Basic ISDN Configuration

 

Basic ISDN Configuration

Introduction

Configuration of ISDN interfaces on Cisco routers are usually considered a straight-forward process, however there are some details which can cause your ISDN dialer interface to fail.

This article will take you through the basic steps of configuring a Cisco Router to work with ISDN. Below is a table of ISDN Switch Types. Before you attempt to configure ISDN you need to ensure that you know which type of ISDN switch you are connecting to at the Telco.

Telco Switch Type

Cisco Keyword

AT&T Basic Rate Switch

basic-5ess

Nortel DMS-100 Basic Rate Switch

basic-dms100

National ISDN-1 switch

basic-ni1

PINX (PBS) Switches with QSIG Signalling per Q.931

basic-qsig

NET3 Switch Type for UK , Europe , Asia & Australia

basic-net3

AT&T 4ESS (ISDN PRI Only)

primary-4ess

AT&T 5ESS (ISDN PRI Only)

primary-5ess

Nortel DMS-100 (ISDN PRI Only)

primary-dms100

National ISDN Switch Type

primary-ni

NTT ISDN PRI Switches ( Japan )

primary-ntt

European and Australian ISDN PRI Switches

primary-net5

QSIG Signaling per Q.931

primary-qsig

No Switch Type

none

For a very basic BRI ISDN configuration on a Cisco Router, you need to perform the following steps (that's providing you are not using DDR - Demand Dial Routing);

• Configure the Switch Type (Using switch-type)

• Configure the ISDN Dialer Map; this basically maps the IP Address of the end router's IP Address with its telephone number (Using dialer-map)

• Configure your ISDN Interface IP Address (using interface & ip address)

• Configure a routing rule (using ip route)

For Example,

• Router(config)# isdn switch-type basic-5ess
• Router(config)# isdn dialer map ip 10.10.10.2 name main_office 66552
• Router(config)# ip route 192.168.0.0 255.255.255.0 10.10.10.2
• Router(config)# interface BRI 0
• Router(config-if)# ip address 10.10.10.1 255.255.255.0

N.B. If using the DMS-100 and National-1 switch types (as well as AT&T 5ESS), you will need to obtain from you telco the Service Profile Identifier (SPIDs) of which you require one per each Bearer Channel. For these switch types, these need to be configured using the isdn spid1 and isdn spid2 commands. The SPIDs are used to authenticate call requests at the telco's switch. The format for using the SPIDs is isdn spid1 spid-number ldn and isdn spid2 spid-number ldn.

Demand Dial Routing

Since ISDN is a circuit switched technology, you usually pay for the amount of time that the line is active. For this reason, you generally do not want to raise the ISDN line for just any type of traffic. In order to accommodate this, you can configure what's called Demand Dial Routing or, DDR for short. The idea behind DDR is that you to specify the ‘interesting traffic' that will be able to raise the ISDN line.

For example, you can set as ‘interesting traffic', all packets destined towards a specific remote network you connect to. If none of the routed traffic that is heading for that network (on the other side of the ISDN line), the line doesn't get raised. This saves on unnecessary costs due to none essential traffic raising the line.

The way you specify ‘interesting traffic' is with the use of Access Control Lists (ACL's). If the traffic destined for the remote network doesn't match this interesting traffic, then the line stays down, otherwise the line is raised and traffic is allowed to travel to the remote network. It's worth mentioning here that although you have defined this “Interesting” traffic, it doesn't mean that other traffic will not be allowed to travel along the ISDN link. If the link is active, any traffic (unless blocked by Access Lists) is allowed to travel the link, therefore keep in mind that the “Interesting” traffic is used only to bring the line up.

You can then create a 2 nd set of ACL's which defining the traffic that can traverse the ISDN line. Also, the DDR works using a counter, like a time-out counter, once the counter reaches a preset configurable threshold, the line will drop again. This threshold is reduced each time “Interesting” traffic is sent over the ISDN Link, therefore it's also only the “Interesting” traffic that is used to maintain the link.

Steps in configuring DDR (assumes part of the basic steps above have already been configured)

• Define Interesting Traffic (Using dialer-list & access-list)

• Assign Interesting Traffic to an Interface (Using dialer-group)

• Define the destination IP address, hostname and telephone number to dial (Using dialer map)

• Define any additional options (Using dialer idle-timeout, dialer fast-idle, dialer load-threshold)

The Additional Options

Command

Description

dialer idle-timeout seconds

Specifies the number of idle time in seconds before the link is dropped.

dialer fast-idle seconds

Specifies the time that a line showing contention can remain idle for before it is dropped in order to allow the other call to be placed.

dialer load-threshold load [outbound | inbound |either]

Specifies the amount of load on the ISDN before the dialer initiates another call to the same destination (i.e. brings up the second channel making the link 128Kbps). The load is a number from 1 to 255, with 255 equalling 100% load. The final parameters specify which direction of traffic to calculate the load from.

For Example;

• Router(config)# isdn dialer map ip 10.10.10.2 name main_office 66552
• Router(config)# access-list 101 permit tcp any any eq http
• Router(config)# dialer-list 1 protocol ip list 101
• Router(config)# interface BRI 0
• Router(config-if)# dialer-group 1

What this example does, is allowing all web traffic (http) from anywhere to anywhere. As you can see, the dialer-list is specifying the 101 access list to dialer-list 1. The dialer-group is then specifying the 1, which links to the dialer-list 1.

Dialer Profiles

On Cisco's website it states that “Dialer Profiles implementation of DRR is based on a separation between the logical and physical interface configurations. Dialer profiles also allow the logical and physical configurations to be bound together dynamically on a per-call basis.”

There are several advantages of Dialer Profiles over Legacy DDR, it is much more scalable then Legacy DDR because Legacy DDR is based on a static binding between the per-destination call and the physical interface configuration. Dialer profiles are point-to-point interfaces which mean we no longer need the layer 3 to layer 2 mapping (layer 2 meaning the telephone number) since the profile can only dial a single location (hence point-to-point).

Since the logical and physical configurations are dynamic, it allows physical interfaces to take on different characteristics based on the logical call requirements that is utilising the physical interface. The final advantage that we will include here is that it enables you to have a backup interface that isn't tying up one of the physical interfaces. Providing a spare interface is available, the backup can be used when it's required.

Dialer Profiles are made up of a Dialer Interface, dialer pool and physical interface. Optionally, you can also have a map class. These are highlighted in the table below

Dialer Interface

This is the logical call requirements to the point-to-point connection. The following parameters can be configured; the IP address of the destination network (ip address address mask ), Layer 2 encapsulation type for example ppp (encapsulation ppp ), PPP authentication (ppp authentication chap & pap ), the remote routers authentication name (dialer remote-name username ), the remote destination to call (dialer string dial-string ), dialer pool mapping to use for calls to this destination (dialer pool number ), assign dialer interface to a dialer group (dialer group number ).

There are other optional parameters such as PPP Mulitlink, idle timeout and more.

Dialer Pool

Used for a dialer interface to reference and be associated to a physical interface

Physical Interface

Here we assign a physical interface into a dialer pool (dialer pool-member number) and assign any additional physical features, such as layer 2 encapsulation (encapsulation ppp ), ppp authentication (authentication chap ), etc..

Map Class (Optional)

Used to create a mapping between the remote IP address and phone number required to dial in order to reach it.

Steps in creating a Dialer Profile (assumes part of the basic steps above have already been configured, such as ISDN type);

• Define a Dialer Interface

• Configure a Dialer String

• Assign Physical Interface to a Dialer Pool, plus any additional Physical attributes

• Other steps from the previous sections may need to be performed, for example; if you wanted to active the link using interesting traffic, DDR needs to be configured.

For Example;

Define Dialer Interface

• Router(config)# interface dialer1
• Router(config-if)# ip address 10.10.10.1 255.255.255.0
• Router(config-if)# encapsulation ppp
• Router(config-if)# ppp authentication chap
• Router(config-if)# dialer remote-name remoteoffice
• Router(config-if)# dialer string 999666
• Router(config-if)# dialer pool 1
• Router(config-if)# dialer-group 1
• Router(config-if)# exit

Configure Physical Interface

• Router(config)# interface bri0
• Router(config-if)# encapsulation ppp
• Router(config-if)# dialer pool-member 1
• Router(config-if)# ppp authentication chap
• Router(config-if)# exit

Configure DDR to specify “Interesting” traffic (http) used to bring the line up

• Router(config)# access-list 101 permit tcp any any eq http
• Router(config)# dialer-list 1 protocol ip list 101

A Simple Example

The following example includes a variety of different items discussed in this document, in order to help you see how they are applied in a real-case scenario. As always, depending on the security policies and network complexity, the configuration can change quite a bit. For simplicity reasons, we've keep the complexity to a minimum.

Our scenario is based upon two routers who occasionally require to connect their networks via an ISDN dial line, in order to transfer data between them.

Router 1 – Remote Office

• Router1(config)#isdn switch-type basic-5ess

Assign the switch type that we are connecting through to on the physical layer, AT&T Basic Rate Switch telco switch

• Router1(config)#interface dialer1

Create a dialer interface called dialer1 ready for configuring it

• Router1(config)#description Outgoing To HQ

Set a description for this interface.

• Router1(config-if)#ip address 10.10.10.1 255.255.255.252

Assign an IP address to the virtual dialer interface

• Router1(config-if)#encapsulation ppp

Configure the encapsulation used when we are connecting through this virtual interface

• Router1(config-if)#ppp authentication chap pap callin

Set our ppp authentication to use chap, pap. The ‘Callin' parameter ensures our router authenticates the remote router (HQ) on an incoming call. Since we are always the calling party, it does not expect the remote (HQ) router to authenticate, making this authentication process a one-way direction. Remote Office authenticates to the HQ router.

• Router1(config-if)#ppp chap hostname remote-office

Set the username for chap authentication protocol

• Router1(config-if)#ppp chap password cisco

Set the password for the chap authentication protocol

• Router1(config-if)#ppp pap sent-username remote-office password cisco

Set the username & password for the pap authentication protocol.

• Router1(config-if)#dialer string 999666

Configure the telephone number to call when connecting through this virtual interface

• Router1(config-if)#dialer pool 1

Assign this virtual interface to use any physical interface that's assigned to pool number 1

• Router1(config-if)#dialer-group 1

This is to define the interesting traffic that can be used to raise this virtual interface. This command is paired with the "dialer-list 1" command later on.

• Router1(config-if)#dialer idle-timeout 300

Disconnect this call after 300 seconds of inactivity

• Router1(config-if)#ppp multilink

Make this link ‘ppp multilink' capable, allowing the aggregation of the two available 64K ISDN lines to a total of 128Kbps.

• Router1(config-if)#dialer load-threshold 125 either

When the incoming or outgoing (either) traffic reaches half of the available bandwidth (125), then bring up the 2 nd ISDN channel.

• Router1(config-if)#exit

Exits out of the interface sub command

• Router1(config)#interface bri0

Enters into the interface sub command on the physical interface bri0/0, ready for configuring the physical characteristics

• Router1(config-if)#encapsulation ppp

Configure the encapsulation for this physical interface. Optional command since we've already included it in the Dialer Interface

• Router1(config-if)#dialer pool-member 1

Assign this physical interface to a pool. This allows this physical interface to be used by any virtual interface in this same pool. In this case, Dialer 1

• Router1(config-if)#ppp authentication chap

Set our ppp authentication to use chap. Optional command since we've already included it in the Dialer Interface

• Router1(config-if)#exit

Exits out of the interface sub command

• Router1(config)#access-list 101 permit tcp any any eq http

Access list used in defining interesting traffic. This line specifies that tcp port 80 traffic from anywhere to anywhere is interesting

Router1(config)#dialer-list 1 protocol ip list 101

Maps the access-list with the dialer-group. As you can see, number 1 is used in the dialer-list and dialer-group statements

Router1(config)#ip route 192.168.0.0 255.255.255.0 10.10.10.2

Creates a route to the 192.168.0.0/24 subnet to go through 10.10.10.2 (will use Dialer1 to get there since it's on the 10.10.10.0 subnet)

Router 2 – HeadQuaters

HQ(config)#username remote-office password cisco

Assign the username & password the remote office router will user while authenticating to this router

• HQ(config)#isdn switch-type basic-5ess

Assign the switch type that we are connecting through to on the physical layer, AT&T Basic Rate Switch telco switch

• HQ(config)#interface dialer1

Create a dialer interface called dialer1 ready for configuring it

• HQ(config)#description Incoming From Remote-Office

Set a description for this interface.

• HQ(config-if)#ip address 10.10.10.2 255.255.255.252

Assign an IP address to the virtual dialer interface

• HQ(config-if)#encapsulation ppp

Configure the encapsulation used when we are connecting through this virtual interface

• HQ(config-if)#ppp authentication chap pap callin

 

Set our ppp authentication to use chap, pap. The ‘Callin' parameter ensures our router authenticates the remote router (Remote Office) on an incoming call. Since we are always the called party, it will expect the remote router to authenticate. Remote Office authenticates to the HQ router. Username & password will be checked against the credentials we provided in the first line of this router's configuration.

• HQ(config-if)#dialer pool 1

Assign this virtual interface to use any physical interface that's assigned to pool number 1

• HQ(config-if)#ppp multilink

Make this link ‘ppp multilink' capable, allowing the aggregation of the two available 64K ISDN lines to a total of 128Kbps.

• HQ(config-if)#exit

Exits out of the interface sub command

• HQ(config)#interface bri0

Enters into the interface sub command on the physical interface bri0/0, ready for configuring the physical characteristics

• HQ(config-if)#encapsulation ppp

Configure the encapsulation for this physical interface. Optional command since we've already included it in the Dialer Interface

• HQ(config-if)#dialer pool-member 1

Assign this physical interface to a pool. This allows this physical interface to be used by any virtual interface in this same pool. In this case, Dialer 1

• HQ(config-if)#exit

Exits out of the interface sub command

• HQ(config)#ip route 192.168.5.0 255.255.255.0 10.10.10.1

Creates a route to the 192.168.5.0/24 subnet to go through 10.10.10.1.

Summary

Here's the complete configuration without comments. You should now be able to understand what each command represents:

Router 1 - Remote Office

• Router1(config)# isdn switch-type basic-5ess
• Router1(config)# interface dialer1
• Router1(config)# description Outgoing To HQ
• Router1(config-if)# ip address 10.10.10.1 255.255.255.252
• Router1(config-if)# encapsulation ppp
• Router1(config-if)# ppp authentication chap pap callin
• Router1(config-if)# ppp chap hostname remote-office
• Router1(config-if)# ppp chap password cisco
• Router1(config-if)# ppp pap sent-username remote-office password cisco
• Router1(config-if)# dialer string 999666
• Router1(config-if)# dialer pool 1
• Router1(config-if)# dialer-group 1
• Router1(config-if)# dialer idle-timeout 300
• Router1(config-if)# ppp multilink
• Router1(config-if)# dialer load-threshold 125 either
• Router1(config-if)# exit
• Router1(config)# interface bri0
• Router1(config-if)# encapsulation ppp
• Router1(config-if)# dialer pool-member 1
• Router1(config-if)# ppp authentication chap
• Router1(config-if)# exit
• Router1(config)# access-list 101 permit tcp any any eq http
• Router1(config)# dialer-list 1 protocol ip list 101
• Router1(config)# ip route 192.168.0.0 255.255.255.0 10.10.10.2

Router 2 - Headquaters

• HQ(config)# username remote-office password cisco
• HQ(config)# isdn switch-type basic-5ess
• HQ(config)# interface dialer1
• HQ(config)# description Incoming From Remote-Office
• HQ(config-if)# ip address 10.10.10.2 255.255.255.252
• HQ(config-if)# encapsulation ppp
• HQ(config-if)# ppp authentication chap pap callin
• HQ(config-if)# dialer pool 1
• HQ(config-if)# ppp multilink
• HQ(config-if)# exit
• HQ(config)# interface bri0
• HQ(config-if)# encapsulation ppp
• HQ(config-if)# dialer pool-member 1
• HQ(config-if)# exit
• HQ(config)# ip route 192.168.5.0 255.255.255.0 10.10.10.1