA and CCNP exams, you need real hands-on experience with real Cisco routers and switches. However, a production network is a really bad place to practice your configurations, but an excellent way to get fired and/or sued. The key to becoming a true CCNA and CCNP is assembling your own Cisco home lab.

You don’t have to spend a lot of money to do so; used Cisco equipment is cheaper than ever. It’s robust as well – I’ve bought literally hundreds of used routers and switches over the years and have had very few problems. I owe much of my skill to practicing configurations and troubleshooting in my own home lab.
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Candidates preparing to pass the CCNP exams are putting together Cisco home labs like never before. With CCNA and CCNP home lab equipment more affordable than ever, candidates have realized the importance of working on real Cisco routers and switches on the way to earning their certifications.

One question I get often from CCNP candidates is goes something like this: “I’m planning on pursuing the CCIE after I get my CCNP. What kind of routers and switches should I buy now in order to use the same equipment in my CCIE home lab?”

This is going to sound strange coming from me, because I’m the #1 proponent of CCNA and CCNP candidates buying their own home labs. When it comes to the CCIE, though, I have to say that you’re probably better off using rack rentals instead of investing big money now to buy more expensive equipment with which to earn your CCNP.
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A Cisco access server is generally the last item a CCNA or CCNP candidate has on their mind when they’re putting together a home lab. The thinking tends to be that since this router isn’t really doing anything in the production part of your practice lab, it’s not really important.

Once you have more than two devices in your home lab, though, you’ll realize that constantly moving the console cable around from one router to another gets very tiresome. That’s what an access server does for a home lab – it allows you to connect your PC to a single device when working in your home lab, with no need to constantly disconnect and reconnect the console cable. The console cable will be connected directly to the access server, and the access server is connected to all the other devices in your home lab. Once you start working with one, you’ll wonder how you got along without it!

The term “access server” is a little misleading. This is not a server in the traditional sense, it’s a Cisco router with asynchronous serial ports. It is these ports that you’ll use to connect to the other devices in your home lab. Two affordable models of access servers are Cisco 2509s and Cisco 2511s. They can be found on ebay as well as other vendors on the Net.

You will also need an octal cable. On one end, the cable has a large connector that will connect to the access server. The other end is actually eight separate cables, each with RJ-45 connectors. These connectors are numbered 1 – 8 and will be connected to the console port on each router and switch. It is important to note the number on each connector you’re connecting to the other lab devices.

Now that you’ve got the physical equipment, let’s take a look at a typical configuration of an access server:

no service password-encryption

no service udp-small-servers

no service tcp-small-servers

! hostname BRYANT_ADVANTAGE_AS4

no ip domain-lookup

ip host r1 2001 10.4.4.4

ip host r2 2002 10.4.4.4

ip host r3 2003 10.4.4.4

ip host sw1 2004 10.4.4.4

ip host sw2 2005 10.4.4.4

ip host FrameSwitch 2006 10.4.4.4

interface Loopback555

ip address 10.4.4.4 255.255.255.0

line con 0

exec-timeout 0 0

logging synchronous\

line 1 16

no exec

transport input all

You can assign any loopback address and number here the important thing to note is that the IP HOST table you will build constantly refers back to the loopback address on the access server.

In this configuration, I have the octal cable’s connector 1 in R1, 2 in R2, 3 in R3, 4 in SW1, 5 in SW2, and 6 in my frame relay switch. The number “2001″ in the first line of the IP HOST table refers to that connector. That’s why it is important to note the number on a given connector you place in the console port of a router or switch.

The asynchonous lines are identified by “line 1 16″. This access server has 16 possible connections many will just have 8, which is usually plenty. Regardless of how many lines you have, you’ll need the commands transport input all and no exec to allow reverse telnet to work effectively.

There’s one more thing to watch out for. When you first connect to the AS for a practice session, you will need to open the line to each device by using the full hostname of the device as shown in the IP HOST table. Here, you would begin by entering R1, R2, R3, SW1, SW2, and FrameSwitch to open the line to each device. After that, you need to enter the line number – 1, 2, 3, 4, 5, and 6. It sounds a little confusing at first, but after just a few minutes of practice you’ll be doing it without even thinking about it.

When you are working in your home lab, you will not go from one device to another that is, when you are done on R1 and want to configure R2, you must go back to the access server and then to R2. The keystroke to do this is . Again, it may sound complicated, but after a little practice you will again do this without thinking about it.

Adding an access server to your CCNA or CCNP home lab may not be on your mind now, but once you add a few more routers or switches to the lab, you’ll want to spend more time configuring and practicing and less time moving a cable around. And once you get one, you’ll wonder how you did without it!

Occasionally, during your CCNA and CCNP studies, you’ll run into a term that just doesn’t quite make sense to you. (Okay, more than occasionally!) One such term is “reverse telnet”. As a Cisco certification candidate, you know that telnet is simply a protocol that allows you to remotely connect to a networking device such as a router or switch. But what is “reverse telnet”, and why is it so important to a Cisco CCNA / CCNP home lab setup?

Where a telnet session is started by a remote user who wants to remotely control a router or switch, a reverse telnet session is started when the host device itself imitates the telnet session.

In a CCNA / CCNP home lab, reverse telnet is configured and used on the access server. The access server isn’t a white box server like most of us are used to; an access server is a Cisco router that allows you to connect to multiple routers and switches with one session without having to move a rollover cable from device to device.

Your access server will use an octal cable to connect to the other routers and switches in your home lab. The octal cable has one large serial connector that will connect to the access server, and eight rj-45 connectors that will connect to your other home lab devices. Your access server then needs an IP Host table in order to perform reverse telnet.

An IP Host table is easy to put together (and you better know how to write one to pass the CCNA!). The IP Host table is used for local name resolution, taking the place of a DNS server. A typical access server IP Host table looks like this:

ip host FRS 2007 100.1.1.1

ip host R3 2003 100.1.1.1

ip host R1 2001 100.1.1.1

ip host R2 2002 100.1.1.1

ip host R4 2004 100.1.1.1

ip host R5 2005 100.1.1.1

ip host SW1 2006 100.1.1.1

interface Loopback0

ip address 100.1.1.1 255.255.255.255

no ip directed-broadcast

This configuration will allow you to use your access server to connect to five routers, a frame relay switch, and a switch without ever moving a cable. When you type “R1″ at the console line, for example, you’ll be connected to R1 via reverse telnet. If you have a smaller lab, an access server is still a real timesaver and an excellent investment. And by getting a static IP address to put on your access server, you can even connect to your home lab from remote locations!