Cisco CCNA Exam Tutorial: Five ISDN Details To Remember

CCNA exam success depends on mastering many technologies that are new to you, and few exam topics have more details than ISDN. 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

Must set encapsulation to PPP before using PPP subcommands

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 authentication pap

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

R1#show isdn status

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!

Cisco CCNA Certification Exam Tutorial: ISDN Details You Must Know

CCNA exam success depends partially on knowing the details of ISDN, and there are plenty of them! To help you review for your CCNA exam, here are a few ISDN details that you must know on exam day. (They help in the real world, too – and there are still plenty of ISDN networks out there!

The Cisco-proprietary version of HDLC is the default encapsulation type for serial and ISDN interfaces.

R2#show interface serial0

Serial0 is up, line protocol is up

Hardware is HD64570

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255

Encapsulation HDLC, loopback not set, keepalive set (10 sec)


While there’s only one D-channel in BRI, PRI (US) and PRI (EU), the bandwidth of that D-channel does vary from BRI to PRI. It’s 16 kbps in BRI and 64 kbps in both PRI versions.

The global command isdn switch-type must be configured before you can even begin to have ISDN work. show isdn status will tell you whether or not you’ve done this correctly.

R2#show isdn status

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

ISDN BRI0 interface

dsl 0, interface ISDN Switchtype = none

Layer 1 Status:

DEACTIVATED

Layer 2 Status:

Layer 2 NOT Activated

Layer 3 Status:

0 Active Layer 3 Call(s)

PAP allows passwords to be different; CHAP requires that they be the same.

PAP requires the “ppp pap sent-username” interface-level command. CHAP has no equivalent command.

Define interesting traffic with dialer-list and link that list to the interface with dialer-group.

R2#conf t

R2(config)#dialer-list 1 proto ip permit

R2(config)#int bri0

R2(config-if)#dialer-group 1

The dialer idle-timeout value is expressed in seconds, not minutes. (Even IOS Help isn’t totally clear on this.)

R2(config)#int bri0

R2(config-if)#dialer-group 1

R2(config-if)#dialer idle-timeout ?

<1-2147483> Idle timeout before disconnecting a call

R2(config-if)#dialer idle-timeout 120


Dialer map maps a remote IP address to a remote phone number. You never dial the local router’s phone number.

dialer load-threshold requires the ppp multilink command to be configured, and the value of dialer load-threshold is expressed as a ratio of 255, NOT 100. For example, if you want the second b-channel to come up when the first reaches 50% of capacity, the value to express with dialer load-threshold would be 50% of 255 – which equals 127.

R2(config)#int bri0

R2(config-if)#encap ppp

R2(config-if)#ppp multilink

R2(config-if)#dialer load-threshold ?

<1-255> Load threshold to place another call


Success on the CCNA exam depends on knowing the details. Keep studying, keep practicing on real Cisco routers and switches, keep a positive attitude, and you're on your way to CCNA exam success!

Cisco CCNA Certification Exam Tutorial: Configuring Dialer Profiles

The most common method of configuring ISDN is with dialer maps, but dial information can also be configured on a logical interface. To pass the CCNA exam, you must know how to configure and troubleshoot both dialer maps and dialer profiles.

Dialer Profiles allow different dialing information to be configured onto logical interfaces. The logical interfaces may have different dialing destinations, different remote router names, etc., but they’ll be using the same physical interface.

Dialer strings are used on dialer profiles. Note that each logical interface has a different IP address, a different remote router to dial, and a different dialer string, but they will be using the same physical interface to dial out. The commands dialer pool and dialer pool-member are used to link the logical and physical interfaces. The number following each command must match for the logical interface to correctly bind to the physical interface.


R1(config)#interface dialer0

R1(config-if)#ip address 172.16.1.1 255.255.255.0

R1(config-if)#encapsulation ppp

<. The encapsulation type is placed on both the logical and physical interfaces. >

R1(config-if)#dialer remote-name Remote0



R1(config-if)#dialer pool 1

< places logical interface into dialer pool >

R1(config-if)#dialer string 5551212

< number dialed to contact router Remote0 >

R1(config-if)#dialer-group 1

< links logical interface to dialer-list 1 >


R1(config)#interface dialer1

R1(config-if)#ip address 172.16.1.2 255.255.255.0

R1(config-if)#encapsulation ppp

R1(config-if)#dialer remote-name Remote1

R1(config-if)#dialer pool 1

R1(config-if)#dialer string 5551234

R1(config-if)#dialer-group 1


R1(config)#interface bri0

R1(config-if)#no ip address

< With dialer profiles, IP addresses are assigned to logical interfaces. >

R1(config-if)#encapsulation ppp

< The encapsulation type is place on both the logical and physical interfaces.>

R1(config-if)#dialer pool-member 1

< The number associated with this command should match the number configured with the dialer pool number on the logical dialer interfaces. >

R1(config-if)#isdn spid1 0835866101

R1(config-if)#isdn spid2 0835866301



When configuring dialer profiles, the encapsulation type should be placed on both the physical BRI interface and the logical dialer interfaces. The SPIDs are configured on the physical interface as well.

Configuring dialer profiles can be a little tricky at first, and the best way to master this skill is to get real hands-on practice in your own CCNA / CCNP home lab or a rack rental service. Either way, hands-on is the best practice. Best of luck in your CCNA studies!

Cisco CCNA / CCNP Certification Exam Tutorial: Dialer Watch

Dialer Watch is a vital part of your CCNA and CCNP studies, particularly for the BCRAN exam, but it's one of the most misunderstood technologies as well. To help you pass the CCNA and CCNP certification exams, here's a detailed look at Dialer Watch.

Dialer Watch allows you to configure a route or routes as "watched" when the watched route leaves the routing table and there is no other valid route to that specific destination, the ISDN link will come up. In the following example, R1 and R2 are connected by both a Frame Relay cloud over the 172.12.123.0 /24 network and an ISDN cloud using the 172.12.12.0 /24 network. The routers are running OSPF over the Frame cloud, and R1 is advertising its loopback of 1.1.1.1/32 as well as an Ethernet segment, 10.1.1.0/24, via OSPF. R2 has both of these routes in its OSPF table, as shown below.

R2#show ip route ospf

1.0.0.0/32 is subnetted, 1 subnets

O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:07, Serial0

10.0.0.0/24 is subnetted, 1 subnets

O 10.1.1.0 [110/128] via 172.12.123.1, 00:00:08, Serial0

We want R2 to place a call to R1 if either the loopback or Ethernet networks leave R2's routing table, but we don't want to have to depend on interesting traffic. That dictates the use of Dialer Watch.

First, configure the list of watched routes with dialer watch-list. Only one of the watched routes needs to leave the routing table for the ISDN link to come up. In this example, R2 will watch both routes from its OSPF routing table.

Be careful with this command. The entries here need to match exactly the routes and masks being watched. Dialer watch-lists use subnet masks, not wildcard masks.

R2(config)#dialer watch-list 5 ip 10.1.1.0 255.255.255.0

R2(config)#dialer watch-list 5 ip 1.1.1.1 255.255.255.255

Configure the dialer watch-group command on the BRI interface, AND frame map statements for the watched routes. As with dialer-list and dialer-group, the group number referenced in the dialer watch-group command must match the number assigned to the dialer watch-list.

The Dialer Watch configuration will not work without frame map statements for each watched route. I repeat this because this is the step a lot of people leave out.

R2(config)#interface bri0

R2(config-if)#dialer watch-group 5

R2(config-if)# dialer map ip 1.1.1.1 255.255.255.255. name R1 5557777 broadcast

R2(config-if)# dialer map ip 10.1.1.0 255.255.255.0 name R1 5557777 broadcast

To test Dialer Watch, the Serial0 interface on R2 will be shut down. Since we're running OSPF, the route table will be updated almost immediately and the ISDN link should come up right after that.

R2(config)#int s0

R2(config-if)#shut


01:12:47: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Serial0 from FULL to DOWN, N
eighbor Down: Interface down or detached


01:12:47: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up

01:12:48: %SYS-5-CONFIG_I: Configured from console by console

01:12:48: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state
to up

01:12:49: %LINK-5-CHANGED: Interface Serial0, changed state to administratively
down


01:12:50: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state
to down

01:12:53: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 5557777 R1

Within five seconds, the ISDN link is up. show dialer verifies that Dialer Watch is the reason the line was brought up.

R2#show dialer

BRI0 - dialer type = ISDN

Dial String Successes Failures Last DNIS Last status

5557777 2 0 00:00:11 successful

0 incoming call(s) have been screened.

0 incoming call(s) rejected for callback.

BRI0:1 - dialer type = ISDN

Idle timer (120 secs), Fast idle timer (20 secs)

Wait for carrier (30 secs), Re-enable (15 secs)

Dialer state is data link layer up

Dial reason: Dialing on watched route loss

Time until disconnect 108 secs

Connected to 5557777 (R1)
A final note regarding Dialer Watch ... it will not work with RIP, but will with all our other dynamic IGPs (IGRP, EIGRP, OSPF).