Cisco CCNP/BCMSN Exam Tutorial: Spanning Tree Protocol (STP) Timers

In your BCMSN / CCNP exam study, it's easy to overlook some of the details of Spanning Tree Protocol (STP). After all, you learned all of that in your CCNA studies, right? Not necessarily! While some of the BCMSN material will be a review for you, there are some details regarding familiar topics that you need to learn. That includes the timers for STP - Hello Time, MaxAge, and Forward Delay.

You may remember these timers from your CCNA studies as well, and you should also remember that these timers should not be changed lightly. What you might not have known is that if you decide to change any and all of these timers, that change must be configured on the root bridge! The root bridge will inform the nonroot switches of the change via BPDUs.

Hello Time is the interval between BPDUs, two seconds by default.

Forward Delay is the length of both the listening and learning STP stages, with a default value of 15 seconds.

Maximum Age, referred to by the switch as MaxAge, is the amount of time a switch will retain a BPDU's contents before discarding it. The default is 20 seconds.

The value of these timers can be changed with the spanning-tree vlan command shown below. Verify the changes with the show spanning-tree command.

SW1(config)#spanning-tree vlan 1 ?

forward-time Set the forward delay for the spanning tree

hello-time Set the hello interval for the spanning tree

max-age Set the max age interval for the spanning tree

priority Set the bridge priority for the spanning tree

root Configure switch as root

SW1(config)#spanning-tree vlan 1 hello-time 5

SW1(config)#spanning-tree vlan 1 max-age 30

SW1(config)#spanning-tree vlan 1 forward-time 20

SW1(config)#^Z

SW1#show spanning-tree vlan 1

VLAN0001

Spanning tree enabled protocol ieee

Root ID Priority 32769

Address 000f.90e1.c240

This bridge is the root

Hello Time 5 sec Max Age 30 sec Forward Delay 20 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)

Address 000f.90e1.c240

Hello Time 5 sec Max Age 30 sec Forward Delay 20 sec

Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

Fa0/11 Desg FWD 19 128.11 P2p

Fa0/12 Desg FWD 19 128.12 P2p

Again, you should always take great care in changing these timers. Those defaults are set for a reason - helping to prevent switching loops!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (http://www.thebryantadvantage.com ), home of free CCNA and CCNP tutorials, and The Ultimate CCNA and CCNP Study Packages. For a copy of his FREE "How To Pass The CCNA" or "CCNP" ebook, visit the website and download copies! Our new RSS feed gives you free Cisco questions daily!

A Night In San Francisco

When traveling to most cities of the world, finding discount accommodations when arriving to your destination is always a possibility. Not so in the diverse and complex city of San Francisco. From the hills of Daly City to the plains of Berkeley, the environments and people are so diverse that your vacation could with one dial of the phone be a paradise made in heaven or an inferno designed in, well you know, that other place.

Families making their hotel reservations at the smaller hotels that dot the periphery of San Francisco might be able to land a bargain, but find they may find themselves sharing accommodations with other crawling critters that would make even the toughest among us parents squirm.

I did this once. I came to San Francisco on a budget and thinking to wing my accommodations; I found a phonebook and booked a motel for $59 a night. Well, the minute I saw the hotel along a poorly lit side street I knew we were in trouble. This hotel was something I would expect to see in medieval Europe, not in actuality. But it was late and we succumbed to our basal instincts for sleep and it was ?just one night.? But the bed was so grimy we actually slept on the floor, and the occasional voices we heard in the alleys were so hair razing I don?t think sleep was an option for any of us - and we were in their ?best room!?

The next day I called numerous hotels in San Francisco city proper. You know, where the Trolley Cars roam and the street performers perform. I found a number of places, ranging from the low 100?s and going up to the high 600?s. We got in our car to make our investigation and inspected one hotel after the other, some dark and dank and some bright and classic. We settled on the Grant Plaza Hotel at $180 a night. We had a rollaway brought for my two children and spent the rest of the day recuperating for our previous nightmare.

That evening, we ventured out to walk around the Streets of San Francisco and I must tell you it was scary. There were destitute people lurking at every doorway, down every alley, sleeping above the street vents for warmth. This was a haven for the homeless and I am not sure where the tourist fits in. It was both sad and a frightening, especially since coming from an isolated small town in Minnesota. I passed out quarters to those asking as if I were passing out flyers for a corner store.

We eventually found a Starbuck near Union Square and quickly ducked in. After a family meeting to discuss our situation, we decided to catch a trolley car and make a dash to the famous Fisherman?s Wharf and Ghirardelli?s Square. We scampered out of Starbucks and caught the next trolley. It was a wild ride straight up and straight down a city built on a mountain. We all screamed with joy. We spent an enchanting evening among all the tourist shops, street performers and family attractions that populate Fisherman?s Wharf. We bought a big pound of fudge at a fudge shop and drank coke and champagne until late in the evening.

Our lesson from all this, at least regarding San Francisco hotel reservations, make your hotel reservations in advance. Don?t waste time looking for a hotel room when you get there, the city is just too big. Since writing this, we went to San Diego. I booked all our accommodations off the Internet which featured discount hotel reservations with pictures and prices clearly explained. Welcome to the 21st century.

Douglas Anchel travels the USA in search of discount hotel reservations and accommodations and writes for All Reservations, Travelviva and World Accommodations

Cisco CCNP/BSCI Exam Tutorial: Configuring Eigrp Packet Authentication

Configuring RIPv2 and EIGRP authentication with key chains can be tricky at first, and the syntax isn't exactly easy to remember. But for BSCI and CCNP exam success, we've got to be able to perform this task.

In a previous tutorial, we saw how to configure RIPv2 packet authentication, with both clear-text and MD5 authentication schemes. EIGRP authentication is much the same, and has the text and MD5 authentication options as well. But EIGRP being EIGRP, the command just has to be a little more detailed!

As with RIPv2, the authentication mode must be agreed upon by the EIGRP neighbors. If one router's interface is configured for MD5 authentication and the remote router's interface is configured for text authentication, the adjacency will fail even if the two interfaces in question are configured to use the same password.

We'll now configure link authentication on the adjacency over an Ethernet segment. Below, you'll see how to configure a key chain called EIGRP on both routers, use key number 1, and use the key-string BSCI. Run show key chain on a router to see all key chains.

R2(config)#key chain EIGRP

R2(config-keychain)#key 1

R2(config-keychain-key)#key-string BSCI

R2#show key chain

Key-chain EIGRP:

key 1 -- text "BSCI"

accept lifetime (always valid) - (always valid) [valid now]

send lifetime (always valid) - (always valid) [valid now]

R3(config)#key chain EIGRP

R3(config-keychain)#key 1

R3(config-keychain-key)#key-string BSCI

R3#show key chain

Key-chain EIGRP:

key 1 -- text "BSCI"

accept lifetime (always valid) - (always valid) [valid now]

send lifetime (always valid) - (always valid) [valid now]

The EIGRP command to apply the key chain is a bit of a pain to remember, because the protocol and AS number is identified in the middle of the command, not the beginning. Also note that two commands are needed - one to name the key chain, another to define the authentication mode in use.

R2(config)#interface ethernet0

R2(config-if)#ip authentication key-chain eigrp 100 EIGRP

R2(config-if)#ip authentication mode eigrp 100 md5

5d07h: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.23.3 (Ethernet0) is down: keychain changed

R3(config)#interface ethernet0

R3(config-if)#ip authentication key-chain eigrp 100 EIGRP

R3(config-if)#ip authentication mode eigrp 100 md5

5d07h: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.23.2 (Ethernet0) is up:

As with RIPv2, the existing adjacency was torn down when one side was configured with authentication. If the key chain is correctly defined and applied on both sides, the adjacency will come back up. Always run show ip eigrp neighbor to make sure the adjacency is present. Learn the details of EIGRP key chains by configuring them on your home lab equipment, and you'll be more than ready for BSCI exam success!

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (http://www.thebryantadvantage.com ), home of free CCNA and CCNP tutorials, and The Ultimate CCNA and CCNP Study Packages. For a copy of his FREE "How To Pass The CCNA" or "CCNP" ebook, visit the website and download your copies!

Cisco Certification: Becoming A Truly Valuable CCNA

I've been active in the Cisco Certification track for four years, working my way from the CCNA to the coveted Cisco Certified Internetwork Expert title, and during that time I've conducted job interviews and casual conversations with hundreds of CCNAs and CCNA candidates.

The CCNA is an exciting beginning to your Cisco career, but just having the certification simply isn't enough. A recruiter or interviewer isn't going to be impressed just with the cert; you've got to have some real-world knowledge to back it up.

I've been down that road myself, and sat on both sides of the CCNA job interview table. With that in mind, I'd like to offer to you some tips on becoming a truly valuable and employable CCNA.

Get some hands-on experience. I know the trap well; you can't get experience until you get a CCNA, and you can't get a CCNA without real experience. Well, actually, you can, but do you want to?

Working on simulators is fine to a certain extent, but don't make the classic mistake of depending on them. I've seen plenty of CCNAs who were put in front of a set of routers and really didn't know what to do or how to put together a simple configuration, and had NO idea how to begin troubleshooting. Simulators fail to help you develop the analytical and logical skills that you must have to be an effective troubleshooter.

There are CCNA classes that offer you the chance to work with industry experts on real Cisco equipment. Beyond that, you can put together your own CCNA rack for less than $500 by buying used routers. Some people think that's a lot of money, but this is the foundation of your career. Treat it that way. The work you do now is the most important work you'll ever do. Do it on real Cisco equipment. The skills I learned as a CCNA helped me all the way up to the CCIE.

Besides, after you get your CCNA (and after that, hopefully you'll choose to pursue the CCNP), you can always get some of your money back by selling the equipment. The hands-on experience you gain this way is invaluable.

(Ill also soon be offering remote equipment access for CCNAs and CCNPs. Watch for it!)

Know binary math. Do NOT go the easy route of memorizing a subnet mask chart for the CCNA exam. I know some people brag about being able to pass the CCNA exam without really understanding binary math. I've seen those people on the other side of the interview table, and they're not laughing when I ask them to do a subnetting question. They're not laughing when they can't explain or create a VLSM scheme. That chart does nothing to help you understand what's going on.

If you can add and know the difference between a one and a zero, you can do binary math. Don't let the name intimidate you. Become a REAL CCNA -- learn binary math !

Run "show" and "debug" commands. No commands help you truly understand how things work in a Cisco network than show and debug commands. As you progress through the Cisco certification ranks, you'll be glad you started using these at the CCNA level.

Do you need to know these commands for the exam? Probably not. Do you need them to be successul in the real world? Absolutely.

The Cisco certification track has been great to me, and it can boost your career as well, whether you stop at the CCNA, CCNP, or go all the way to the CCIE. It's the skills you develop today that will truly make you a networking engineer. Don't take shortcuts or get the attitude of "just passing the exam".

It's what you achieve after the exam that counts, and it's the work you put in before passing the exam that makes those achievements possible.

Good luck !

Chris Bryant, CCIE #12933

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage. The Bryant Advantage's website offers FREE ebooks and tutorials for the CCNA and CCNP exams, FREE subscriptions to "Cisco Certification Central", and sells the best CCNA and CCNP prep courses and books on the market today. Visit his site at www.thebryantadvantage.com today!

chris@thebryantadvantage.com

Cisco CCNA Certification: Don't Overreact To Exam Version Changes

CCNA Certification: Don't Overreact To Exam Version Changes

Whenever a Cisco exam version changes, there's always a lot of chatter about it on the web. The CCNA exams are no exception.

One comment I see often goes like this: " I hear Cisco is going to change Intro / ICND / CCNA exam versions soon, so I'm not going to start studying yet. I'll wait until the new exam comes out." Do not let this happen to you. While some large publishers would have you think these exams change tremendously from one version to another ("updated for the latest exams!"), the simple fact is that the Intro, ICND, and CCNA Composite exams simply don't change much from version to version.Sure, the questions change.

The only people who should be nervous about that are those who are trying to braindump their way to a technical certification.The topics covered on the CCNA exams don't change much at all.

You know you're going to have to demonstrate knowledge of LAN switching, ISDN, Frame Relay, routing protocol behavior, RIP, IGRP, EIGRP, and OSPF. Perhaps some of the more advanced topics will change, but these will be minor changes at best. Cisco announces these changes on their website well in advance (and you'll read about them here, too), so you won't be left with no time to study.

Whatever you do, don't fall into the "version change". Don't spend $100 - $150 to hurry up and take an exam before you're ready because of an upcoming version change.When you're ready, you're ready. Time spent learning is never wasted. Get started NOW.

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage. The Bryant Advantage's website offers FREE ebooks and tutorials for the CCNA and CCNP exams, FREE subscriptions to "Cisco Certification Central", and sells the best CCNA and CCNP prep courses and books on the market today. Visit his site at www.thebryantadvantage.com today!

chris@thebryantadvantage.comCisco Certification: Becoming A Truly Valuable CCNAChris Bryant

I've been active in the Cisco Certification track for four years, working my way from the CCNA to the coveted Cisco Certified Internetwork Expert title, and during that time I've conducted job interviews and casual conversations with hundreds of CCNAs and CCNA candidates.

The CCNA is an exciting beginning to your Cisco career, but just having the certification simply isn't enough. A recruiter or interviewer isn't going to be impressed just with the cert; you've got to have some real-world knowledge to back it up.

I've been down that road myself, and sat on both sides of the CCNA job interview table. With that in mind, I'd like to offer to you some tips on becoming a truly valuable and employable CCNA.

Get some hands-on experience. I know the trap well; you can't get experience until you get a CCNA, and you can't get a CCNA without real experience. Well, actually, you can, but do you want to?

Working on simulators is fine to a certain extent, but don't make the classic mistake of depending on them. I've seen plenty of CCNAs who were put in front of a set of routers and really didn't know what to do or how to put together a simple configuration, and had NO idea how to begin troubleshooting. Simulators fail to help you develop the analytical and logical skills that you must have to be an effective troubleshooter.

There are CCNA classes that offer you the chance to work with industry experts on real Cisco equipment. Beyond that, you can put together your own CCNA rack for less than $500 by buying used routers. Some people think that's a lot of money, but this is the foundation of your career. Treat it that way. The work you do now is the most important work you'll ever do. Do it on real Cisco equipment. The skills I learned as a CCNA helped me all the way up to the CCIE.

Besides, after you get your CCNA (and after that, hopefully you'll choose to pursue the CCNP), you can always get some of your money back by selling the equipment. The hands-on experience you gain this way is invaluable.

(Ill also soon be offering remote equipment access for CCNAs and CCNPs. Watch for it!)

Know binary math. Do NOT go the easy route of memorizing a subnet mask chart for the CCNA exam. I know some people brag about being able to pass the CCNA exam without really understanding binary math. I've seen those people on the other side of the interview table, and they're not laughing when I ask them to do a subnetting question. They're not laughing when they can't explain or create a VLSM scheme. That chart does nothing to help you understand what's going on.

If you can add and know the difference between a one and a zero, you can do binary math. Don't let the name intimidate you. Become a REAL CCNA -- learn binary math !

Run "show" and "debug" commands. No commands help you truly understand how things work in a Cisco network than show and debug commands. As you progress through the Cisco certification ranks, you'll be glad you started using these at the CCNA level.

Do you need to know these commands for the exam? Probably not. Do you need them to be successul in the real world? Absolutely.

The Cisco certification track has been great to me, and it can boost your career as well, whether you stop at the CCNA, CCNP, or go all the way to the CCIE. It's the skills you develop today that will truly make you a networking engineer. Don't take shortcuts or get the attitude of "just passing the exam".

It's what you achieve after the exam that counts, and it's the work you put in before passing the exam that makes those achievements possible.

Good luck !

Chris Bryant, CCIE #12933

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage. The Bryant Advantage's website offers FREE ebooks and tutorials for the CCNA and CCNP exams, FREE subscriptions to "Cisco Certification Central", and sells the best CCNA and CCNP prep courses and books on the market today. Visit his site at www.thebryantadvantage.com today!

chris@thebryantadvantage.comCisco Certification: Debug and Show commands for ISDNChris Bryant

The major reason I recommend getting your hands on real Cisco equipment rather than a simulator is that real Cisco routers give you the chance to practice and learn show and debug commands.

The knowledge you acquire from debugs is invaluable. Frankly, its this knowledge that puts you above the average CCNA who doesnt have that hands-on experience. Watching debugs in action also gives you a head start on the CCNP. Since 90 95% of CCNAs go on to pursue the CCNP, its a great idea to get started with debugs now.

Dont make the mistake of waiting until youre studying for your CCNP and CCIE to start learning debugs and shows. The work you do for the CCNA is the foundation for everything youll do in the future.

Never, ever, ever practice debugs on a production network. There are debugs that will give you so much information that the router actually becomes overloaded and then locks up. Never practice debugs on a production network.

Its important to know the proper show and debug commands for ISDN for several reasons. First, by watching ISDN in operation, you can see its processes and better understand whats going on. Secondly, its difficult if not impossible to properly troubleshoot ISDN without knowing the proper show and debug commands. (Its easy to overlook an ISDN authentication error just by looking at the configuration, but running debug ppp negotiation will quickly show you where the problem lies.)

Lets take a look at the ISDN show and debug commands that every CCNA and CCNP should know.

Show ISDN Status

If you only know one ISDN show command, its got to be this one. Always use this command after configuring your ISDN switch type and any necessary SPIDs. The command will show you the switch type (and will also show you if you did not configure a switch type), and whether the SPIDs you entered are valid:

R1#show isdn statusGlobal ISDN Switchtype = basic-ni    ISDN BRI0 interface        dsl 0, interface ISDN Switchtype = basic-ni    Layer 1 Status:        ACTIVE    Layer 2 Status:        TEI = 91, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED        TEI = 92, Ces = 2, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED    Spid Status:        TEI 91, ces = 1, state = 5(init)            spid1 configured, no LDN, spid1 sent, spid1 valid               Endpoint ID Info: epsf = 0, usid = 1, tid = 1        TEI 92, ces = 2, state = 5(init)            spid2 configured, no LDN, spid2 sent, spid2 valid               Endpoint ID Info: epsf = 0, usid = 3, tid = 1    Layer 3 Status:        0 Active Layer 3 Call(s)    Activated dsl 0 CCBs = 0    The Free Channel Mask:  0x80000003

Once in a while, youll get this output from show isdn status:

R2#show isdn statusThe current ISDN Switchtype = basic-ni1ISDN BRI0 interface    Layer 1 Status:        ACTIVE    Layer 2 Status:        Layer 2 NOT Activated    Spid Status:        TEI Not Assigned, ces = 1, state = 3(await establishment)            spid1 configured, no LDN, spid1 NOT sent, spid1 NOT valid        TEI Not Assigned, ces = 2, state = 1(terminal down)            spid2 configured, no LDN, spid2 NOT sent, spid2 NOT valid

Check your running configuration, and if the SPIDs look good, simply close the BRI interface and open it again. Then run show ISDN status again. If you then see spids are valid, youre ready to proceed. If you still see a message that the spids are invalid, youve most likely mistyped the SPID.

Show Access-List

Whats this command got to do with ISDN? Everything.

Remember how the ISDN link comes up in the first place? Interesting traffic. By default, there is no interesting traffic. You define interesting traffic with the dialer-list and dialer-group commands, AND the access-list command. If you have a problem with your link never coming up or with it coming up and staying up, use this command to see what traffic has been defined as interesting.

Show Dialer

Another helpful command to determine why an ISDN link is coming up and staying up. This command shows you how many successful calls and failed calls have taken place, what the current idle-timer value is (by default, its 120 seconds), and most importantly, what the source and destination was for the current interesting traffic:

R1#show dialerBRI0 - dialer type = ISDNDial String      Successes   Failures    Last called   Last status8358662                  1                 0           00:00:59       successful0 incoming call(s) have been screened.0 incoming call(s) rejected for callback.BRI0:1 - dialer type = ISDNIdle timer (120 secs), Fast idle timer (20 secs)Wait for carrier (30 secs), Re-enable (15 secs)Dialer state is data link layer upDial reason: ip (s=172.12.21.1, d=172.12.21.2)   Time until disconnect 62 secs   Connected to 8358662 (R2)

Here, you can see that the idle-timer value is at its default, that there are 62 seconds left until the link comes down (unless interesting traffic resets the timer), and that the source of the interesting traffic was 172.12.21.1 and the destination is 172.12.21.2. If that destination is the multicast address of a routing protocol say, 224.0.0.5 for OSPF you know what traffic is keeping the line up.

Show ISDN History

Want to see what calls have been made in the last 15 minutes? Just run this command. Its helpful if youre walking in to an ISDN troubleshooting situation and want to see what calls have been made.

R1#show isdn history-------------------------------------------------------------------------------- ISDN CALL HISTORY--------------------------------------------------------------------------------History table has a maximum of 100 entries.History table data is retained for a maximum of 15 Minutes.--------------------------------------------------------------------------------Call Calling Called Remote Seconds Seconds Seconds ChargesType Number .Number Name Used Left Idle Units/Currency-----------------------------------------------------------------------------------------------------------Out 8358662 R2 121 0Out 8358662 R2 121 0-----------------------------------------------------------------------------------------------------

Debug PPP Negotiation

Not only do you need to know this command for your CCNA and CCNP exams, you MUST know it to be an effective ISDN troubleshooter.

When PPP authentication is first configured, its simple to mistype a password, or forget to configure a username / password combination. Instead of continually reading your running configuration to see what the problem is, run debug ppp negotiation and send a ping to bring the line up. Youll quickly see where the problem is.

R2#debug ppp negotiationPPP protocol negotiation debugging is onR2#ping 172.12.21.1BR0:1 PPP: Phase is AUTHENTICATING, by bothBR0:1 CHAP: O CHALLENGE id 1 len 23 from "R2"BR0:1 CHAP: I CHALLENGE id 1 len 23 from "R1"BR0:1 CHAP: O RESPONSE id 1 len 23 from "R2"BR0:1 CHAP: I SUCCESS id 1 len 4BR0:1 CHAP: I RESPONSE id 1 len 23 from "R1"BR0:1 CHAP: O SUCCESS id 1 len 4

By mastering these simple ISDN show and debug commands, you increase your chances of passing the CCNA and CCNP exams greatly, and vastly improve your on-the-job skills.

I recommend you visit www.cisco.com/univercd for more show and debug commands. Learning to negotiate this valuable online resource will truly help you become more valuable on the job. Every Cisco router and switch command, along with default and usage guidelines, can be found on this site. Ill have an article posted soon to help you learn your way around the site; in the meantime, I urge you to begin adding this website to your troubleshooting toolkit.

Chris Bryant
CCIE #12933

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage. The Bryant Advantage's website offers FREE ebooks and tutorials for the CCNA and CCNP exams, FREE subscriptions to "Cisco Certification Central", and sells the best CCNA and CCNP prep courses and books on the market today. Visit his site at www.thebryantadvantage.com today!

chris@thebryantadvantage.comCisco Certification: Configuring CHAP on ISDNChris Bryant, CCIE #12933

Cisco Certification: The Cisco Three-Layered Hierarchical Model

Cisco Certification: The Cisco Three-Layered Hierarchical Model

 by: www.SemSim.com

Cisco has defined a hierarchical model known as the hierarchical internetworking model. This model simplifies the task of building a reliable, scalable, and less expensive hierarchical internetwork because rather than focusing on packet construction, it focuses on the three functional areas, or layers, of your network:

Core layer: This layer is considered the backbone of the network and includes the high-end switches and high-speed cables such as fiber cables. This layer of the network does not route traffic at the LAN. In addition, no packet manipulation is done by devices in this layer. Rather, this layer is concerned with speed and ensures reliable delivery of packets.

Distribution layer: This layer includes LAN-based routers and layer 3 switches. This layer ensures that packets are properly routed between subnets and VLANs in your enterprise. This layer is also called the Workgroup layer.

Access layer: This layer includes hubs and switches. This layer is also called the desktop layer because it focuses on connecting client nodes, such as workstations to the network. This layer ensures that packets are delivered to end user computers.

Figure INT.2.1 displays the three layers of the Cisco hierarchical model.

When you implement these layers, each layer might comprise more than two devices or a single device might function across multiple layers.The benefits of the Cisco hierarchical model include:

High Performance: You can design high performance networks, where only certain layers are susceptible to congestion.

Efficient management & troubleshooting: Allows you to efficiently organize network management and isolate causes of network trouble.

Policy creation: You can easily create policies and specify filters and rules.

Scalability: You can grow the network easily by dividing your network into functional areas.

Behavior prediction: When planning or managing a network, the model allows you determine what will happen to the network when new stresses are placed on it.

Core Layer

The core layer is responsible for fast and reliable transportation of data across a network. The core layer is often known as the backbone or foundation network because all other layers rely upon it. Its purpose is to reduce the latency time in the delivery of packets. The factors to be considered while designing devices to be used in the core layer are:

High data transfer rate: Speed is important at the core layer. One way that core networks enable high data transfer rates is through load sharing, where traffic can travel through multiple network connections.

Low latency period: The core layer typically uses high-speed low latency circuits which only forward packets and do not enforcing policy.

High reliability: Multiple data paths ensure high network fault tolerance; if one path experiences a problem, then the device can quickly discover a new route.

At the core layer, efficiency is the key term. Fewer and faster systems create a more efficient backbone. There are various equipments available for the core layer. Examples of core layer Cisco equipment include:

Cisco switches such as 7000, 7200, 7500, and 12000 (for WAN use)

Catalyst switches such as 6000, 5000, and 4000 (for LAN use)

T-1 and E-1 lines, Frame relay connections, ATM networks, Switched Multimegabit Data Service (SMDS)

Distribution Layer

The distribution layer is responsible for routing. It also provides policy-based network connectivity, including:

Packet filtering (firewalling): Processes packets and regulates the transmission of packets based on its source and destination information to create network borders.

QoS: The router or layer 3 switches can read packets and prioritize delivery, based on policies you set.

Access Layer Aggregation Point: The layer serves the aggregation point for the desktop layer switches.

Control Broadcast and Multicast: The layer serves as the boundary for broadcast and multicast domains.

Application Gateways: The layer allows you to create protocol gateways to and from different network architectures.

The distribution layer also performs queuing and provides packet manipulation of the network traffic.

It is at this layer where you begin to exert control over network transmissions, including what comes in and what goes out of the network. You will also limit and create broadcast domains, create virtual LANs, if necessary, and conduct various management tasks, including obtaining route summaries. In a route summary, you consolidate traffic from many subnets into a core network connection. In Cisco routers, the command to obtain a routing summary is:

show ip route summary

You can practice viewing routing information using a free CCNA exam router simulator available from SemSim.com. You can also determine how routers update each other's routing tables by choosing specific routing protocols.

Examples of Cisco-specific distribution layer equipment include 2600,4000, 4500 series routers

Access Layer

The access layer contains devices that allow workgroups and users to use the services provided by the distribution and core layers. In the access layer, you have the ability to expand or contract collision domains using a repeater, hub, or standard switch. In regards to the access layer, a switch is not a high-powered device, such as those found at the core layer.

Rather, a switch is an advanced version of a hub.

A collision domain describes a portion of an Ethernet network at layer 1 of the OSI model where any communication sent by a node can be sensed by any other node on the network. This is different from a broadcast domain which describes any part of a network at layer 2 or 3 of the OSI model where a node can broadcast to any node on the network.

At the access layer, you can:

Enable MAC address filtering: It is possible to program a switch to allow only certain systems to access the connected LANs.

Create separate collision domains: A switch can create separate collision domains for each connected node to improve performance.

Share bandwidth: You can allow the same network connection to handle all data.

Handle switch bandwidth: You can move data from one network to another to perform load balancing

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