ISDN isn’t just for your CCNA exam studies, though. While ISDN is dismissed by many, the fact is that there are many small and mid-size networks out there that use ISDN as their backup to frame relay. Some of these companies have spoke networks that use ISDN to connect to their hub as well, so it’s a great idea to know ISDN configuration and troubleshooting for your real-world career as well as passing the CCNA. With that in mind, let’s take a look at five common ISDN errors and how to avoid them.

With dialer map statements, remember that the phone number you put in the dialer map is the phone number of the remote router, not the local one. Look at it this way – if you want to call a friend on your cell, you don’t pick up your cell and dial your own number!

Speaking of dialer map statements, don’t forget the all-important broadcast option at the end of the command:

R1(config-if)#dialer map ip 172.12.21.1 name R2 broadcast 5555555

The router will accept that command without the “broadcast” option, but routing protocol updates and hellos would not be able to travel across the line. (This command is also needed in frame relay map statements to allow broadcasts and multicasts to be transmitted.)

PAP is PPP’s clear-text authentication scheme, and clear text is a really bad idea. But if you do have to configure it, don’t forget that PAP requires additional configuration -the ppp pap sent-username command.

R1(config-if)#ppp pap sent-username R1 password CISCO

R1(config-if)#

The error message we got while configuring the sent-username command is another important reminder – by default, a BRI line is running HDLC, not PPP. Since HDLC doesn’t allow us to use either PAP or CHAP, we’ll need to set the link to PPP with the encapsulation ppp command.

R1(config-if)#encapsulation ppp

R1(config-if)#ppp pap sent-username R1 password CISCO

But before we configure any of this information, we should configure the ISDN switch-type. Why? Because without the switch-type configuration, it doesn’t matter that we avoid the other four errors – the line will not come up. Configure the switch-type with the “isdn switch-type” command, and then verify it with “show isdn status”.

R1(config)#isdn switch-type basic-ni

Global ISDN Switchtype = basic-ni (output of this command cut here for clarity)

If you forget this part of the configuration, the output of show isdn status wastes no time in reminding you!

R1#show isdn status

**** No Global ISDN Switchtype currently defined ****

ISDN is an important part of your CCNA studies, and this knowledge still comes in handy in production networks as well. Keep studying, notice the details, run those debugs, and you’ll be a CCNA before you know it!

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

Between RIP, IGRP, EIGRP, OSPF, and switching, there are hundreds of details you’ve got to absorb! It’s easy to spend all your time on those topics and not pay proper attention to “easier” technologies, and then all of a sudden on exam day you can’t quite remember the details of those particular services.

One setup you’ve got to be more than familiar with is directly connecting serial interfaces on Cisco routers. This is also a valuable skill to have in your home lab, since it allows you to add segments to your network setup.

A Cisco serial interface is operating as a DTE by default. The problem is that when you take a cable and connect two routers directly by their serial interfaces (with a DTE/DCE cable, that is!), they’re both waiting for the other to send them a clock rate. One of the interfaces must act as the DCE and that interface must send the clock rate.

If you can see the DTE/DCE cable, you can tell by looking which router has the DCE interface connected to it – the letters “DTE” or “DCE” will either be molded into the connector itself, or if it’s an older cable there should be a little piece of tape on the cable that tells you what the interface type is. But what if you have no access to the cable, or there are other cables all around it and you can’t see what type it is?

Run the command “show controller serial x”, with x representing the interface number the cable’s connected to. There will be quite a bit of output from this command, but the information you need is right at the top:

R1#show controller serial 1

buffer size 1524 HD unit 1, V.35 DTE cable

I left off the 16 or so rows of information that comes after this, but this is the information we need right now. If R1′s got the DTE cable end, the other router should have the DCE end:

R3#show controller serial 1

buffer size 1524 HD unit 1, V.35 DCE cable

We know now that R3 needs to supply a clock rate to R1. There’s a hint of a problem in just that little bit of command output – do you see what it is? Let’s run show interface serial1 to get more information.

R3#show int s1

Serial1 is up, line protocol is down

The line protocol is down because there is no clockrate being supplied by R3. If there has been, we would have seen that in the output of show controllers serial 1.

This is simple enough to fix, though! We’ll use the command clockrate 56000 on R3′s serial1 interface, and the line protocol will soon come up.

R3(config)#int s1

1w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up

This is a simple concept, but there are a few details you must keep in mind! For a home lab configuration, you’ll need a DTE/DCE cable to make this work. If you cannot see the cable connectors, run show controllers serial x to see if the router has the DTE or DCE end of the cable attached. On the interface with the DCE attached, use the clockrate command to bring the line protocol up. It’s just that simple!

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This involves manipulating the routers configuration register, and that is enough to make some CCNA candidates and network administrators really nervous!

Its true that setting the configuration register to the wrong value can damage the router, but if you do the proper research before starting the password recovery process, youll be fine.

Despite what some books say, there is no “one size fits all” approach to Cisco password recovery. What works on a 2500 router may not work on other routers and switches. There is a great master Cisco document out on the Web that you should bookmark today. Just put “cisco password recovery” in your favorite search engine and you should find it quickly.

The following procedure describes the process in recovering from a lost password on a Cisco 2500 router. As always, dont practice this at home. It is a good idea to get some practice with this technique in your CCNA / CCNP home lab, though!

The password recovery method examined here is for 2500 routers.

An engineer who finds themselves locked out of a router can view and change the password by changing the configuration register.

The router must first be rebooted and a “break” performed within the first 60 seconds of the boot process. This break sequence can also vary depending on what program is used to access the router, but is the usual key combination.

The router will now be in ROM Monitor mode. From the rom monitor prompt, change the default configuration register of 0×2102 to 0×2142 with the o/r 0×2142 command. Reload the router with the letter i. (As you can see, ROM Monitor mode is a lot different than working with the IOS!)

This particular config register setting will cause the router to ignore the contents of NVRAM. Your startup configuration is still there, but it will be ignored on reload.

When the router reloads, you’ll be prompted to enter Setup mode. Answer “N”, and type enable at the router> prompt.

Be careful here. Type configure memory or copy start run. Do NOT type write memory or copy run start!

Enter the command show running-config. You’ll see the passwords in either their encrypted or unencrypted format.

Type config t, then use the appropriate command to set a new enable secret or enable password.

Don’t forget to change the configuration register setting back to the original value! The command config-register 0×2102 will do the job. Save this change with write memory or copy run start, and then run reload one more time to restart the router.

This process sounds hard, but its really not. You just have to be careful, particularly when youre copying the startup config over the running config. You dont want to get that backwards! So take your time, check the online Cisco documentation before starting, get some practice with this procedure with lab equipment, and youll be ready for success on the CCNA exam and in your production network!

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To help you review these important concepts, heres a quick look at the basics of RIPv1, RIPv2, IGRP, and EIGRP.

RIPv1: Broadcasts updates every 30 seconds to the address 255.255.255.255. RIPv1 is a classful protocol, and it does not recognize VLSM, nor does it carry subnet masking information in its routing updates.

Update contains entire RIP routing table. Uses Bellman-Ford algorithm. Allows equal-cost load-balancing by default. Max hop count is 15. Does not support clear-text or MD5 authentication of routing updates. Updates carry 25 routes maximum.

RIPv2: Multicasts updates every 30 seconds to the address 224.0.0.9. RIPv2 is a classless protocol, allowing the use of subnet masks.

Update contains entire RIP routing table. Uses Bellman-Ford algorithm. Allows equal-cost load-balancing by default. Max hop count is 15. Supports clear-text and MD5 authentication of routing updates. Updates carry 25 routes maximum.

IGRP: Broadcasts updates every 90 seconds to the address 255.255.255.255. IGRP is a Cisco-proprietary protocol, and is also a classful protocol and does not recognize subnet masking.

Update contains entire routing table. Uses Bellman-Ford algorithm. Equal-cost load-balancing on by default; unequal-cost load-sharing can be used with the variance command. Max hop count is 100.

EIGRP: Multicasts full routing table only when an adjacency is first formed. Multicasts updates only when there is a change in the network topology, and then only advertises the change.

Multicasts to 224.0.0.10 and allows the use of subnet masks. Uses DUAL routing algorithm. Unequal-cost load-sharing available with the variance command.

By mastering the basics of these protocols, youre laying the foundation for success in the exam room and when working on production networks.

Pay attention to the details and the payoff is “CCNA” and “CCNP” behind your name!

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The Forward Explicit Congestion Notification (FECN, pronounced “feckon”) bit is set to zero by default, and will be set to 1 if congestion was experienced by the frame in the direction in which the frame was traveling. A DCE (frame relay switch) will set this bit, and a DTE (router) will receive it, and see that congestion was encountered along the frames path.

If network congestion exists in the opposite direction in which the frame was traveling, the Backward Explicit Congestion Notification (BECN, pronounced “beckon”) will be set to 1 by a DCE.

If this is your first time working with BECNs and FECNs, you might wonder why the BECN even exists – after all, why send a “backwards” notification? The BECN is actually the most important part of this entire process, since its the BECN bit that indicates to the sender that it needs to slow down!

For example, frames sent from Kansas City to Green Bay encounter congestion in the FR cloud. A Frame Switch sets the FECN bit to 1. In order to alert KC that its sending data too fast, GB will send return frames with the BECN bit set. When KC sees the BECN bit is set to 1, the KC router knows that the congestion occurred when frames were sent from KC to GB.

Frame Relay BECN Adaptive Shaping allows a router to dynamically throttle back on its transmission rate if it receives frames from the remote host with the BECN bit set. In this case, KC sees that the traffic it’s sending to GB is encountering congestion, because the traffic coming back from GB has the BECN bit set. If BECN Adaptive Shaping is running on KC, that router will adjust to this congestion by slowing its transmission rate. When the BECNs stop coming in from GB, KC will begin to send at a faster rate.

BECN Adaptive Shaping is configured as follows:

KC(config-if)#frame-relay adaptive-shaping becn

To see how many frames are coming in and going out with the BECN and FECN bits set, run show frame pvc.

R3#show frame pvc

pvc create time 01:26:27, last time pvc status changed 01:26:27

Just watch the “in”s and “out”s of BECN, FECN, and DE in both the exam room and your production networks!

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

Frame Relay VCs use Data-Link Connection Identifiers (DLCI – pronounced “del-see”) as their addresses. Unlike other Cisco technologies, VCs have only a single DLCI in their header. They do not have a source and destination.

DLCIs have local significance only. DLCI numbers are not advertised to other routers, and other routers can use the same DLCI numbers without causing connectivity issues.

Cisco uses the term global addressing to describe a technique by which a router in a frame relay network is reached via the same DLCI number from each router in the network. For example, in a 25-router network, the same DLCI number would be used to reach “Router A” by each router.

Global Addressing is an organizational tool that does not affect the fact that DLCIs have local significance only.

The locally significant DLCI must be mapped to the destination router’s IP address. There are two options for this, Inverse ARP and static mapping.

In both of the following examples, the single physical Serial interface on Router 1 is configured with two logical connections through the frame relay cloud, one to Router 2 and one to Router 3.

Inverse ARP runs by default once Frame Relay is enabled, and starts working as soon as you open the interface. By running show frame-relay map after enabling Frame Relay, two dynamic mappings are shown on this router. If a dynamic mapping is shown, Inverse ARP performed it.

R1#show frame map

                    broadcast,, status defined, active

Static mappings require the use of a frame map statement. To use static mappings, turn Inverse ARP off with the no frame-relay inverse-arp statement, and configure a frame map statement for each remote destination that maps the local DLCI to the remote IP address. Frame Relay requires the broadcast keyword to send broadcasts to the remote device.

R1#conf t

R1(config-if)#frame map ip 200.1.1.3 123 broadcast

The syntax of the frame map statement maps the remote IP address to the local DLCI. Broadcasts will not be transmitted by default; the broadcast option must be configured.

R1#show frame map

                      CISCO, status defined, active

Hands-on practice is the best way to prepare for CCNA exam success. Working with Frame Relay in a lab environment practically guarantees that youll truly master the concepts shown here – and then youre on your way to the CCNA and becoming a master network engineer.

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

Therefore, you had better know the basics!

The use of “host” and “any” confuses some newcomers to ACLs, so lets take a look at that first.

It is acceptable to configure a wildcard mask of all ones or all zeroes. A wildcard mask of 0.0.0.0 means the address specified in the ACL line must be matched exactly a wildcard mask of 255.255.255.255 means that all addresses will match the line.

Wildcard masks have the option of using the word host to represent a wildcard mask of 0.0.0.0. Consider a configuration where only packets from IP source 10.1.1.1 should be allowed and all other packets denied. The following ACLs both do that.

R3#conf t

R3(config)#access-list 7 permit host 10.1.1.1

The keyword any can be used to represent a wildcard mask of 255.255.255.255.

R3(config)#access-list 15 permit any

Another often overlooked detail is the order of the lines in an ACL. Even in a two- or three-line ACL, the order of the lines in an ACL is vital.

Consider a situation where packets sourced from 172.18.18.0 /24 will be denied, but all others will be permitted. The following ACL would do that.

R3#conf t

R3(config)#access-list 15 permit any

The previous example also illustrates the importance of configuring the ACL with the lines in the correct order to get the desired results. What would be the result if the lines were reversed?

R3#conf t

R3(config)#access-list 15 deny 172.18.18.0 0.0.0.255

If the lines were reversed, traffic from 172.18.18.0 /24 would be matched against the first line of the ACL. The first line is “permit any”, meaning all traffic is permitted. The traffic from 172.18.18.0/24 matches that line, the traffic is permitted, and the ACL stops running. The statement denying the traffic from 172.18.18.0 is never run.

The key to writing and troubleshoot access lists is to take just an extra moment to read it over and make sure its going to do what you intend it to do. Its better to realize your mistake on paper instead of once the ACLs been applied to an interface!

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

A few minutes before youre scheduled to be there, you decide theres something really good on TV youd like to watch. Or you decide to go to the gym, or play a game, or do anything else except go see the client.

Even if you werent going to get fired for not showing up, its certainly unfair to the client. Youve got a professional obligation, and you should be there on time.

Now, whats this got to do with you becoming a CCNA or CCNP? Plenty. Because when it comes to your study time, youre the client. You owe it to yourself to show up.

You would never blow off an appointment to meet a client to get some important work done.

First, though, you have to make that appointment with yourself! Schedule your CCNA / CCNP study time, and keep that appointment as you would with a client.

Turn off the TV, your cell, your iPod, and everything else electronic that you carry around. Believe it or not, the world can survive with being in contact with you for an hour or so! You might even like it!

Getting certified isnt about how many hours, days, or weeks you spend studying. Its about how much quality time you put in.

Be honest with yourself and realize that youre better off with 45 minutes of uninterrupted study as you would be with three hours of constantly interrupted study.

Dont blow off an appointment to yourself, either. Schedule the time, be there on time, get your study done, and youre one step closer to your CCNA and CCNP!

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

And of course, it\s not enough to notice these details – you\ve got to know what to do about them!

A Cisco router is a DTE by default, but directly connecting two DTEs with a DCE/DTE cable is not enough. In the following example, R1 and R3 are directly connected at their Serial1 interfaces. The line goes up briefly after being opened, but the line protocol goes down after about 30 seconds.

R3(config-if)#int s1

2d18h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down

The problem is that one of the routers needs to act as the DCE in order for the line protocol to come up and stay up. If this were your CCNA / CCNP home lab, you could just go over and look at the DTE/DCE cable to see which router had the DCE end of the cable attached. In this example, though, we don\t have physical access to the routers. How can we tell which router has the DCE end of the cable attached?

R3#show controller serial 1

buffer size 1524 HD unit 1, V.35 DCE cable

The show controller command gives us this information. (There\s a lot more output that this with this command, but it\s unimportant for our purposes.) The router with the DCE end of the cable needs to supply a clock rate to the DTE, and we\ll do just that with the interface-level clockrate command.

R3#conf t

2d18h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up

In just a few seconds, the line protocol goes up and stays up.

When troubleshooting a connection, always run show interface first. If you see the combination shown below, the connection is physically fine but logically down. That\s generally the result of a needed keepalive not being present. With Frame Relay, it\s probably an LMI issue, but with directly connected serial interfaces the issue is most likely the DCE end of the connection not supplying clockrate.

R3#show interface serial 1

Serial1 is up, line protocol is down

Troubleshooting is a big part of the job, and it\s a big part of the Cisco CCNA and CCNP programs as well. Know your show and debug commands and you\re on your way to passing the CCNA!

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!

Whether you’re planning on passing the CCNA exam or just brushing up on your networking skills, this VTP tutorial will help you learn the basics of this important protocol.

VTP allows switches to advertise VLAN information between other members of the same VTP domain. VTP allows a consistent view of the switched network across all switches. When a VLAN is created on one switch in a VTP server, all other VTP devices in the domain are notified of that VLAN’s existence. VTP servers will know about every VLAN, even VLANs that have no members on that switch.

Switches run VTP in one of three modes. In server mode, VLANs can be created, modified, and deleted on a VTP server. When these actions are taken, the changes are advertised to all switches in the VTP domain. VTP Servers keep VLAN configuration information upon reboot.

In client mode, the switch cannot modify, create, or delete VLANs. VTP clients cannot retain VLAN configuration information upon reboot; they have to obtain this information from a VTP server.

In real-world networks, this is generally done to centralize the creation and deletion of VLANs. An interesting side effect of the server/client methodology is that if a VLAN is only to have ports on the VTP client switch, the VLAN must still first be created on the VTP server. The VTP client will learn about the VLAN from the VTP server, and ports can then be placed into that VLAN.

The third VTP mode is transparent mode. VTP switches in this mode ignore VTP messages. They do forward the VTP advertisements received from other switches. VLANs can be created, deleted, and modified on a transparent server, but those changes are not advertised to the other switches in the VTP domain.

For switches running VTP to successfully exchange VLAN information, three things have to happen. I’ve listed them for you in the order that you’ll see them in the real world.

The VTP domain name must match. This is case-sensitive. “CISCO” and “cisco” are two different domains.

To distribute information about a newly-created VLAN, the switch upon which that VLAN is created must be in Server mode.

Learning VTP isn’t just a good idea for passing your CCNA exams, it’s a skill you must have to be effective in configuring and troubleshooting VLANs. I wish you the best in both of these pursuits!

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

For a FREE copy of his latest e-books, “How To Pass The CCNA” or “How To Pass The CCNP”, and for free daily exam question, visit the website and download your copies!