To pass your CCNA exam and earn this coveted certification, you've got to master the seven layers of the OSI model and what each layer does. For those of you taking the two-exam path, you can expect quite a few OSI model questions on the Intro exam. In this seven-part series, we'll spend some time taking a look at each of the OSI model layers, starting with the Physical layer.
Often, CCNA candidates ask if the OSI model has any practical uses for network administrators. I used to wonder the same thing, and I can now tell you that the answer is definitely yes!
The OSI model isn't something you want to memorize and then forget about, as using the OSI model gives you a structured approach for troubleshooting. Whenever a network device isn't working properly, I always say to "start at the physical layer". The Physical layer is Layer One of the OSI model, and this is where troubleshooting should always start. Is the device on? Is it properly connected? If everything is fine at Layer One, you just move up to Layer Two, and continue in this structured fashion until the problem is identified.
The Physical layer is the layer at which bits are transmitted over the physical media. There is no routing or switching going on at this layer. The data has been broken down into more manageable pieces until the data takes the form of ones and zeroes at the Physical layer.
Even though there's no routing or switching at the Physical layer, CCNA candidates should be familiar with a couple of network devices that work at Layer One. A repeater is a device that regenerates an electrical signal, allowing the signal to travel longer distances without fading. (The process of an electrical signal gradually fading in strength over distance is "attenuation".) A hub is basically a multiport repeater, and both of these devices are considered Physical layer devices. Ethernet and Token Ring both operate at the Physical layer as well.
Learning the OSI model's Physical layer isn't just important in your CCNA exam studies, it's the first step in any network troubleshooting. After all, your network's end users are going to have a tough time sending print jobs to a printer that's turned off!
Showing posts with label ethernet. Show all posts
Showing posts with label ethernet. Show all posts
Wednesday, December 24, 2008
Cisco CCNA Certification: Defining And Creating Collision Domains
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.
Now that we know what a collision is, and what CSMA/CD is, we need to be able to define a collision domain. A collision domain is any area where a collision can theoretically take place, so only one device can transmit at a time in a collision domain.
In another free CCNA certification tutorial, we saw that broadcast domains were defined by routers (default) and switches if VLANs have been defined. Hubs and repeaters did nothing to define broadcast domains. Well, they don't do anything here, either. Hubs and repeaters do not define collision domains.
Switches do, however. A Cisco switchport is actually its own unshared collision domain! Therefore, if we have 20 host devices connected to separate switchports, we have 20 collision domains. All 20 devices can transmit simultaneously with no danger of collisions. Compare this to hubs and repeaters - if you have five devices connected to a single hub, you still have one large collision domain, and only one device at a time can transmit.
Mastering the definition and creation of collision domains and broadcast domains is an important step toward earning your CCNA and becoming an effective network administrator. Best of luck to you in both these worthwhile pursuits!
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.
Now that we know what a collision is, and what CSMA/CD is, we need to be able to define a collision domain. A collision domain is any area where a collision can theoretically take place, so only one device can transmit at a time in a collision domain.
In another free CCNA certification tutorial, we saw that broadcast domains were defined by routers (default) and switches if VLANs have been defined. Hubs and repeaters did nothing to define broadcast domains. Well, they don't do anything here, either. Hubs and repeaters do not define collision domains.
Switches do, however. A Cisco switchport is actually its own unshared collision domain! Therefore, if we have 20 host devices connected to separate switchports, we have 20 collision domains. All 20 devices can transmit simultaneously with no danger of collisions. Compare this to hubs and repeaters - if you have five devices connected to a single hub, you still have one large collision domain, and only one device at a time can transmit.
Mastering the definition and creation of collision domains and broadcast domains is an important step toward earning your CCNA and becoming an effective network administrator. Best of luck to you in both these worthwhile pursuits!
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!
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 Tutorial: Home Lab Assembly Case Study
Part of your CCNA / CCNP education is deciding what network topology to use when you're putting together your home lab. Some of you are starting with one or two routers or switches, while others are starting with more. A customer recently sent me a list of his Cisco routers and switches that he has available for a home lab and asked for my help in coming up with the best way to use them.
There is no "right" or "wrong" answer to this question; again, part of the learning process is configuring and reconfiguring the physical topology of your lab. Let's look at the routers and switches he has available, including the interfaces on each, and come up with one possible CCNA / CCNP home lab setup.
The equipment list:
Two 3620 routers. Each has 1 serial port and 2 ethernet ports.
One 3640 router. This has two ethernet cards, each with two ports, and two AUI ports.
Three 2503s, my personal favorite for home labs! These have 1 AUI port, 2 serial interfaces, and one BRI interface apiece.
One 2524 router. This has one serial port, 1 ethernet port, and one BRI interface.
One 4500 router. This has eight BRI ports, 2 ethernet ports, and more importantly, four serial ports.
He also has a 5200 access server, an ISDN simulator, one 2924 switch, and one 1924 switch.
Now, if you don't have this much equipment to work with, don't panic! Most CCNA / CCNP candidates don't; this is more of an exercise in looking at what you do have and using it to the utmost.
As I've mentioned in many of my CCNA / CCNP home lab articles, an access server is a great thing to have. All he needs is an octal cable to connect his AS to the other devices we choose to use, and he's all set. (If you need an access server sample configuration, there is one on my website in the Home Lab section.)
A frame relay switch is also great to have, and the 4500 will make a great FR switch. Having a frame relay cloud in your CCNA / CCNP home lab is a great way to get experience configuring and troubleshooting frame relay, an essential skill for CCNA success.
I would put both of the 3620s on the frame relay cloud via the Serial interface, as well as two of the 2503s. That gives you four routers that will be using frame relay to communicate, and that's the most we can have since the 4500 has four serial ports. The 4500 will need to be configured as a frame relay switch and connected to the other routers via a DTE/DCE cable. (Again, if you need a frame relay switch configuration, the one I use in my pods is on the website in the same place as the access server configuration.)
The two 2503s that are on the frame relay cloud should also be connected via their BRI interfaces. The home lab also includes an ISDN simulator, which is necessary to allow routers to communicate via their BRI interfaces. Just get a couple of straight-through cables to connect those two routers to the ISDN simulator and that segment is ready to go. (Remember that you can't connect Cisco routers directly via their BRI interfaces.)
All of the routers in this lab have at least one ethernet or AUI port, so we can connect them all to either one of the switches. The switches should be connected via at least two crossover cables to allow practice with trunking, root bridge election, and VLANs. Having two switches really does add quite a bit to a CCNA / CCNP home lab's capabilities. You can experiment with different subnets and vlans with as well. Don't be afraid to dive in - that's what a home lab is all about!
So now we've got four routers connected via frame relay, two via ISDN, and the others via ethernet segments. Two of the routers that are not using their serial interfaces should be connected directly via their serial ports. For this, you'll just need another DTE/DCE cable. Knowing how to bring up the line between two directly connected serial ports is an important CCNA skill, and so is troubleshooting it. You should be able to bring such a connection up with your eyes closed, and once you work with your own CCNA / CCNP home lab, you'll be able to!
Also, don't forget to add a loopback interface to each one of your routers. I like to use 1.1.1.1 for R1, 2.2.2.2 for R2, and so on. Advertising loopbacks is another great way to get practice with RIP, OSPF, EIGRP, IGRP, and static routing.
We've taken a pile of routers and switches and turned them into a fantastic CCNA / CCNP home lab. Whether you're working with two Cisco devices or ten, coming up with your own home lab topology is a great learning experience and the beginning of developing your analytical and troubleshooting skills.
There is no "right" or "wrong" answer to this question; again, part of the learning process is configuring and reconfiguring the physical topology of your lab. Let's look at the routers and switches he has available, including the interfaces on each, and come up with one possible CCNA / CCNP home lab setup.
The equipment list:
Two 3620 routers. Each has 1 serial port and 2 ethernet ports.
One 3640 router. This has two ethernet cards, each with two ports, and two AUI ports.
Three 2503s, my personal favorite for home labs! These have 1 AUI port, 2 serial interfaces, and one BRI interface apiece.
One 2524 router. This has one serial port, 1 ethernet port, and one BRI interface.
One 4500 router. This has eight BRI ports, 2 ethernet ports, and more importantly, four serial ports.
He also has a 5200 access server, an ISDN simulator, one 2924 switch, and one 1924 switch.
Now, if you don't have this much equipment to work with, don't panic! Most CCNA / CCNP candidates don't; this is more of an exercise in looking at what you do have and using it to the utmost.
As I've mentioned in many of my CCNA / CCNP home lab articles, an access server is a great thing to have. All he needs is an octal cable to connect his AS to the other devices we choose to use, and he's all set. (If you need an access server sample configuration, there is one on my website in the Home Lab section.)
A frame relay switch is also great to have, and the 4500 will make a great FR switch. Having a frame relay cloud in your CCNA / CCNP home lab is a great way to get experience configuring and troubleshooting frame relay, an essential skill for CCNA success.
I would put both of the 3620s on the frame relay cloud via the Serial interface, as well as two of the 2503s. That gives you four routers that will be using frame relay to communicate, and that's the most we can have since the 4500 has four serial ports. The 4500 will need to be configured as a frame relay switch and connected to the other routers via a DTE/DCE cable. (Again, if you need a frame relay switch configuration, the one I use in my pods is on the website in the same place as the access server configuration.)
The two 2503s that are on the frame relay cloud should also be connected via their BRI interfaces. The home lab also includes an ISDN simulator, which is necessary to allow routers to communicate via their BRI interfaces. Just get a couple of straight-through cables to connect those two routers to the ISDN simulator and that segment is ready to go. (Remember that you can't connect Cisco routers directly via their BRI interfaces.)
All of the routers in this lab have at least one ethernet or AUI port, so we can connect them all to either one of the switches. The switches should be connected via at least two crossover cables to allow practice with trunking, root bridge election, and VLANs. Having two switches really does add quite a bit to a CCNA / CCNP home lab's capabilities. You can experiment with different subnets and vlans with as well. Don't be afraid to dive in - that's what a home lab is all about!
So now we've got four routers connected via frame relay, two via ISDN, and the others via ethernet segments. Two of the routers that are not using their serial interfaces should be connected directly via their serial ports. For this, you'll just need another DTE/DCE cable. Knowing how to bring up the line between two directly connected serial ports is an important CCNA skill, and so is troubleshooting it. You should be able to bring such a connection up with your eyes closed, and once you work with your own CCNA / CCNP home lab, you'll be able to!
Also, don't forget to add a loopback interface to each one of your routers. I like to use 1.1.1.1 for R1, 2.2.2.2 for R2, and so on. Advertising loopbacks is another great way to get practice with RIP, OSPF, EIGRP, IGRP, and static routing.
We've taken a pile of routers and switches and turned them into a fantastic CCNA / CCNP home lab. Whether you're working with two Cisco devices or ten, coming up with your own home lab topology is a great learning experience and the beginning of developing your analytical and troubleshooting skills.
Cisco CCNA / CCNP Home Lab Tutorial: Using 2520 Routers
I know from experience that part of the excitement and anxiety of putting together your own CCNA / CCNP home lab is deciding what to buy! While you can make a workable home lab out of almost any combination of Cisco routers and switches, some routers are better suited for home lab work than others because they can fill multiple roles.
My personal favorite is the Cisco 2520. This router has four serial interfaces, making it an ideal frame relay switch. Don't forget that just because you're using a router as a frame switch, you can still use its routing capabilities. One setup I use is to use three of the four serial interfaces for frame switching and the fourth interface as a point-to-point network with another router. All you need is some DTE/DCE cables and you're all set.
The 2520 also comes with one ethernet interface and an ISDN interface, so that gives you even more options. Even if you're not planning to run ISDN in your home lab right now, you may choose to do so in the future - and with a 2520, you've already got the right router to do so. Keep in mind that if you are going to run ISDN in your home lab, you’ll need an ISDN device such as an ISDN simulator in your lab. (ISDN simulators are physical devices and are plentiful on ebay – they’re no relation to “router simulators”.)
Again, I want to reiterate that you can work any Cisco router into a CCNA / CCNP home lab - there's no "right" or "wrong" combination of equipment. But as with anything else, some combinations are better than others, so consider adding some 2520s to your home lab! This router gives you a great combination of interfaces and capabilities, plus the most important factor of all - real hands-on experience during your CCNA and CCNP exam preparation!
My personal favorite is the Cisco 2520. This router has four serial interfaces, making it an ideal frame relay switch. Don't forget that just because you're using a router as a frame switch, you can still use its routing capabilities. One setup I use is to use three of the four serial interfaces for frame switching and the fourth interface as a point-to-point network with another router. All you need is some DTE/DCE cables and you're all set.
The 2520 also comes with one ethernet interface and an ISDN interface, so that gives you even more options. Even if you're not planning to run ISDN in your home lab right now, you may choose to do so in the future - and with a 2520, you've already got the right router to do so. Keep in mind that if you are going to run ISDN in your home lab, you’ll need an ISDN device such as an ISDN simulator in your lab. (ISDN simulators are physical devices and are plentiful on ebay – they’re no relation to “router simulators”.)
Again, I want to reiterate that you can work any Cisco router into a CCNA / CCNP home lab - there's no "right" or "wrong" combination of equipment. But as with anything else, some combinations are better than others, so consider adding some 2520s to your home lab! This router gives you a great combination of interfaces and capabilities, plus the most important factor of all - real hands-on experience during your CCNA and CCNP exam preparation!
Cisco CCNA / CCNP Home Lab Tutorial: The 2503 Router
I know from experience that part of the excitement and anxiety of putting together your own CCNA / CCNP home lab is deciding what to buy! While you can make a workable home lab out of almost any combination of Cisco routers and switches, some routers are better suited for home lab work than others because they can fill multiple roles.
When you buy CCNA or CCNP "lab kits" - bundles of routers and switches - you can get a little confused about whether you're getting a good deal. One router I get asked about quite a bit is the 2503.
2503 routers are fantastic for CCNA and CCNP home labs. They come with two serial interfaces, allowing you to connect one interface directly to another router (you'll need a DTE/DCE cable for that, too) while connecting another to a frame relay switch if you like. If you don't have a frame relay switch, you can connect a 2503 directly to two other routers via the serial interfaces.
You also have an AUI port, which requires a transceiver to operate as your Ethernet interface. Transceivers are pretty cheap and readily available from Cisco resellers and ebay vendors, so pick one up for each 2503 you decide to buy.
2503 routers also come with a BRI interface. Even though you may not have an ISDN simulator right now, you may choose to add one later. That makes a 2503 a great bet for future lab expansion.
All in all, 2503 routers are great for your CCNA / CCNP home lab. They run about $125 each on ebay, or less, so they're also very affordable. There's no better training than training on your own CCNA or CCNP home lab, and you can always sell the equipment later if you like. Basically, you're renting the routers and switches, and the experience you get by working with the real deal is invaluable.
When you buy CCNA or CCNP "lab kits" - bundles of routers and switches - you can get a little confused about whether you're getting a good deal. One router I get asked about quite a bit is the 2503.
2503 routers are fantastic for CCNA and CCNP home labs. They come with two serial interfaces, allowing you to connect one interface directly to another router (you'll need a DTE/DCE cable for that, too) while connecting another to a frame relay switch if you like. If you don't have a frame relay switch, you can connect a 2503 directly to two other routers via the serial interfaces.
You also have an AUI port, which requires a transceiver to operate as your Ethernet interface. Transceivers are pretty cheap and readily available from Cisco resellers and ebay vendors, so pick one up for each 2503 you decide to buy.
2503 routers also come with a BRI interface. Even though you may not have an ISDN simulator right now, you may choose to add one later. That makes a 2503 a great bet for future lab expansion.
All in all, 2503 routers are great for your CCNA / CCNP home lab. They run about $125 each on ebay, or less, so they're also very affordable. There's no better training than training on your own CCNA or CCNP home lab, and you can always sell the equipment later if you like. Basically, you're renting the routers and switches, and the experience you get by working with the real deal is invaluable.
Cisco CCNA / CCNP Home Lab Tutorial: The 2503 Router
I know from experience that part of the excitement and anxiety of putting together your own CCNA / CCNP home lab is deciding what to buy! While you can make a workable home lab out of almost any combination of Cisco routers and switches, some routers are better suited for home lab work than others because they can fill multiple roles.
When you buy CCNA or CCNP "lab kits" - bundles of routers and switches - you can get a little confused about whether you're getting a good deal. One router I get asked about quite a bit is the 2503.
2503 routers are fantastic for CCNA and CCNP home labs. They come with two serial interfaces, allowing you to connect one interface directly to another router (you'll need a DTE/DCE cable for that, too) while connecting another to a frame relay switch if you like. If you don't have a frame relay switch, you can connect a 2503 directly to two other routers via the serial interfaces.
You also have an AUI port, which requires a transceiver to operate as your Ethernet interface. Transceivers are pretty cheap and readily available from Cisco resellers and ebay vendors, so pick one up for each 2503 you decide to buy.
2503 routers also come with a BRI interface. Even though you may not have an ISDN simulator right now, you may choose to add one later. That makes a 2503 a great bet for future lab expansion.
All in all, 2503 routers are great for your CCNA / CCNP home lab. They run about $125 each on ebay, or less, so they're also very affordable. There's no better training than training on your own CCNA or CCNP home lab, and you can always sell the equipment later if you like. Basically, you're renting the routers and switches, and the experience you get by working with the real deal is invaluable.
When you buy CCNA or CCNP "lab kits" - bundles of routers and switches - you can get a little confused about whether you're getting a good deal. One router I get asked about quite a bit is the 2503.
2503 routers are fantastic for CCNA and CCNP home labs. They come with two serial interfaces, allowing you to connect one interface directly to another router (you'll need a DTE/DCE cable for that, too) while connecting another to a frame relay switch if you like. If you don't have a frame relay switch, you can connect a 2503 directly to two other routers via the serial interfaces.
You also have an AUI port, which requires a transceiver to operate as your Ethernet interface. Transceivers are pretty cheap and readily available from Cisco resellers and ebay vendors, so pick one up for each 2503 you decide to buy.
2503 routers also come with a BRI interface. Even though you may not have an ISDN simulator right now, you may choose to add one later. That makes a 2503 a great bet for future lab expansion.
All in all, 2503 routers are great for your CCNA / CCNP home lab. They run about $125 each on ebay, or less, so they're also very affordable. There's no better training than training on your own CCNA or CCNP home lab, and you can always sell the equipment later if you like. Basically, you're renting the routers and switches, and the experience you get by working with the real deal is invaluable.
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