ccna4 module 4

8/14/2019 CCNA4 Module 4

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Module 4: ISDN and DDR

Module 4: ISDN and DDR........................................................................................1

Module Overview....................................................................................................2

[4.1] ISDN Concepts............................................................................................2

[4.1.1] Introducing ISDN...................................................................................2

[4.1.2] ISDN standards and access methods....................................................3

[4.1.3] ISDN 3-layer model and protocols........................................................4

[4.1.4] ISDN functions......................................................................................6

[4.1.5] ISDN reference points...........................................................................6

[4.1.6] Determining th e router ISDN interface..............................................7

[4.1.7] ISDN s w itch types.................................................................................7

[4.2] ISDN Configuration.....................................................................................8

[4.2.1] Configurin g ISDN BRI............................................................................8

[4.2.2] Configuring ISD N PRI............................................................................9

[4.2.3] Verifying ISDN configuration...............................................................10

[4.2.4] Troublesho o ting the ISDN configuration..............................................11

[4.3] DDR Configuration.....................................................................................12

[4.3.1] DDR operation....................................................................................12

[4.3.2] Configuring legacy DDR......................................................................12[4.3.3] Defining static routes for DDR............................................................12

[4.3.4] Specifying interest ing traffic for DDR..................................................13

[4.3.5] Configuring DDR dialer information....................................................13

[4.3.6] Dialer p r o files.....................................................................................15

[4.3.7] Configuring dialer profiles...................................................................16

[4.3.8] Verifying DDR configuration................................................................16

[4.3.9] Troubleshooting the DDR configuration..............................................17

Module Summary.................................................................................................17


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Module Overview

Integrated Services Digital Network (ISDN) is a network that provides end-to-enddigital connectivity to support a wide range of services including voice and dataservices.

ISDN allows multiple digital channels to operate simultaneously through the sameregular phone wiring used for analog lines, but ISDN transmits a digital signalrather than analog. Latency is much lower on an ISDN line than on an analog line.

Dial-on-demand routing (DDR) is a technique developed by Cisco that allows theuse of existing telephone lines to form a wide-area network (WAN), instead ofusing separate, dedicated lines. Public switched telephone networks (PSTNs) areinvolved in this process.

DDR is used when a constant connection is not needed, thus reducing costs. DDRdefines the process of a router connecting using a dialup network when there istraffic to send, and then disconnecting when the transfer is complete.

Students completing this module should be able to:

* Define the ISDN standards used for addressing, concepts, and signaling

* Describe how ISDN uses the physical and data link layers

* List the interfaces and reference points for ISDN

* Configure the router ISDN interface

* Determine what traffic is allowed when configuring DDR

* Configure static routes for DDR

* Choose the correct encapsulation type for DDR

* Be able to determine and apply an access list affecting DDR traffic

* Configure dialer interfaces

[4.1] ISDN Concepts


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[4.1.1] Introducing ISDN

There are several WAN technologies used to provide network access from remotelocations. One of these technologies is ISDN. ISDN can be used as a solution tothe low bandwidth problems that small offices or dial-in users have withtraditional telephone dial-in services.

The traditional PSTN was based on an analog connection between the customerpremises and the local exchange, also called the local loop.

The analog circuits introduce limitations on the bandwidth that can be obtainedon the local loop. Circuit restrictions do not permit analog bandwidths greaterthan approximately 3000 Hz. ISDN technology permits the use of digital data onthe local loop, providing better access speeds for the remote users.

Telephone companies developed ISDN with the intention of creating a totallydigital network. ISDN allows digital signals to be transmitted over existingtelephone wiring. This became possible when the telephone company switcheswere upgraded to handle digital signals. ISDN is generally used fortelecommuting and networking small and remote offices into the corporate LAN.

Telephone companies developed ISDN as part of an effort to standardizesubscriber services. This included the User-Network Interface (UNI), better known

as the local loop. The ISDN standards define the hardware and call setupschemes for end-to-end digital connectivity. These standards help achieve thegoal of worldwide connectivity by ensuring that ISDN networks easilycommunicate with one another. In an ISDN network, the digitizing function isdone at the user site rather than the telephone company.

ISDN brings digital connectivity to remote sites. The following list provides someof the benefits of ISDN:

* Carries a variety of user traffic signals, including data, voice, and video

* Offers much faster call setup than modem connections

* B channels provide a faster data transfer rate than modems

* B channels are suitable for negotiated Point-to-Point Protocol (PPP) links

ISDN is a versatile service able to carry voice, video, and data traffic. It is possibleto use multiple channels to carry different types of traffic over a singleconnection.


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ISDN uses out-of-band signaling, the delta (D channel), for call setup andsignaling. To make a normal telephone call, the user dials the number one digit ata time. Once all the numbers are received, the call can be placed to the remoteuser. ISDN delivers the numbers to the switch at D-channel rates, thus reducingthe time it takes to set up the call.

ISDN also provides more bandwidth than a traditional 56 kbps dialup connection.ISDN uses bearer channels, also called B channels, as clear data paths. Each Bchannel provides 64 kbps of bandwidth. With multiple B channels, ISDN offersmore bandwidth for WAN connections than some leased services. An ISDNconnection with two B channels would provide a total usable bandwidth of 128kbps.

Each ISDN B channel can make a separate serial connection to any other site in

the ISDN network. Since PPP operates over both synchronous and asynchronousserial links, ISDN lines can be used in conjunction with PPP encapsulation.

[4.1.2] ISDN standards and access methods

Work on standards for ISDN began in the late 1960s. A comprehensive set ofISDN recommendations was published in 1984 and is continuously updated bythe International Telecommunication Union Telecommunication StandardizationSector (ITU-T), formerly known as the Consultative Committee for International

Telegraph and Telephone (CCITT). The ISDN standards are a set of protocols thatencompass digital telephony and data communications. The ITU-T groups andorganizes the ISDN protocols according to the following general topic areas:

* E Protocols Recommend telephone network standards for ISDN. Forexample, the E.164 protocol describes international addressing for ISDN.

* I Protocols Deal with concepts, terminology, and general methods. TheI.100 series includes general ISDN concepts and the structure of other I-seriesrecommendations. I.200 deals with service aspects of ISDN. I.300 describes

network aspects. I.400 describes how the UNI is provided.* Q Protocols Cover how switching and signaling should operate. The term

signaling in this context means the process of establishing an ISDN call.

ISDN standards define two main channel types, each with a different transmissionrate. The bearer channel, or B channel, is defined as a clear digital path of 64kbps. It is said to be clear because it can be used to transmit any type of digitizeddata in full-duplex mode. For example, a digitized voice call can be transmittedon a single B channel. The second channel type is called a delta channel, or Dchannel. There can either be 16 kbps for the Basic Rate Interface (BRI) or 64 kbpsfor the Primary Rate Interface (PRI).


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The D channel is used to carry control information for the B channel.

When a TCP connection is established, there is an exchange of information calledthe connection setup. This information is exchanged over the path on which the

data will eventually be transmitted. Both the control information and the datashare the same pathway. This is called in-band signaling. ISDN however, uses aseparate channel for control information, the D channel. This is called out-of-bandsignaling.

ISDN specifies two standard access methods, BRI and PRI. A single BRI or PRIinterface provides a multiplexed bundle of B and D channels.

BRI uses two 64 kbps B channels plus one 16kbps D channel. BRI operates withmany Cisco routers. Because it uses two B channels and one D channel, BRI issometimes referred to as 2B+D.

The B channels can be used for digitized speech transmission. In this case,specialized methods are used for the voice encoding. Also, the B channels can beused for relatively high-speed data transport. In this mode, the information iscarried in frame format, using either high-level data link control (HDLC) or PPP asthe Layer 2 protocol. PPP is more robust than HDLC because it provides amechanism for authentication and negotiation of compatible link and protocolconfiguration.

ISDN is considered a circuit-switched connection. The B channel is the elementalcircuit-switching unit.

The D channel carries signaling messages, such as call setup and teardown, tocontrol calls on B channels. Traffic over the D channel employs the Link AccessProcedure on the D Channel (LAPD) protocol. LAPD is a data link layer protocolbased on HDLC.

In North America and Japan, PRI offers twenty-three 64 kbps B channels and one64 kbps D channel. A PRI offers the same service as a T1 or DS1 connection. InEurope and much of the rest of the world, PRI offers 30 B channels and one Dchannel in order to offer the same level of service as an E1 circuit. PRI uses aData Service Unit/Channel Service Unit (DSU/CSU) for T1/E1 connections.

[4.1.3] ISDN 3-layer model and protocols


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ISDN utilizes a suite of ITU-T standards spanning the physical, data link, andnetwork layers of the OSI reference model:

* The ISDN BRI and PRI physical layer specifications are defined in ITU-T I.430

and I.431, respectively.

* The ISDN data link specification is based on LAPD and is formally specified inthe following:

o ITU-T Q.920

o ITU-T Q.921

o ITU-T Q.922

o ITU-T Q.923

* The ISDN network layer is defined in ITU-T Q.930, also known as I.450 andITU-T Q.931, also known as I.451. These standards specify user-to-user, circuit-switched, and packet-switched connections.

BRI service is provided over a local copper loop that traditionally carries analogphone service. While there is only one physical path for a BRI, there are threeseparate information paths, 2B+D. Information from the three channels ismultiplexed into the one physical path.

ISDN physical layer, or Layer 1, frame formats differ depending on whether theframe is outbound or inbound. If the frame is outbound, it is sent from theterminal to the network. Outbound frames use the TE frame format. If the frameis inbound, it is sent from the network to the terminal. Inbound frames use the NTframe format.

Each ISDN BRI frame contains two sub-frames each containing the following:

* 8 bits from the B1 channel

* 8 bits from the B2 channel

* 2 bits from the D channel

* 6 bits of overhead

ISDN BRI frames therefore comprise 48 bits. Four thousand of these frames are

transmitted every second. Each B channel, B1and B2, has a capacity of 8 * 4000* 2 = 64 kbps, while channel D has a capacity of 2 * 4000 * 2 = 16 kbps. This


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accounts for 144 kbps (B1 + B2 + D) of the total ISDN BRI physical interface bitrate of 192 kbps. The remainder of the data rate are the overhead bits that arerequired for transmission: 6 * 4000 * 2 = 48 kbps.

The overhead bits of an ISDN sub-frame are used as follows:

* Framing bit Provides synchronization

* Load balancing bit- Adjusts the average bit value

* Echo of previous D channel bits Used for contention resolution whenseveral terminals on a passive bus contend for a channel

* Activation bit Activates devices

* Spare bit Unassigned

Note that the physical bit rate for the BRI interface is 48*4000 = 192 kbps. Theeffective rate is 144 kbps = 64 kbps + 64 kbps + 16 kbps (2B+D).

Layer 2 of the ISDN signaling channel is LAPD. LAPD is similar to HDLC. LAPD isused across the D channel to ensure that control and signaling information isreceived and flows properly.

The LAPD flag and control fields are identical to those of HDLC. The LAPD addressfield is 2 bytes long.

The first address field byte contains the service access point identifier (SAPI),which identifies the portal at which LAPD services are provided to Layer 3. Thecommand/response bit (C/R), indicates whether the frame contains a commandor a response. The second byte contains the terminal endpoint identifier (TEI).Each piece of terminal equipment on the customer premises needs a uniqueidentifier. The TEI may be statically assigned at installation, or the switch maydynamically assign it when the equipment is started up. If the TEI is staticallyassigned during installation, the TEI is a number ranging from 0 to 63.Dynamically assigned TEIs range from 64 to 126. A TEI of 127, or all 1s, indicatesa broadcast.

[4.1.4] ISDN functions

Several exchanges must occur for one router to connect to another using ISDN.To establish an ISDN call, the D channel is used between the router and the ISDNswitch. Signal System 7 (SS7) signaling is used between the switches within the

service provider network.


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The D channel between the router and the ISDN switch is always up. Q.921describes the ISDN data-link processes of LAPD, which functions like Layer 2processes in the OSI reference model. The D channel is used for call controlfunctions such as call setup, signaling, and termination. These functions are

implemented in the Q.931 protocol. Q.931 specifies OSI reference model Layer 3functions. The Q.931 standard recommends a network layer connection betweenthe terminal endpoint and the local ISDN switch, but it does not impose an end-to-end recommendation. Because some ISDN switches were developed beforeQ.931 was standardized, the various ISDN providers and switch types can and douse various implementations of Q.931. Because switch types are not standard,routers must have commands in their configuration specifying the ISDN switch towhich they are connecting.

The following sequence of events occurs during the establishment of a BRI or PRI

call:

1. The D channel is used to send the called number to the local ISDN switch.

2. The local switch uses the SS7 signaling protocol to set up a path and passthe called number to the remote ISDN switch.

3. The remote ISDN switch signals the destination over the D channel.

4. The destination ISDN NT-1 device sends the remote ISDN switch a call-connect message.

5. The remote ISDN switch uses SS7 to send a call-connect message to the localswitch.

6. The local ISDN switch connects one B channel end-to-end, leaving the otherB channel available for a new conversation or data transfer. Both B channels canbe used simultaneously

[4.1.5] ISDN reference points

ISDN standards define functional groups as devices or pieces of hardware thatenable the user to access the services of the BRI or PRI. Vendors can createhardware that supports one or more functions. ISDN specifications define fourreference points that connect one ISDN device to another.

Each device in an ISDN network performs a specific task to facilitate end-to-endconnectivity.

To connect devices that perform specific functions, the interface between the two

devices needs to be well defined. These interfaces are called reference points.


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The reference points that affect the customer side of the ISDN connection are asfollows:

* R References the connection between a non-ISDN compatible device

Terminal Equipment type 2 (TE2) and a Terminal Adapter (TA), for example an RS-232 serial interface.

* S References the points that connect into the customer switching deviceNetwork Termination type 2 (NT2) and enables calls between the various types ofcustomer premises equipment.

* T Electrically identical to the S interface, it references the outboundconnection from the NT2 to the ISDN network or Network Termination type 1(NT1).

* U References the connection between the NT1 and the ISDN network

owned by the telephone company.

Because the S and T references are electrically similar, some interfaces arelabeled S/T interfaces. Although they perform different functions, the port iselectrically the same and can be used for either function.

[4.1.6] Determining the router ISDN interface

In North America, the NT1 is part of the Customer Premise Equipment (CPE). Thismeans that the customer must supply an NT1 device or a device with integratedNT1 functionality. In North America, ISDN routers are typically equipped with ISDNBRI-U interface cards to provide NT1 functionality. In Europe, the service providersupplies a separate NT1 device. Therefore, the customer supplies an ISDNcapable device to connect to the NT1, such as a router with an ISDN BRI-STinterface.

To select a Cisco router with the appropriate ISDN interface, do the following:


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1. Determine whether the router supports ISDN BRI. Look on the back of therouter for a BRI connector or a BRI WAN Interface Card (WIC).

2. Determine the provider of the NT1. An NT1 terminates the local loop to the

central office (CO) of the ISDN service provider. In North America, the NT1 is partof the Customer Premise Equipment (CPE). This means that the customer mustsupply an NT1 device or a device with integrated NT1 functionality. In NorthAmerica, ISDN routers are typically equipped with ISDN BRI-U interface cards toprovide NT1 functionality. In Europe, the service provider supplies a separate NT1device. Therefore, the customer supplies an ISDN capable device to connect tothe NT1, such as a router with an ISDN BRI-ST interface.

3. If the NT1 is built into the CPE, the router should have a U interface. If therouter has an S/T interface, then it will need an external NT1 to connect to theISDN provider.

If the router has a connector labeled BRI then it is already ISDN-enabled. With anative ISDN interface already built in, the router is a TE1 and will need to connectto an NT1. If the router has a U interface, it also has a built-in NT1.

If the router does not have a connector labeled BRI, and it is a fixed-configuration, or non-modular router, then it must use an existing serial interface.With non-native ISDN interfaces such as serial interfaces, an external TA devicemust be attached to the serial interface to provide BRI connectivity. If the router

is modular it may be possible to upgrade to a native ISDN interface, providing ithas an available slot.

[4.1.7] ISDN switch types

Routers must be configured to identify the type of switch with which they willcommunicate. Available ISDN switch types vary, depending in part on the countryin which the switch is being used. As a consequence of various implementationsof Q.931, the D channel signaling protocol used on ISDN switches varies from

vendor to vendor.

Services offered by ISDN carriers vary considerably from country to country orregion to region.

Like modems, each switch type operates slightly differently, and has a specificset of call setup requirements. Before the router can be connected to an ISDNservice, it must be configured for the switch type used at the CO. This informationmust be specified during router configuration so the router can communicatewith the switch, place ISDN network level calls, and send data.


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In addition to knowing the switch type the service provider is using, it may alsobe necessary to know what service profile identifiers (SPIDs) are assigned by thetelco. A SPID is a number provided by the ISDN carrier to identify the lineconfiguration of the BRI service. SPIDs allow multiple ISDN devices, such as voiceand data equipment, to share the local loop. SPIDs are required by DMS-100 andNational ISDN-1 switches.

SPIDs are used only in North America and Japan. The ISDN carrier provides a SPIDto identify the line configuration of the ISDN service. In many cases whenconfiguring a router, the SPIDs will need to be entered.

Each SPID points to line setup and configuration information. SPIDs are a series ofcharacters that usually resemble telephone numbers. SPIDs identify each Bchannel to the switch at the central office. Once identified, the switch links the

available services to the connection. Remember, ISDN is typically used for dialupconnectivity. The SPIDs are processed when the router initially connects to theISDN switch. If SPIDs are necessary, but are not configured correctly, theinitialization will fail, and the ISDN services cannot be used.

[4.2] ISDN Configuration

[4.2.1] Configuring ISDN BRI

The command isdn switch-type switch-type can be configured at the global orinterface command mode to specify the provider ISDN switch.

Configuring the isdn switch-type command in the global configuration mode setsthe ISDN switch type identically for all ISDN interfaces. Individual interfaces maybe configured, after the global configuration command, to reflect an alternateswitch type.

When the ISDN service is installed, the service provider will issue informationabout the switch type and SPIDs. SPIDs are used to define the services availableto individual ISDN subscribers. Depending on the switch type, these SPIDs mayhave to be added to the configuration. National ISDN-1 and DMS-100 ISDNswitches require SPIDs to be configured, but the AT&T 5ESS switch does not.SPIDs must be specified when using the Adtran ISDN simulator.

The format of the SPIDs can vary depending on the ISDN switch type and specificprovider requirements. Use the isdn spid1 and isdn spid2 interface configurationmode commands to specify the SPID required by the ISDN network when therouter initiates a call to the local ISDN exchange.


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Configuration of ISDN BRI is a mix of global and interface commands.

To configure the ISDN switch type, use the isdn switch-type command in globalconfiguration mode:

Router(config)#isdn switch-type switch-type

The argument switch-type indicates the service provider switch type. To disablethe switch on the ISDN interface, specify isdn switch-type none . The followingexample configures the National ISDN-1 switch type in the global configurationmode:

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

To define SPIDs use the isdn spid# command in interface configuration mode.This command is used to define the SPID numbers that have been assigned forthe B channels:

Router(config-if)#isdn spid1 spid-number [ldn ]

Router(config-if)#isdn spid2 spid-number [ldn ]

The optional ldn argument defines a local dial directory number. On mostswitches, the number must match the called party information coming in fromthe ISDN switch. SPIDs are specified in interface configuration mode. To enterinterface configuration mode, use the interface bri command in the globalconfiguration mode:

Router(config)#interface bri slot/port

Router(config)#interface bri0/0

Router(config-if)#isdn spid1 51055540000001 5554000

Router(config-if)#isdn spid2 51055540010001 5554001


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[4.2.2] Configuring ISDN PRI

ISDN PRI is delivered over a leased T1 or E1 line. The main PRI configuration

tasks are as follows:

1. Specify the correct PRI switch type that the router interfaces with at the COof the ISDN provider.

2. Specify the T1/E1 controller, framing type, and line coding for the facility ofthe ISDN provider.

3. Set a PRI group timeslot for the T1/E1 facility and indicate the speed used.

Because routers connect to PRI using T1/E1, there is no "interface pri" command.Instead, the physical interface on the router that connects to the leased line iscalled a T1 controller, or an E1 controller, if an E1 line is being used. Thiscontroller must be configured properly in order to communicate with the carriernetwork. The ISDN PRI D and PRI B channels are configured separately from thecontroller, using the interface serial command.

Use the isdn switch-type command to specify the ISDN switch used by theprovider to which the PRI connects. As with BRI, this command can be issued

globally or in interface configuration mode. The table shows the switch typesavailable for ISDN PRI configuration:

Router(config)#isdn switch-type primary-net5

Configuring a T1 or E1 controller is done in four parts:

1. From global configuration mode, specify the controller and the slot/port inthe router where the PRI card is located:

Router(config)#controller {t1 | e1} {slot/port}

Router(config-controller)#

2. Configure the framing, line coding, and clocking, as dictated by the service

provider. The framing command is used to select the frame type used by the PRIservice provider. For T1, use the following command syntax:


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Router(config-controller)#framing {sf | esf}

For E1 lines, use the framing command with the following options:

Router(config-controller)#framing {crc4 | no-crc4} [australia]

Use the linecode command to identify the physical-layer signaling method onthe digital facility of the provider:

Router(config-controller)#linecode {ami | b8zs| hdb3}

In North America, the B8ZS signaling method is used for T1 carrier facilities. Itallows a full 64 kbps for each ISDN channel. In Europe, it is typically HDB3encoding that is used.

3. Configure the specified interface for PRI operation and the number of fixedtimeslots that are allocated on the digital facility of the provider:

Router(config-controller)#pri-group [timeslots range]

For T1, the range of timeslots used is 1-24. For E1 the range of timeslots usedis 1-31.

4. Specify an interface for PRI D-channel operation. The interface is a serialinterface to a T1/E1 on the router:

Router(config)#interface serial{slot/port: | unit:}{23 | 15}

Within an E1 or T1 facility, the channels start numbering at 1. The numberingranges from 1 to 31 for E1 and 1 to 24 for T1. Serial interfaces in the Cisco routerstart numbering at 0. Therefore, channel 16, the E1 signaling channel, is channel15 on the interface. Channel 24, the T1 signaling channel, becomes channel 23on the interface. Thus, interface serial 0/0:23 refers to the D channel of a T1 PRI.

Subinterfaces, commonly used with Frame Relay, are designated with a dot, or

period. For example, serial 0/0.16 is a subinterface. Do not confuse the channels


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of a T1 or E1 with subinterfaces. Channels use a colon instead of a dot to indicatethe channel number:

* S0/0.23 refers to a subinterface

* S0/0:23 refers to a channel

[4.2.3] Verifying ISDN configuration

Several show commands can be used to verify that the ISDN configuration hasbeen implemented correctly.

To confirm BRI operations, use the show isdn status command to inspect thestatus of the BRI interfaces. This command can be used after configuring theISDN BRI to verify that the TE1, or router, is communicating correctly with theISDN switch. In the Figure

output, the TEIs have been successfully negotiated and ISDN Layer 3 is ready tomake or receive calls.

Verify that Layer 1 Status is ACTIVE, and that the Layer 2 Status stateMULTIPLE_FRAME_ESTABLISHED appears. This command also displays the number

of active calls.

The show isdn active command displays current call information, including allof the following:

* Called number

* Time until the call is disconnected

* Advice of charge (AOC)

* Charging units used during the call

* Whether the AOC information is provided during calls or at end of calls

The show dialer command displays information about the dialer interface:

* Current call status

* Dialup timer values


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* Dial reason

* Remote device that is connected

The show interface bri0/0 displays statistics for the BRI interface configured onthe router. Channel specific information is displayed by putting the channelnumber at the end of the command. In this case, the show interface bri0/0:1command shows the following:

* The B channel is using PPP encapsulation.

* LCP has negotiated and is open.

* There are two NCPs running, IPCP and Cisco Discovery Protocol ControlProtocol (CDPCP).

[4.2.4] Troubleshooting the ISDN configuration

The following commands are used to debug and troubleshoot the ISDNconfiguration:

* The debug isdn q921 command shows data link layer, or Layer 2, messages

on the D channel between the router and the ISDN switch. Use this command ifthe show isdn status command does not show Layer 1 as ACTIVE and Layer 2 asMULTIPLE_FRAME_ESTABLISHED.

* The debug isdn q931 command shows the exchange of call setup andteardown messages of the Layer 3 ISDN connection.

* The debug ppp authentication command displays the PPP authenticationprotocol messages, including Challenge Handshake Authentication Protocol(CHAP) packet exchanges and Password Authentication Protocol (PAP) exchanges.

* The debug ppp negotiation command displays information on PPP traffic and

exchanges while the PPP components are negotiated. This includes LCP,authentication, and NCP exchanges. A successful PPP negotiation will first openthe LCP state, then authenticate, and finally negotiate NCP.

* The debug ppp error command displays protocol errors and error statisticsassociated with PPP connection negotiation and operation. Use the debug pppcommands to troubleshoot a Layer 2 problem if the show isdn status commanddoes not indicate an ISDN problem.


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[4.3] DDR Configuration

[4.3.1] DDR operation

Dial-on-demand routing (DDR) is triggered when traffic that matches a predefinedset of criteria is queued to be sent out a DDR-enabled interface. The traffic thatcauses a DDR call to be placed is referred to as interesting traffic. Once therouter has transmitted the interesting traffic, the call is terminated.

The key to efficient DDR operation is in the definition of interesting traffic.Interesting traffic is defined with the dialer-list command. Dialer lists can allow alltraffic from a specific protocol to bring up a DDR link, or they can query an accesslist to see what specific types of traffic should bring up the link. Dialer lists do notfilter traffic on an interface. Even traffic that is not interesting will be forwarded ifthe connection to the destination is active.

DDR is implemented in Cisco routers in the following steps:

1. The router receives traffic, performs a routing table lookup to determine ifthere is a route to the destination, and identifies the outbound interface.

2. If the outbound interface is configured for DDR, the router does a lookup todetermine if the traffic is interesting.

3. The router identifies the dialing information necessary to make the call usinga dialer map to access the next-hop router.

4. The router then checks to see if the dialer map is in use. If the interface iscurrently connected to the desired remote destination, the traffic is sent. If theinterface is not currently connected to the remote destination, the router sendscall-setup information through the BRI using the D channel.

5. After the link is enabled, the router transmits both interesting anduninteresting traffic. Uninteresting traffic can include data and routing updates.

6. The idle timer starts and runs as long as no interesting traffic is seen during

the idle timeout period and disconnects the call based on the idler timerconfiguration.

The idle timer setting specifies the length of time the router should remainconnected if no interesting traffic has been sent. Once a DDR connection isestablished, any traffic to that destination will be permitted. However, onlyinteresting traffic resets the idle timer.

[4.3.2] Configuring legacy DDR


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Legacy DDR is a term used to define a very basic DDR configuration in which asingle set of dialer parameters is applied to an interface. If multiple unique dialerconfigurations are needed on one interface, then dialer profiles should be used.

To configure legacy DDR perform the following steps:

* Define static routes

* Specify interesting traffic

* Configure the dialer information

[4.3.3] Defining static routes for DDR

To forward traffic, routers need to know what route to use for a given destination.When a dynamic routing protocol is used, the DDR interface will dial the remotesite for every routing update or hello message if these packets are defined asinteresting traffic. To prevent the frequent or constant activation of the DDR link,configure the necessary routes statically.

To configure a static route for IP use the following command:

Router(config)#ip route net-prefix mask {address | interface } [distance ][permanent]

The Central router has a static route to network 10.40.0.0 on the Home router.

The Home router has two static routes defined for the two subnets on the CentralLAN. If the network attached to the Home router is a stub network, then all non-local traffic should be sent to Central. A default route is a better choice for theHome router in this instance.

Home(config)#ip route 0.0.0.0 0.0.0.0 10.1.0.2

When configuring static routes, consider the following:

* By default, a static route will take precedence over a dynamic routebecause of its lower administrative distance. Without additionalconfiguration, a dynamic route to a network will be ignored if a staticroute is present in the routing table for the same network.


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* To reduce the number of static route entries, define a summarized or defaultstatic route.

[4.3.4] Specifying interesting traffic for DDR

DDR calls are triggered by interesting traffic. This traffic can be defined as any ofthe following:

* IP traffic of a particular protocol type

* Packets with a particular source address or destination

* Other criteria as defined by the network administrator

Use the dialer-list command to identify interesting traffic. The command syntax isas follows:

Router(config)#dialer-list dialer-group-num protocol protocol-name {permit |deny | list access-list-number }

The dialer-group-num is an integer between 1 and 10 that identifies the dialer listto the router. The command dialer-list 1 protocol ip permit will allow all IP trafficto trigger a call.

Instead of permitting all IP traffic, a dialer list can point to an access list in orderto specify exactly what types of traffic should bring up the link. The reference toaccess list 101 in dialer list 2 prevents FTP and Telnet traffic from activating theDDR link. Any other IP packet is considered interesting, and will therefore initiatethe DDR link.

[4.3.5] Configuring DDR dialer information

There are several steps involved in configuring the DDR interface. PPP isconfigured on the dialer interface using the same commands that enable PPP ona serial interface. HDLC is the default encapsulation for an ISDN interface on aCisco router, but most networks employ PPP for circuit-switched connections.Because of its robustness, interoperability, and additional features such asauthentication, PPP is the data link protocol in use on the B channels of mostrouters. To configure PPP on the DDR interface use the following commands:

Home(config)#username Central password cisco


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Home(config)#interface bri0/0

Home(config-if)#encapsulation ppp

Home(config-if)#ppp authentication chap

Home(config-if)#ip address 10.1.0.1 255.255.255.0

A dialer list specifying the interesting traffic for this DDR interface needs to be

associated with the DDR interface. This is done using the dialer-group group-number command:

Home(config-if)#dialer-group 1

In the command, group-number specifies the number of the dialer group to whichthe interface belongs. The group number can be an integer from 1 to 10. Thisnumber must match the dialer-list group-number . Each interface can have onlyone dialer group. However, the same dialer list can be assigned to multipleinterfaces with the dialer-group command.

The correct dialing information for the remote DDR interface needs to bespecified. This is done using the dialer map command.


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The dialer map command maps the remote protocol address to a telephonenumber. This command is necessary to dial multiple sites.

Router(config-if)#dialer map protocol next-hop-address [name hostname ]

[speed 56 | 64] [broadcast] dial-string

If dialing only one site, use an unconditional dialer string command that alwaysdials the one phone number regardless of the traffic destination. This step isunique to legacy DDR. Although the information is always required, the steps toconfigure destination information are different when using dialer profiles insteadof legacy DDR.

The dialer idle-timeout seconds command may be used to specify the number ofidle seconds before a call is disconnected.

The seconds represent the number of seconds until a call is disconnected afterthe last interesting packet is sent. The default is 120.

[4.3.6] Dialer profiles

Legacy DDR is limited because the configuration is applied directly to aphysical interface. Since the IP address is applied directly to the interface, then

only DDR interfaces configured in that specific subnet can establish a DDRconnection with that interface. This means that there is a one-to-onecorrespondence between the two DDR interfaces at each end of the link.

Dialer profiles remove the configuration from the interface receiving or makingcalls and only bind the configuration to the interface on a per-call basis. Dialerprofiles allow physical interfaces to dynamically take on different characteristicsbased on incoming or outgoing call requirements. Dialer profiles can do all ofthe following:

* Define encapsulation and access control lists* Determine minimum or maximum calls* Turn features on or off

Dialer profiles aid in the design and deployment of more complex and scalablecircuit-switched internetworks by implementing a more scalable DDR model inCisco routers and access servers. Dialer profiles separate the logical portion ofDDR, such as the network layer, encapsulation, and dialer parameters, from thephysical interface that places or receives calls.

Using dialer profiles, the following tasks may be performed:


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* Configure B channels of an ISDN interface with different IP subnets.* Use different encapsulations on the B channels of an ISDN interface.

* Set different DDR parameters for the B channels of an ISDNinterface.

* Eliminate the waste of ISDN B channels by letting ISDN BRIs belongto multiple dialer pools.

A dialer profile consists of the following elements:

* Dialer interface A logical entity that uses a per-destination dialer profile.

* Dialer pool Each dialer interface references a dialer pool, which is a groupof one or more physical interfaces associated with a dialer profile.

* Physical interfaces Interfaces in a dialer pool are configured forencapsulation parameters and to identify the dialer pools to which the interfacebelongs. PPP authentication, encapsulation type, and multilink PPP are allconfigured on the physical interface.

Like legacy DDR, dialer profiles activate when interesting traffic is queued to besent out a DDR interface. First, an interesting packet is routed to a remoteDDR IP address. The router then checks the configured dialer interfaces forone that shares the same subnet as the remote DDR IP address. If oneexists, the router looks for an unused physical DDR interface in the dialerpool. The configuration from the dialer profile is then applied to theinterface and the router attempts to create the DDR connection. When theconnection is terminated, the interface is returned to the dialer pool for thenext call.

[4.3.7] Configuring dialer profiles

Multiple dialer interfaces may be configured on a router. Each dialer interface isthe complete configuration for a destination. The interface dialer commandcreates a dialer interface and enters interface configuration mode.

To configure the dialer interface, perform the following tasks:

1. Configure one or more dialer interfaces with all the basic DDR commands:* IP address* Encapsulation type and authentication* Idle-timer* Dialer-group for interesting traffic

2. Configure a dialer string and dialer remote-name to specify the remote routername and phone number to dial it. The dialer pool associates this logicalinterface with a pool of physical interfaces.

3. Configure the physical interfaces and assign them to a dialer pool using the

dialer pool-member command.


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An interface can be assigned to multiple dialer pools by using multiple dialerpool-member commands. If more than one physical interface exists in thepool, use the priority option of the dialer pool-member command to set thepriority of the interface within a dialer pool. If multiple calls need to beplaced and only one interface is available, then the dialer pool with thehighest priority is the one that dials out.

A combination of any of these interfaces may be used with dialer pools:

* Synchronous Serial* Asynchronous Serial* BRI* PRI

[4.3.8] Verifying DDR configuration

The show dialer interface [BRI] command displays information in the same formatas the legacy DDR statistics on incoming and outgoing calls.

The message "Dialer state is data link layer up" suggests that the dialer came upproperly and interface BRI 0/0:1 is bound to the profile dialer1.

The show isdn active command displays information about the current activeISDN calls.

In this output, the ISDN call is outgoing to a remote router named Seattle.

The show isdn status command displays information about the three layers of theBRI interface.

In this output, ISDN Layer 1 is active, ISDN Layer 2 is established with SPID1 andSPID2 validated, and there is one active connection on Layer 3.

[4.3.9] Troubleshooting the DDR configuration

There are two major types of DDR problems. Either a router is not dialing when itshould, or it is constantly dialing when it should not. Several debugcommands can be used to help troubleshoot problems with a DDRconfiguration.

In the following lines, the seventh and eighth most significant hexadecimalnumbers indicate the type of message.

* 0x05 indicates a call setup message* 0x02 indicates a call proceeding message* 0x07 indicates a call connect message* 0x0F indicates a connect acknowledgment (ack) message

The debug isdn q931 command is useful for viewing Layer 2 ISDN call setupexchanges for both outgoing and incoming calls. The i = field in theQ.921 payload field is the hexadecimal value of a Q.931 message.

The debug dialer [events | packets] command is useful for troubleshooting DDRconnectivity. The debug dialer events command sends a message to the


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console indicating when a DDR link has connected and what traffic causedit to connect.

If a router is not configured correctly for DDR, then the output of thecommand will usually indicate the source of the problem. If there is nodebug output, then the router is not aware of any interesting traffic. Anincorrectly configured dialer or access list may be the cause.

Not all DDR problems result in an interface failing to dial. Routing protocolscan cause an interface to continuously dial, even if there is nouser data to send. An interface that is constantly going up and down issaid to be flapping. The debug dialer packet command sends a messageto the console every time a packet is sent out a DDR interface. Use thisdebug command to see exactly what traffic is responsible for a flappingDDR interface.

If a router is not connecting when it should, then it is possible that an ISDNproblem is the cause, as opposed to a DDR problem. The remote router

may be incorrectly configured, or there could be a problem with the ISDNcarrier network. Use the isdn call interface command to force the localrouter to attempt to dial into the remote router.

If the routers cannot communicate using this command, then the lack ofconnectivity is an ISDN problem, not a DDR problem. However, ifthe routers can communicate, then both the toll network and theISDN configurations on the routers are working properly. In thiscase, the problem is most likely an error in the DDR configurationon either router.

In some cases it is useful to reset the connection between the router and thelocal ISDN switch. The clear interface bri command clears currently

established connections on the interface and resets the interface with theISDN switch. This command forces the router to renegotiate its SPIDs withthe ISDN switch, and is sometimes necessary after making changes to theisdn spid1 and isdn spid2 commands on an interface.

Module Summary

ISDN refers to a set of communication protocols proposed by telephonecompanies to permit telephone networks to carry integrated voice, video,

and data services. ISDN permits communication over high-quality, high-speed, digital communication channels.

DDR is used in order to save the costs of a dedicated WAN line for organizationsand companies that do not need a permanent connection. It can also beused as a backup by organizations that use the dedicated line for criticalapplications.

An understanding of the following key points should have been achieved:

* ISDN carries data, voice, and video* ISDN uses standards for addressing, concepts, and signaling

* ISDN uses the physical and data-link layers* Interfaces and reference points for ISDN


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* Router configuration for ISDN* Which traffic is allowed when configuring DDR* Static routes for DDR* The correct encapsulation type for DDR* Access lists affecting DDR traffic* Dialer interfaces

ccna4 module 4

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