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From the CCNA to the CCIE, ISDN is one of the most important technolgies you’ll work with. It’s also very common in the field ISDN is frequently used as a backup connection in case an organization’s Frame Relay connections go down. Therefore, it’s important to know ISDN basics not only for your particular exam, but for job success.

ISDN is used between two Cisco routers that have BRI or PRI interfaces. Basically, with ISDN one of the routers places a phone call to the other router. It is vital to understand not only what causes one router to dial another, but what makes the link go down.

Why? Since ISDN is basically a phone call from one router to another, you’re getting billed for that phone call — by the minute. If one of your routers dials another, and never hangs up, the connection can theoretically last for days or weeks. The network manager then receives an astronomical phone bill, which leads to bad things for everyone involved!

Cisco routers use the concept of interesting traffic to decide when one router should call another. By default, there is no interesting traffic, so if you don’t define any, the routers will never call each other.

Interesting traffic is defined with the dialer-list command. This command offers many options, so you can tie interesting traffic down not only to what protocols can bring the link up, but what the source, destination, or even port number must be for the line to come up.

One common misconception occurs once that link is up. Interesting traffic is required to bring the link up, but by default, any traffic can then cross the ISDN link.

What makes the link come down? Again, the concept of interesting traffic is used. Cisco routers have an idle-timeout setting for their dialup interfaces. If interesting traffic does not cross the link for the amount of time specified by the idle-timeout, the link comes down.

To summarize: Interesting traffic brings the link up by default, any traffic can cross the link once it’s up a lack of interesting traffic is what brings the link down.

Just as important is knowing what keeps the link up once it is dialed. Why? Because ISDN acts as a phone call between two routers, and it’s billed that way to your client. The two routers that are connected by this phone call may be located in different area codes, so now we’re talking about a long distance phone call.

If your ISDN link does not have a reason to disconnect, the connection could theoretically last for days or weeks before someone realizes what’s going on. This is particularly true when the ISDN link is used as a backup for another connection type, as is commonly the case with Frame Relay. When the Frame Relay goes down, the backup ISDN link comes up when the Frame Relay link comes back not billed for all that time.

To understand why an ISDN link stays up when it’s not needed, we have to understand why it stays up period. Cisco’s ISDN interfaces use the idle-timeout to determine when an ISDN link should be torn down. By default, this value is two minutes, and it also uses the concept of interesting traffic.

Once interesting traffic brings the link up, by default all traffic can cross the link. However, only interesting traffic resets the idle-timeout. If no interesting traffic crosses the link for two minutes, the idle-timer hits zero and the link comes down.

If the protocol running over the ISDN link is RIP version 2 or EIGRP, the most efficient way to prevent the routing updates from keeping the line up is expressly prohibiting their multicast routing update address in the access-list that is defining interesting traffic. Do not prevent them from crossing the link entirely, or the protocol obviously won’t work correctly.

With OSPF, Cisco offers the ip ospf demand-circuit interface-level command. The OSPF adjacency will form over the ISDN link, but once formed, the Hello packets will be suppressed. However, the adjacency will not be lost. A check of the adjacency table with show ip ospf adjacency will show the adjacency remains at Full, even though Hellos are no longer being sent across the link. The ISDN link can drop without the adjacency being lost. When the link is needed, the adjacency is still in place and data can be sent without waiting for OSPF to go through the usual steps of forming an adjacency.

This OSPF command is vital for Cisco certification candidates at every level, but is particularly important for CCNA candidates. Learn this command now, get used to the fact that the adjacency stays up even though Hellos are suppressed, and add this valuable command to your Cisco toolkit.

One myth about ISDN is that Cisco Discovery Packets keep an ISDN link up. CDP is a Cisco-proprietary protocol that runs between directly connected Cisco devices. There is a school of thought that CDP packets have to be disabled on a BRI interface in order to prevent the link from staying up or dialing when it’s not really needed. I’ve worked with ISDN for years in the field and in the lab, and I’ve never seen CDP bring up an ISDN link. Try it yourself the next time you’re working on a practice rack!

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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.

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 $1000 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.

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 with your Cisco Certification.

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There’s quite an emphasis on security in today’s networks, and that’s reflected in Cisco’s certification tracks. Cisco offers a CCIE Security track and the Cisco Certified Security Professional (CCSP) intermediate-level certification, but there is no real equivalent to the CCNA on the security side. Instead, Cisco offers several different Security Associate certifications.

The good news is that you’ve got a lot of security specializations from which to choose; the bad news is that you’ve got a lot of choices! In choosing a specialization, take some time to choose a certification that will be of practical use to you in your current position or in your “dream job”.

One of the more popular Security Associate certifications is the Cisco VPN Specialist certification. This two-exam track consists of a Securing Cisco Network Devices (SND, 642-551) exam and a Cisco Secure Virtual Private Networks (CSVPN, 642-511) exam. To earn the Cisco VPN Specialist exam, you must hold a valid CCNA certification.

What should you expect on these exams? On the SND exam, expect to be grilled on basic security features on both switches and routers, as well as VPN 3000 concentrators, PIXes, and IDS/IPS Sensors. You’ll need to be ready to configure and troubleshoot basic AAA configurations, access-lists, syslog, AutoSecure, and much more. You should also be solid with IPSec.

IPSec will also be part of your CSVPN exam. As you’d expect, you’ll also be expected to be quite good with the VPN 3000 Concentrator series, including browser configuration, creating users and group, the Windows VPN Software Client, and more.

This is a demanding certification that is an excellent addition to your resume and your skill set. For the latest on this and other Cisco certifications, you should regularly visit the Learning & Events section of Cisco’s website. As a Cisco certification candidate, it’s your responsibility to stay current of any additions and changes to Cisco’s certification paths – and it’s good for your career!

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Preparing to pass the CCNA exam and earn this important Cisco certification? Route summarization is just one of the many skills you’ll have to master in order to earn your CCNA. Whether it’s RIP version 2, OSPF, or EIGRP, the CCNA exam will demand that you can flawlessly configure route summarization.

Route summarization isn’t just important for the CCNA exam. It’s a valuable skill to have in the real world as well. Correctly summarizing routes can lead to smaller routing tables that are still able to route packets accurately – what I like to call “concise and complete” routing tables.

The first skill you’ve got to have in order to work with route summarization is binary math; more specifically, you must be able to take multiple routes and come up with both a summary route and mask to advertise to downstream routers. Given the networks 100.16.0.0 /16, 100.17.0.0 /16, 100.18.0.0 /16, and 100.19.0.0 /16, could you quickly come up with both the summary address and mask? All you need to do is break the four network numbers down into binary strings. We know the last two octets will all convert to the binary string 00000000, so in this article we’ll only illustrate how to convert the first and second octet from decimal to binary.

100 16 = 01100100 00010000

100 17 = 01100100 00010001

100 18 = 01100100 00010010

100 19 = 01100100 00010011

To come up with the summary route, just work from left to right and draw a line where the four networks no longer have a bit in common. For these four networks, that point comes between the 14th and 15th bits. This leaves us with this string: 01100100 000100xx. All you need to do is convert that string back to decimal, which gives us 100 for the first octet and 16 for the second. (The two x values are bits on the right side of the line, which aren’t used in calculating the summary route.) Since we know that zero is the value for the last two octets, the resulting summary network number is 100.16.0.0.

But we’re not done! We now have to come up with the summary mask to advertise along with the summary route. To arrive at the summary route, write out a mask in binary with a “1″ for every bit to the left of the line we drew previously, and a “0″ for every bit to the right. That gives us the following string:

11111111 11111100 00000000 00000000

Converting that to dotted decimal, we arrive at the summary mask 255.252.0.0. The correct summary network and mask to advertise are 100.16.0.0 252.0.0.0.

For the CCNA exam, emphasis is put on knowing how to advertise these summary routes in RIPv2 and EIGRP. For both of these protocols, route summarization happens at the interface level – it’s not configured under the protocol. On the interface that should advertise the summary route, use the command “ip summary-address”. Here are examples of how the above summary route would be configured on ethernet0 in both RIPv2 and EIGRP.

R1(config-if)#ip summary-address rip 100.16.0.0 255.252.0.0

R1(config-if)#ip summary-address eigrp 100 100.16.0.0 255.252.0.0

The main difference between the two is that the EIGRP command must specify the AS number – that’s what the “100″ is in the middle of the EIGRP command. Since RIPv2 does not use AS numbers, there’s no additional value needed in the configuration.

For OSPF, the commands differ. If you’re configuring inter-area route summarization, use the “area range” command; if you are summarizing routes that are being redistributed into OSPF, use the summary-address command under the OSPF routing process on the ASBR. Neither of these are interface-level commands.

I speak from experience when I tell you that practice makes perfect on the CCNA exam, especially with binary and summarization questions. The great thing about these questions is that there are no grey areas with these questions – you either know how to do it or you don’t. And with practice and an eye for detail, you can master these skills, pass the exam, and become a CCNA. Here’s to your success!

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• MEANING OF ROUTE SUMMARIZATION

As a CCNA candidate, you most likely have some background in PC hardware and workstation support. If so, you’re already familiar with loopback interfaces, particularly 127.0.0.1, the loopback address assigned to a PC.

When you’re learning all about the different physical interfaces for your CCNA exam – serial, ethernet, and BRI, among others – there’s one logical interface you need to know about, and that is – you guessed it! – the loopback interface.

What isn’t as immediately apparent is why we use loopback interfaces on routers and switches to begin with. Many of the Cisco router features that can use loopbacks are intermediate and advanced features that you’ll learn about in your CCNP and CCIE studies, but these features all come back to one basic concept: If the loopback interface on a router is down, that means the router is unavailable as a whole.

In contrast, a physical interface being down does not mean the router itself is out of commission. A router’s ethernet port can go down, but the other physical interfaces on that router are still operational. Since a loopback interface is logical, there’s nothing physical that can go wrong with it.

As I mentioned, you’ll learn different Cisco router and switch features that utilize loopback interfaces as you climb the Cisco certification ladder. There’s one misconception about Cisco loopback interfaces that you want to get clear on now, though. You’re probably familiar with loopback interfaces on a PC, and may even know that the address range 127.0.0.0 is reserved for loopback addressing.

Note that this reserved address range does not apply to loopbacks on Cisco devices, however. If you attempt to assign an address from this range to a Cisco loopback interface, you get this result:

R1#conf t

Enter configuration commands, one per line. End with CNTL/Z.

R1(config)#interface loopback0

R1(config-if)#ip address 127.0.0.2 255.255.255.0

Not a valid host address – 127.0.0.2

R1(config-if)#ip address 127.1.1.1 255.255.255.0

Not a valid host address – 127.1.1.1

The range 127.0.0.0 is reserved for host loopbacks (such as PCs), not routers or switches. The most commonly used address from this range is 127.0.0.1 – if you can’t ping that on a workstation, that means you can’t ping yourself, which means there’s a problem with the TCP/IP install itself.

Keep these details in mind on the exam and in the workplace, and you’re on your way to CCNA exam success!

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