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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CCNA / CCNP candidates are going to be drilled by Cisco when it comes to troubleshooting questions. You’re going to have to be able to analyze configurations to see what the problem is (and if there is a problem in the first place), determine the meaning of different debug outputs, and show the ability not just to configure a router or switch, but troubleshoot one.

That’s just as it should be, because CCNAs and CCNPs will find themselves doing a lot of troubleshooting in their careers. Troubleshooting isn’t something that can just be learned from a book; you’ve got to have some experience working with routers and switches. The only real way to learn how to troubleshoot is to develop that ability while working on live equipment.
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As your CCNA / CCNP home lab expands, an access server such as the Cisco 2509 or 2511 is one of the best investments you can make. In this article, we’ll look at the basic configuration for an access server and discuss how to connect to the other routers and switches in your pod through the AS.

Here’s part of a configuration from one of my access servers:

ip host FRS 2006 100.1.1.1

ip host SW2 2005 100.1.1.1

ip host SW1 2004 100.1.1.1

ip host R2 2002 100.1.1.1

ip host R1 2001 100.1.1.1

ip host R3 2003 100.1.1.1

interface Loopback0

ip address 100.1.1.1 255.255.255.255

no ip directed-broadcast

This is an IP Host table, and this is what makes the entire AS setup work. Your PC will connect to the access server, and the access server is in turn physically connected to your other routers and switches via an octal cable. One end of the octal cable splices off into eight separate cables, each terminated with an Rj-45 connector. That connector will be placed into the console port of one of your home lab devices. In this configuration, I have connector 1 connected to the console port of R1, connector 2 to R2, connector 3 to R3, connector 4 to Sw1, and so forth. (The connectors are physically numbered as well.)

The IP Host table entries here are linked to the loopback address shown. The loopback can be any address, but it must match the address in the IP Host table. This allows you to create reverse telnet sessions to the routers and switches.

To open the reverse telnet sessions upon opening a connection to the AS, type the entire name of the device and press the enter key twice. A connection to that device will now be visible, as shown here:

Access_Server#r1

Trying R1 (100.1.1.1, 2001)… Open

R1#

To get back to the access server, use the key combination followed by pressing the “x” key. Keep doing this until you’ve opened a connection to every router and switch in your pod.

Once you’ve opened the lines, you will not use the full device name to connect to the home lab devices. You should press only the number corresponding to the reverse telnet session you opened. For instance, in this configuration I opened telnet session 1 to R1, session 2 to R2, and session 3 to R3. Once I opened those sessions, I just use those numbers to reconnect to the devices, as shown here:

Access_server#1

[Resuming connection 1 to r1 ... ]

R1#

Access_server#2

[Resuming connection 2 to r2 ... ]

R2#

Access_server#3

[Resuming connection 3 to r3 ... ]

R3#

If you type the full hostname again after initially opening the connection, you will see this message:

Access_server#r1

Trying R1 (100.1.1.1, 2001)…

% Connection refused by remote host

The connection is refused because you already have an open connection to that router.

There’s one more important part of an access server config your CCNA / CCNP home lab will need:

line 1 8

no exec

transport input all

The line numbers may differ according to your access server, but “no exec” is very important here. This will stop rogue EXEC sessions from refusing connections that it shouldn’t be refusing. Without this command, you’ll commonly see “connection refused by remote host” when you shouldn’t be. That message is the most common error you’ll see on an access server, and it’s there because you already have an open connection or you left “no exec” out of your configuration. “No exec” isn’t mandatory, but it will help you keep your sanity!

A Cisco home lab is an invaluable study tool when you’re preparing for CCNA and CCNP exam success. Once you’ve gotten a couple of routers and switches, you’ll quickly get tired of moving that blue console cable every time you want to configure a different device. The solution to this problem is purchasing and configuring an access server (AS).

For those of you new to access servers, note that these are not white boxes running Microsoft operating systems. These are Cisco routers that allow you to connect to all the routers and switches in your home lab without moving a cable. You can physically or logically connect to the access server and work with all your devices from there.

When you’re pricing access servers, please remember that you do NOT need an expensive AS. Right now on ebay there are access servers costing up to $5000 – this is NOT what you want to buy. What you’re looking for is something like a 2509 or 2511, which is going to run you anywhere from $100 – $200. It’s money well spent, because once you get an AS, you’ll really wonder how you ever did without it.

The only additional hardware you need is the cable that will physically connect your AS to the other routers and switches in your home lab. The cable you need is called an octal cable, so named because one end of this cable is actually eight ends, all terminated with a numbered RJ-45 connector.

The large end of the cable is going to be connected to the AS itself. The cable will connect to a port on the AS that will have “async 1-8″ directly above the physical port. It is this port that makes an AS different from other Cisco routers.

Once you’ve got your AS and this cable, you’re ready to configure your AS. Connect the cable to the AS as described above, and then you will connect one of the RJ-45 connectors to the console port of each one of your routers and switches. Make sure to note the number that’s on the cable itself right below the connector, because that’s very important. In the next part of this home lab tutorial, I’ll tell you exactly how to configure your access server for best results, along with a few troubleshooting tips.

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!