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] [4.1.7] [4.2]
Determining the router ISDN interface..............................................7 ISDN switch types.................................................................................7
ISDN Configuration.....................................................................................8
[4.2.1]
Configuring ISDN BRI............................................................................8
[4.2.2]
Configuring ISDN PRI............................................................................9
[4.2.3]
Verifying ISDN configuration...............................................................10
[4.2.4]
Troubleshooting 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 interesting traffic for DDR..................................................13
[4.3.5]
Configuring DDR dialer information....................................................13
[4.3.6]
Dialer profiles.....................................................................................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
Module Overview Integrated Services Digital Network (ISDN) is a network that provides end-to-end digital connectivity to support a wide range of services including voice and data services.
ISDN allows multiple digital channels to operate simultaneously through the same regular phone wiring used for analog lines, but ISDN transmits a digital signal rather 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 the use of existing telephone lines to form a wide-area network (WAN), instead of using separate, dedicated lines. Public switched telephone networks (PSTNs) are involved in this process.
DDR is used when a constant connection is not needed, thus reducing costs. DDR defines the process of a router connecting using a dialup network when there is traffic 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
[4.1.1]
Introducing ISDN
There are several WAN technologies used to provide network access from remote locations. One of these technologies is ISDN. ISDN can be used as a solution to the low bandwidth problems that small offices or dial-in users have with traditional telephone dial-in services.
The traditional PSTN was based on an analog connection between the customer premises and the local exchange, also called the local loop. The analog circuits introduce limitations on the bandwidth that can be obtained on the local loop. Circuit restrictions do not permit analog bandwidths greater than approximately 3000 Hz. ISDN technology permits the use of digital data on the local loop, providing better access speeds for the remote users.
Telephone companies developed ISDN with the intention of creating a totally digital network. ISDN allows digital signals to be transmitted over existing telephone wiring. This became possible when the telephone company switches were upgraded to handle digital signals. ISDN is generally used for telecommuting and networking small and remote offices into the corporate LAN.
Telephone companies developed ISDN as part of an effort to standardize subscriber services. This included the User-Network Interface (UNI), better known as the local loop. The ISDN standards define the hardware and call setup schemes for end-to-end digital connectivity. These standards help achieve the goal of worldwide connectivity by ensuring that ISDN networks easily communicate with one another. In an ISDN network, the digitizing function is done at the user site rather than the telephone company.
ISDN brings digital connectivity to remote sites. The following list provides some of 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 possible to use multiple channels to carry different types of traffic over a single connection.
ISDN uses out-of-band signaling, the delta (D channel), for call setup and signaling. To make a normal telephone call, the user dials the number one digit at a time. Once all the numbers are received, the call can be placed to the remote user. ISDN delivers the numbers to the switch at D-channel rates, thus reducing the 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 B channel provides 64 kbps of bandwidth. With multiple B channels, ISDN offers more bandwidth for WAN connections than some leased services. An ISDN connection with two B channels would provide a total usable bandwidth of 128 kbps.
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 asynchronous serial 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 of ISDN recommendations was published in 1984 and is continuously updated by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), formerly known as the Consultative Committee for International Telegraph and Telephone (CCITT). The ISDN standards are a set of protocols that encompass digital telephony and data communications. The ITU-T groups and organizes the ISDN protocols according to the following general topic areas:
* E Protocols — Recommend telephone network standards for ISDN. For example, the E.164 protocol describes international addressing for ISDN. * I Protocols — Deal with concepts, terminology, and general methods. The I.100 series includes general ISDN concepts and the structure of other I-series recommendations. 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 transmission rate. The bearer channel, or B channel, is defined as a clear digital path of 64 kbps. It is said to be clear because it can be used to transmit any type of digitized data in full-duplex mode. For example, a digitized voice call can be transmitted on a single B channel. The second channel type is called a delta channel, or D channel. There can either be 16 kbps for the Basic Rate Interface (BRI) or 64 kbps for the Primary Rate Interface (PRI).
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 called the connection setup. This information is exchanged over the path on which the data will eventually be transmitted. Both the control information and the data share the same pathway. This is called in-band signaling. ISDN however, uses a separate channel for control information, the D channel. This is called out-of-band signaling.
ISDN specifies two standard access methods, BRI and PRI. A single BRI or PRI interface provides a multiplexed bundle of B and D channels.
BRI uses two 64 kbps B channels plus one 16kbps D channel. BRI operates with many Cisco routers. Because it uses two B channels and one D channel, BRI is sometimes 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 be used for relatively high-speed data transport. In this mode, the information is carried in frame format, using either high-level data link control (HDLC) or PPP as the Layer 2 protocol. PPP is more robust than HDLC because it provides a mechanism for authentication and negotiation of compatible link and protocol configuration.
ISDN is considered a circuit-switched connection. The B channel is the elemental circuit-switching unit.
The D channel carries signaling messages, such as call setup and teardown, to control calls on B channels. Traffic over the D channel employs the Link Access Procedure on the D Channel (LAPD) protocol. LAPD is a data link layer protocol based on HDLC.
In North America and Japan, PRI offers twenty-three 64 kbps B channels and one 64 kbps D channel. A PRI offers the same service as a T1 or DS1 connection. In Europe and much of the rest of the world, PRI offers 30 B channels and one D channel in order to offer the same level of service as an E1 circuit. PRI uses a Data Service Unit/Channel Service Unit (DSU/CSU) for T1/E1 connections.
[4.1.3]
ISDN 3-layer model and protocols
ISDN utilizes a suite of ITU-T standards spanning the physical, data link, and network 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 in the 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 and ITU-T Q.931, also known as I.451. These standards specify user-to-user, circuitswitched, and packet-switched connections.
BRI service is provided over a local copper loop that traditionally carries analog phone service. While there is only one physical path for a BRI, there are three separate information paths, 2B+D. Information from the three channels is multiplexed into the one physical path.
ISDN physical layer, or Layer 1, frame formats differ depending on whether the frame is outbound or inbound. If the frame is outbound, it is sent from the terminal to the network. Outbound frames use the TE frame format. If the frame is inbound, it is sent from the network to the terminal. Inbound frames use the NT frame 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
accounts for 144 kbps (B1 + B2 + D) of the total ISDN BRI physical interface bit rate of 192 kbps. The remainder of the data rate are the overhead bits that are required 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 when several 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. The effective 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 is used across the D channel to ensure that control and signaling information is received and flows properly.
The LAPD flag and control fields are identical to those of HDLC. The LAPD address field 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. The command/response bit (C/R), indicates whether the frame contains a command or a response. The second byte contains the terminal endpoint identifier (TEI). Each piece of terminal equipment on the customer premises needs a unique identifier. The TEI may be statically assigned at installation, or the switch may dynamically assign it when the equipment is started up. If the TEI is statically assigned 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, indicates a 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 ISDN switch. Signal System 7 (SS7) signaling is used between the switches within the service provider network.
The D channel between the router and the ISDN switch is always up. Q.921 describes the ISDN data-link processes of LAPD, which functions like Layer 2 processes in the OSI reference model. The D channel is used for call control functions such as call setup, signaling, and termination. These functions are implemented in the Q.931 protocol. Q.931 specifies OSI reference model Layer 3 functions. The Q.931 standard recommends a network layer connection between the terminal endpoint and the local ISDN switch, but it does not impose an endto-end recommendation. Because some ISDN switches were developed before Q.931 was standardized, the various ISDN providers and switch types can and do use various implementations of Q.931. Because switch types are not standard, routers must have commands in their configuration specifying the ISDN switch to which 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 pass the 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 callconnect message. 5. The remote ISDN switch uses SS7 to send a call-connect message to the local switch. 6. The local ISDN switch connects one B channel end-to-end, leaving the other B channel available for a new conversation or data transfer. Both B channels can be used simultaneously
[4.1.5]
ISDN reference points
ISDN standards define functional groups as devices or pieces of hardware that enable the user to access the services of the BRI or PRI. Vendors can create hardware that supports one or more functions. ISDN specifications define four reference points that connect one ISDN device to another. Each device in an ISDN network performs a specific task to facilitate end-to-end connectivity.
To connect devices that perform specific functions, the interface between the two devices needs to be well defined. These interfaces are called reference points.
The reference points that affect the customer side of the ISDN connection are as follows:
* R — References the connection between a non-ISDN compatible device Terminal Equipment type 2 (TE2) and a Terminal Adapter (TA), for example an RS232 serial interface. * S — References the points that connect into the customer switching device Network Termination type 2 (NT2) and enables calls between the various types of customer premises equipment. * T — Electrically identical to the S interface, it references the outbound connection 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 are labeled S/T interfaces. Although they perform different functions, the port is electrically 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). This means that the customer must supply an NT1 device or a device with integrated NT1 functionality. In North America, ISDN routers are typically equipped with ISDN BRI-U interface cards to provide NT1 functionality. In Europe, the service provider supplies a separate NT1 device. Therefore, the customer supplies an ISDN capable device to connect to the NT1, such as a router with an ISDN BRI-ST interface.
To select a Cisco router with the appropriate ISDN interface, do the following:
1. Determine whether the router supports ISDN BRI. Look on the back of the router 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 part of the Customer Premise Equipment (CPE). This means that the customer must supply an NT1 device or a device with integrated NT1 functionality. In North America, ISDN routers are typically equipped with ISDN BRI-U interface cards to provide NT1 functionality. In Europe, the service provider supplies a separate NT1 device. Therefore, the customer supplies an ISDN capable device to connect to the 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 the router has an S/T interface, then it will need an external NT1 to connect to the ISDN provider.
If the router has a connector labeled BRI then it is already ISDN-enabled. With a native ISDN interface already built in, the router is a TE1 and will need to connect to 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 fixedconfiguration, or non-modular router, then it must use an existing serial interface. With non-native ISDN interfaces such as serial interfaces, an external TA device must 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 it has an available slot.
[4.1.7]
ISDN switch types
Routers must be configured to identify the type of switch with which they will communicate. Available ISDN switch types vary, depending in part on the country in which the switch is being used. As a consequence of various implementations of 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 or region to region. Like modems, each switch type operates slightly differently, and has a specific set of call setup requirements. Before the router can be connected to an ISDN service, it must be configured for the switch type used at the CO. This information must be specified during router configuration so the router can communicate with the switch, place ISDN network level calls, and send data.
In addition to knowing the switch type the service provider is using, it may also be necessary to know what service profile identifiers (SPIDs) are assigned by the telco. A SPID is a number provided by the ISDN carrier to identify the line configuration of the BRI service. SPIDs allow multiple ISDN devices, such as voice and data equipment, to share the local loop. SPIDs are required by DMS-100 and National ISDN-1 switches.
SPIDs are used only in North America and Japan. The ISDN carrier provides a SPID to identify the line configuration of the ISDN service. In many cases when configuring a router, the SPIDs will need to be entered.
Each SPID points to line setup and configuration information. SPIDs are a series of characters that usually resemble telephone numbers. SPIDs identify each B channel to the switch at the central office. Once identified, the switch links the available services to the connection. Remember, ISDN is typically used for dialup connectivity. The SPIDs are processed when the router initially connects to the ISDN switch. If SPIDs are necessary, but are not configured correctly, the initialization will fail, and the ISDN services cannot be used.
[4.2] [4.2.1]
ISDN Configuration Configuring ISDN BRI
The command isdn switch-type switch-type can be configured at the global or interface command mode to specify the provider ISDN switch.
Configuring the isdn switch-type command in the global configuration mode sets the ISDN switch type identically for all ISDN interfaces. Individual interfaces may be configured, after the global configuration command, to reflect an alternate switch type.
When the ISDN service is installed, the service provider will issue information about the switch type and SPIDs. SPIDs are used to define the services available to individual ISDN subscribers. Depending on the switch type, these SPIDs may have to be added to the configuration. National ISDN-1 and DMS-100 ISDN switches 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 specific provider requirements. Use the isdn spid1 and isdn spid2 interface configuration mode commands to specify the SPID required by the ISDN network when the router initiates a call to the local ISDN exchange.
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 global configuration mode:
Router(config)#isdn switch-type switch-type
The argument switch-type indicates the service provider switch type. To disable the switch on the ISDN interface, specify isdn switch-type none . The following example configures the National ISDN-1 switch type in the global configuration mode:
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 for the 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 most switches, the number must match the called party information coming in from the ISDN switch. SPIDs are specified in interface configuration mode. To enter interface configuration mode, use the interface bri command in the global configuration 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
[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 CO of the ISDN provider. 2. Specify the T1/E1 controller, framing type, and line coding for the facility of the 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 is called a T1 controller, or an E1 controller, if an E1 line is being used. This controller must be configured properly in order to communicate with the carrier network. The ISDN PRI D and PRI B channels are configured separately from the controller, using the interface serial command.
Use the isdn switch-type command to specify the ISDN switch used by the provider to which the PRI connects. As with BRI, this command can be issued globally or in interface configuration mode. The table shows the switch types available 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 in the 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 PRI service provider. For T1, use the following command syntax:
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 on the 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. It allows a full 64 kbps for each ISDN channel. In Europe, it is typically HDB3 encoding that is used. 3. Configure the specified interface for PRI operation and the number of fixed timeslots 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 used is 1-31. 4. Specify an interface for PRI D-channel operation. The interface is a serial interface 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 numbering ranges from 1 to 31 for E1 and 1 to 24 for T1. Serial interfaces in the Cisco router start numbering at 0. Therefore, channel 16, the E1 signaling channel, is channel 15 on the interface. Channel 24, the T1 signaling channel, becomes channel 23 on 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
of a T1 or E1 with subinterfaces. Channels use a colon instead of a dot to indicate the 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 has been implemented correctly.
To confirm BRI operations, use the show isdn status command to inspect the status of the BRI interfaces. This command can be used after configuring the ISDN BRI to verify that the TE1, or router, is communicating correctly with the ISDN switch. In the Figure output, the TEIs have been successfully negotiated and ISDN Layer 3 is ready to make or receive calls.
Verify that Layer 1 Status is ACTIVE, and that the Layer 2 Status state MULTIPLE_FRAME_ESTABLISHED appears. This command also displays the number of active calls.
The show isdn active command displays current call information, including all of 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
* Dial reason * Remote device that is connected
The show interface bri0/0 displays statistics for the BRI interface configured on the router. Channel specific information is displayed by putting the channel number at the end of the command. In this case, the show interface bri0/0:1 command 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 Control Protocol (CDPCP).
[4.2.4]
Troubleshooting the ISDN configuration
The following commands are used to debug and troubleshoot the ISDN configuration:
* 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 if the show isdn status command does not show Layer 1 as ACTIVE and Layer 2 as MULTIPLE_FRAME_ESTABLISHED. * The debug isdn q931 command shows the exchange of call setup and teardown messages of the Layer 3 ISDN connection. * The debug ppp authentication command displays the PPP authentication protocol 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 open the LCP state, then authenticate, and finally negotiate NCP. * The debug ppp error command displays protocol errors and error statistics associated with PPP connection negotiation and operation. Use the debug ppp commands to troubleshoot a Layer 2 problem if the show isdn status command does not indicate an ISDN problem.
[4.3] [4.3.1]
DDR Configuration DDR operation
Dial-on-demand routing (DDR) is triggered when traffic that matches a predefined set of criteria is queued to be sent out a DDR-enabled interface. The traffic that causes a DDR call to be placed is referred to as interesting traffic. Once the router 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 all traffic from a specific protocol to bring up a DDR link, or they can query an access list to see what specific types of traffic should bring up the link. Dialer lists do not filter traffic on an interface. Even traffic that is not interesting will be forwarded if the 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 if there 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 to determine if the traffic is interesting. 3. The router identifies the dialing information necessary to make the call using a 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 is currently connected to the desired remote destination, the traffic is sent. If the interface is not currently connected to the remote destination, the router sends call-setup information through the BRI using the D channel. 5. After the link is enabled, the router transmits both interesting and uninteresting 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 timer configuration.
The idle timer setting specifies the length of time the router should remain connected if no interesting traffic has been sent. Once a DDR connection is established, any traffic to that destination will be permitted. However, only interesting traffic resets the idle timer.
[4.3.2]
Configuring legacy DDR
Legacy DDR is a term used to define a very basic DDR configuration in which a single set of dialer parameters is applied to an interface. If multiple unique dialer configurations 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 remote site for every routing update or hello message if these packets are defined as interesting 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 Central LAN. If the network attached to the Home router is a stub network, then all nonlocal traffic should be sent to Central. A default route is a better choice for the Home 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 route because of its lower administrative distance. Without additional configuration, a dynamic route to a network will be ignored if a static route is present in the routing table for the same network.
* To reduce the number of static route entries, define a summarized or default static route.
[4.3.4]
Specifying interesting traffic for DDR
DDR calls are triggered by interesting traffic. This traffic can be defined as any of the 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 is as 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 list to the router. The command dialer-list 1 protocol ip permit will allow all IP traffic to trigger a call. Instead of permitting all IP traffic, a dialer list can point to an access list in order to specify exactly what types of traffic should bring up the link. The reference to access list 101 in dialer list 2 prevents FTP and Telnet traffic from activating the DDR link. Any other IP packet is considered interesting, and will therefore initiate the DDR link.
[4.3.5]
Configuring DDR dialer information
There are several steps involved in configuring the DDR interface. PPP is configured on the dialer interface using the same commands that enable PPP on a serial interface. HDLC is the default encapsulation for an ISDN interface on a Cisco router, but most networks employ PPP for circuit-switched connections. Because of its robustness, interoperability, and additional features such as authentication, PPP is the data link protocol in use on the B channels of most routers. To configure PPP on the DDR interface use the following commands:
Home(config)#username Central password cisco
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 groupnumber command:
Home(config-if)#dialer-group 1
In the command, group-number specifies the number of the dialer group to which the interface belongs. The group number can be an integer from 1 to 10. This number must match the dialer-list group-number . Each interface can have only one dialer group. However, the same dialer list can be assigned to multiple interfaces with the dialer-group command.
The correct dialing information for the remote DDR interface needs to be specified. This is done using the dialer map command.
The dialer map command maps the remote protocol address to a telephone number. 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 always dials the one phone number regardless of the traffic destination. This step is unique to legacy DDR. Although the information is always required, the steps to configure destination information are different when using dialer profiles instead of legacy DDR.
The dialer idle-timeout seconds command may be used to specify the number of idle seconds before a call is disconnected. The seconds represent the number of seconds until a call is disconnected after the 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 a physical interface. Since the IP address is applied directly to the interface, then only DDR interfaces configured in that specific subnet can establish a DDR connection with that interface. This means that there is a one-to-one correspondence between the two DDR interfaces at each end of the link.
Dialer profiles remove the configuration from the interface receiving or making calls and only bind the configuration to the interface on a per-call basis. Dialer profiles allow physical interfaces to dynamically take on different characteristics based on incoming or outgoing call requirements. Dialer profiles can do all of the 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 scalable circuit-switched internetworks by implementing a more scalable DDR model in Cisco routers and access servers. Dialer profiles separate the logical portion of DDR, such as the network layer, encapsulation, and dialer parameters, from the physical interface that places or receives calls.
Using dialer profiles, the following tasks may be performed:
* 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 ISDN interface. * Eliminate the waste of ISDN B channels by letting ISDN BRIs belong to 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 group of one or more physical interfaces associated with a dialer profile. * Physical interfaces — Interfaces in a dialer pool are configured for encapsulation parameters and to identify the dialer pools to which the interface belongs. PPP authentication, encapsulation type, and multilink PPP are all configured on the physical interface.
Like legacy DDR, dialer profiles activate when interesting traffic is queued to be sent out a DDR interface. First, an interesting packet is routed to a remote DDR IP address. The router then checks the configured dialer interfaces for one that shares the same subnet as the remote DDR IP address. If one exists, the router looks for an unused physical DDR interface in the dialer pool. The configuration from the dialer profile is then applied to the interface and the router attempts to create the DDR connection. When the connection is terminated, the interface is returned to the dialer pool for the next call.
[4.3.7]
Configuring dialer profiles
Multiple dialer interfaces may be configured on a router. Each dialer interface is the complete configuration for a destination. The interface dialer command creates 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 router name and phone number to dial it. The dialer pool associates this logical interface with a pool of physical interfaces. 3. Configure the physical interfaces and assign them to a dialer pool using the dialer pool-member command.
An interface can be assigned to multiple dialer pools by using multiple dialer pool-member commands. If more than one physical interface exists in the pool, use the priority option of the dialer pool-member command to set the priority of the interface within a dialer pool. If multiple calls need to be placed and only one interface is available, then the dialer pool with the highest 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 format as the legacy DDR statistics on incoming and outgoing calls. The message "Dialer state is data link layer up" suggests that the dialer came up properly and interface BRI 0/0:1 is bound to the profile dialer1. The show isdn active command displays information about the current active ISDN 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 the BRI interface. In this output, ISDN Layer 1 is active, ISDN Layer 2 is established with SPID1 and SPID2 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 it should, or it is constantly dialing when it should not. Several debug commands can be used to help troubleshoot problems with a DDR configuration. In the following lines, the seventh and eighth most significant hexadecimal numbers indicate the type of message. * * * *
0x05 0x02 0x07 0x0F
indicates indicates indicates indicates
a a a a
call setup message call proceeding message call connect message connect acknowledgment (ack) message
The debug isdn q931 command is useful for viewing Layer 2 ISDN call setup exchanges for both outgoing and incoming calls. The “i =” field in the Q.921 payload field is the hexadecimal value of a Q.931 message. The debug dialer [events | packets] command is useful for troubleshooting DDR connectivity. The debug dialer events command sends a message to the
console indicating when a DDR link has connected and what traffic caused it to connect. If a router is not configured correctly for DDR, then the output of the command will usually indicate the source of the problem. If there is no debug output, then the router is not aware of any interesting traffic. An incorrectly configured dialer or access list may be the cause. Not all DDR problems result in an interface failing to dial. Routing protocols can cause an interface to continuously dial, even if there is no user data to send. An interface that is constantly going up and down is said to be flapping. The debug dialer packet command sends a message to the console every time a packet is sent out a DDR interface. Use this debug command to see exactly what traffic is responsible for a flapping DDR interface. If a router is not connecting when it should, then it is possible that an ISDN problem is the cause, as opposed to a DDR problem. The remote router may be incorrectly configured, or there could be a problem with the ISDN carrier network. Use the isdn call interface command to force the local router to attempt to dial into the remote router. If the routers cannot communicate using this command, then the lack of connectivity is an ISDN problem, not a DDR problem. However, if the routers can communicate, then both the toll network and the ISDN configurations on the routers are working properly. In this case, the problem is most likely an error in the DDR configuration on either router. In some cases it is useful to reset the connection between the router and the local ISDN switch. The clear interface bri command clears currently established connections on the interface and resets the interface with the ISDN switch. This command forces the router to renegotiate its SPIDs with the ISDN switch, and is sometimes necessary after making changes to the isdn spid1 and isdn spid2 commands on an interface.
Module Summary ISDN refers to a set of communication protocols proposed by telephone companies to permit telephone networks to carry integrated voice, video, and data services. ISDN permits communication over high-quality, highspeed, digital communication channels. DDR is used in order to save the costs of a dedicated WAN line for organizations and companies that do not need a permanent connection. It can also be used as a backup by organizations that use the dedicated line for critical applications. 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
* * * * * *
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
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] [4.1.7] [4.2]
Determining the router ISDN interface..............................................7 ISDN switch types.................................................................................7
ISDN Configuration.....................................................................................8
[4.2.1]
Configuring ISDN BRI............................................................................8
[4.2.2]
Configuring ISDN PRI............................................................................9
[4.2.3]
Verifying ISDN configuration...............................................................10
[4.2.4]
Troubleshooting 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 interesting traffic for DDR..................................................13
[4.3.5]
Configuring DDR dialer information....................................................13
[4.3.6]
Dialer profiles.....................................................................................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
Module Overview Integrated Services Digital Network (ISDN) is a network that provides end-to-end digital connectivity to support a wide range of services including voice and data services.
ISDN allows multiple digital channels to operate simultaneously through the same regular phone wiring used for analog lines, but ISDN transmits a digital signal rather 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 the use of existing telephone lines to form a wide-area network (WAN), instead of using separate, dedicated lines. Public switched telephone networks (PSTNs) are involved in this process.
DDR is used when a constant connection is not needed, thus reducing costs. DDR defines the process of a router connecting using a dialup network when there is traffic 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
[4.1.1]
Introducing ISDN
There are several WAN technologies used to provide network access from remote locations. One of these technologies is ISDN. ISDN can be used as a solution to the low bandwidth problems that small offices or dial-in users have with traditional telephone dial-in services.
The traditional PSTN was based on an analog connection between the customer premises and the local exchange, also called the local loop. The analog circuits introduce limitations on the bandwidth that can be obtained on the local loop. Circuit restrictions do not permit analog bandwidths greater than approximately 3000 Hz. ISDN technology permits the use of digital data on the local loop, providing better access speeds for the remote users.
Telephone companies developed ISDN with the intention of creating a totally digital network. ISDN allows digital signals to be transmitted over existing telephone wiring. This became possible when the telephone company switches were upgraded to handle digital signals. ISDN is generally used for telecommuting and networking small and remote offices into the corporate LAN.
Telephone companies developed ISDN as part of an effort to standardize subscriber services. This included the User-Network Interface (UNI), better known as the local loop. The ISDN standards define the hardware and call setup schemes for end-to-end digital connectivity. These standards help achieve the goal of worldwide connectivity by ensuring that ISDN networks easily communicate with one another. In an ISDN network, the digitizing function is done at the user site rather than the telephone company.
ISDN brings digital connectivity to remote sites. The following list provides some of 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 possible to use multiple channels to carry different types of traffic over a single connection.
ISDN uses out-of-band signaling, the delta (D channel), for call setup and signaling. To make a normal telephone call, the user dials the number one digit at a time. Once all the numbers are received, the call can be placed to the remote user. ISDN delivers the numbers to the switch at D-channel rates, thus reducing the 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 B channel provides 64 kbps of bandwidth. With multiple B channels, ISDN offers more bandwidth for WAN connections than some leased services. An ISDN connection with two B channels would provide a total usable bandwidth of 128 kbps.
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 asynchronous serial 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 of ISDN recommendations was published in 1984 and is continuously updated by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), formerly known as the Consultative Committee for International Telegraph and Telephone (CCITT). The ISDN standards are a set of protocols that encompass digital telephony and data communications. The ITU-T groups and organizes the ISDN protocols according to the following general topic areas:
* E Protocols — Recommend telephone network standards for ISDN. For example, the E.164 protocol describes international addressing for ISDN. * I Protocols — Deal with concepts, terminology, and general methods. The I.100 series includes general ISDN concepts and the structure of other I-series recommendations. 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 transmission rate. The bearer channel, or B channel, is defined as a clear digital path of 64 kbps. It is said to be clear because it can be used to transmit any type of digitized data in full-duplex mode. For example, a digitized voice call can be transmitted on a single B channel. The second channel type is called a delta channel, or D channel. There can either be 16 kbps for the Basic Rate Interface (BRI) or 64 kbps for the Primary Rate Interface (PRI).
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 called the connection setup. This information is exchanged over the path on which the data will eventually be transmitted. Both the control information and the data share the same pathway. This is called in-band signaling. ISDN however, uses a separate channel for control information, the D channel. This is called out-of-band signaling.
ISDN specifies two standard access methods, BRI and PRI. A single BRI or PRI interface provides a multiplexed bundle of B and D channels.
BRI uses two 64 kbps B channels plus one 16kbps D channel. BRI operates with many Cisco routers. Because it uses two B channels and one D channel, BRI is sometimes 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 be used for relatively high-speed data transport. In this mode, the information is carried in frame format, using either high-level data link control (HDLC) or PPP as the Layer 2 protocol. PPP is more robust than HDLC because it provides a mechanism for authentication and negotiation of compatible link and protocol configuration.
ISDN is considered a circuit-switched connection. The B channel is the elemental circuit-switching unit.
The D channel carries signaling messages, such as call setup and teardown, to control calls on B channels. Traffic over the D channel employs the Link Access Procedure on the D Channel (LAPD) protocol. LAPD is a data link layer protocol based on HDLC.
In North America and Japan, PRI offers twenty-three 64 kbps B channels and one 64 kbps D channel. A PRI offers the same service as a T1 or DS1 connection. In Europe and much of the rest of the world, PRI offers 30 B channels and one D channel in order to offer the same level of service as an E1 circuit. PRI uses a Data Service Unit/Channel Service Unit (DSU/CSU) for T1/E1 connections.
[4.1.3]
ISDN 3-layer model and protocols
ISDN utilizes a suite of ITU-T standards spanning the physical, data link, and network 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 in the 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 and ITU-T Q.931, also known as I.451. These standards specify user-to-user, circuitswitched, and packet-switched connections.
BRI service is provided over a local copper loop that traditionally carries analog phone service. While there is only one physical path for a BRI, there are three separate information paths, 2B+D. Information from the three channels is multiplexed into the one physical path.
ISDN physical layer, or Layer 1, frame formats differ depending on whether the frame is outbound or inbound. If the frame is outbound, it is sent from the terminal to the network. Outbound frames use the TE frame format. If the frame is inbound, it is sent from the network to the terminal. Inbound frames use the NT frame 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
accounts for 144 kbps (B1 + B2 + D) of the total ISDN BRI physical interface bit rate of 192 kbps. The remainder of the data rate are the overhead bits that are required 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 when several 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. The effective 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 is used across the D channel to ensure that control and signaling information is received and flows properly.
The LAPD flag and control fields are identical to those of HDLC. The LAPD address field 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. The command/response bit (C/R), indicates whether the frame contains a command or a response. The second byte contains the terminal endpoint identifier (TEI). Each piece of terminal equipment on the customer premises needs a unique identifier. The TEI may be statically assigned at installation, or the switch may dynamically assign it when the equipment is started up. If the TEI is statically assigned 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, indicates a 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 ISDN switch. Signal System 7 (SS7) signaling is used between the switches within the service provider network.
The D channel between the router and the ISDN switch is always up. Q.921 describes the ISDN data-link processes of LAPD, which functions like Layer 2 processes in the OSI reference model. The D channel is used for call control functions such as call setup, signaling, and termination. These functions are implemented in the Q.931 protocol. Q.931 specifies OSI reference model Layer 3 functions. The Q.931 standard recommends a network layer connection between the terminal endpoint and the local ISDN switch, but it does not impose an endto-end recommendation. Because some ISDN switches were developed before Q.931 was standardized, the various ISDN providers and switch types can and do use various implementations of Q.931. Because switch types are not standard, routers must have commands in their configuration specifying the ISDN switch to which 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 pass the 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 callconnect message. 5. The remote ISDN switch uses SS7 to send a call-connect message to the local switch. 6. The local ISDN switch connects one B channel end-to-end, leaving the other B channel available for a new conversation or data transfer. Both B channels can be used simultaneously
[4.1.5]
ISDN reference points
ISDN standards define functional groups as devices or pieces of hardware that enable the user to access the services of the BRI or PRI. Vendors can create hardware that supports one or more functions. ISDN specifications define four reference points that connect one ISDN device to another. Each device in an ISDN network performs a specific task to facilitate end-to-end connectivity.
To connect devices that perform specific functions, the interface between the two devices needs to be well defined. These interfaces are called reference points.
The reference points that affect the customer side of the ISDN connection are as follows:
* R — References the connection between a non-ISDN compatible device Terminal Equipment type 2 (TE2) and a Terminal Adapter (TA), for example an RS232 serial interface. * S — References the points that connect into the customer switching device Network Termination type 2 (NT2) and enables calls between the various types of customer premises equipment. * T — Electrically identical to the S interface, it references the outbound connection 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 are labeled S/T interfaces. Although they perform different functions, the port is electrically 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). This means that the customer must supply an NT1 device or a device with integrated NT1 functionality. In North America, ISDN routers are typically equipped with ISDN BRI-U interface cards to provide NT1 functionality. In Europe, the service provider supplies a separate NT1 device. Therefore, the customer supplies an ISDN capable device to connect to the NT1, such as a router with an ISDN BRI-ST interface.
To select a Cisco router with the appropriate ISDN interface, do the following:
1. Determine whether the router supports ISDN BRI. Look on the back of the router 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 part of the Customer Premise Equipment (CPE). This means that the customer must supply an NT1 device or a device with integrated NT1 functionality. In North America, ISDN routers are typically equipped with ISDN BRI-U interface cards to provide NT1 functionality. In Europe, the service provider supplies a separate NT1 device. Therefore, the customer supplies an ISDN capable device to connect to the 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 the router has an S/T interface, then it will need an external NT1 to connect to the ISDN provider.
If the router has a connector labeled BRI then it is already ISDN-enabled. With a native ISDN interface already built in, the router is a TE1 and will need to connect to 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 fixedconfiguration, or non-modular router, then it must use an existing serial interface. With non-native ISDN interfaces such as serial interfaces, an external TA device must 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 it has an available slot.
[4.1.7]
ISDN switch types
Routers must be configured to identify the type of switch with which they will communicate. Available ISDN switch types vary, depending in part on the country in which the switch is being used. As a consequence of various implementations of 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 or region to region. Like modems, each switch type operates slightly differently, and has a specific set of call setup requirements. Before the router can be connected to an ISDN service, it must be configured for the switch type used at the CO. This information must be specified during router configuration so the router can communicate with the switch, place ISDN network level calls, and send data.
In addition to knowing the switch type the service provider is using, it may also be necessary to know what service profile identifiers (SPIDs) are assigned by the telco. A SPID is a number provided by the ISDN carrier to identify the line configuration of the BRI service. SPIDs allow multiple ISDN devices, such as voice and data equipment, to share the local loop. SPIDs are required by DMS-100 and National ISDN-1 switches.
SPIDs are used only in North America and Japan. The ISDN carrier provides a SPID to identify the line configuration of the ISDN service. In many cases when configuring a router, the SPIDs will need to be entered.
Each SPID points to line setup and configuration information. SPIDs are a series of characters that usually resemble telephone numbers. SPIDs identify each B channel to the switch at the central office. Once identified, the switch links the available services to the connection. Remember, ISDN is typically used for dialup connectivity. The SPIDs are processed when the router initially connects to the ISDN switch. If SPIDs are necessary, but are not configured correctly, the initialization will fail, and the ISDN services cannot be used.
[4.2] [4.2.1]
ISDN Configuration Configuring ISDN BRI
The command isdn switch-type switch-type can be configured at the global or interface command mode to specify the provider ISDN switch.
Configuring the isdn switch-type command in the global configuration mode sets the ISDN switch type identically for all ISDN interfaces. Individual interfaces may be configured, after the global configuration command, to reflect an alternate switch type.
When the ISDN service is installed, the service provider will issue information about the switch type and SPIDs. SPIDs are used to define the services available to individual ISDN subscribers. Depending on the switch type, these SPIDs may have to be added to the configuration. National ISDN-1 and DMS-100 ISDN switches 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 specific provider requirements. Use the isdn spid1 and isdn spid2 interface configuration mode commands to specify the SPID required by the ISDN network when the router initiates a call to the local ISDN exchange.
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 global configuration mode:
Router(config)#isdn switch-type switch-type
The argument switch-type indicates the service provider switch type. To disable the switch on the ISDN interface, specify isdn switch-type none . The following example configures the National ISDN-1 switch type in the global configuration mode:
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 for the 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 most switches, the number must match the called party information coming in from the ISDN switch. SPIDs are specified in interface configuration mode. To enter interface configuration mode, use the interface bri command in the global configuration 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
[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 CO of the ISDN provider. 2. Specify the T1/E1 controller, framing type, and line coding for the facility of the 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 is called a T1 controller, or an E1 controller, if an E1 line is being used. This controller must be configured properly in order to communicate with the carrier network. The ISDN PRI D and PRI B channels are configured separately from the controller, using the interface serial command.
Use the isdn switch-type command to specify the ISDN switch used by the provider to which the PRI connects. As with BRI, this command can be issued globally or in interface configuration mode. The table shows the switch types available 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 in the 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 PRI service provider. For T1, use the following command syntax:
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 on the 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. It allows a full 64 kbps for each ISDN channel. In Europe, it is typically HDB3 encoding that is used. 3. Configure the specified interface for PRI operation and the number of fixed timeslots 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 used is 1-31. 4. Specify an interface for PRI D-channel operation. The interface is a serial interface 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 numbering ranges from 1 to 31 for E1 and 1 to 24 for T1. Serial interfaces in the Cisco router start numbering at 0. Therefore, channel 16, the E1 signaling channel, is channel 15 on the interface. Channel 24, the T1 signaling channel, becomes channel 23 on 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
of a T1 or E1 with subinterfaces. Channels use a colon instead of a dot to indicate the 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 has been implemented correctly.
To confirm BRI operations, use the show isdn status command to inspect the status of the BRI interfaces. This command can be used after configuring the ISDN BRI to verify that the TE1, or router, is communicating correctly with the ISDN switch. In the Figure output, the TEIs have been successfully negotiated and ISDN Layer 3 is ready to make or receive calls.
Verify that Layer 1 Status is ACTIVE, and that the Layer 2 Status state MULTIPLE_FRAME_ESTABLISHED appears. This command also displays the number of active calls.
The show isdn active command displays current call information, including all of 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
* Dial reason * Remote device that is connected
The show interface bri0/0 displays statistics for the BRI interface configured on the router. Channel specific information is displayed by putting the channel number at the end of the command. In this case, the show interface bri0/0:1 command 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 Control Protocol (CDPCP).
[4.2.4]
Troubleshooting the ISDN configuration
The following commands are used to debug and troubleshoot the ISDN configuration:
* 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 if the show isdn status command does not show Layer 1 as ACTIVE and Layer 2 as MULTIPLE_FRAME_ESTABLISHED. * The debug isdn q931 command shows the exchange of call setup and teardown messages of the Layer 3 ISDN connection. * The debug ppp authentication command displays the PPP authentication protocol 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 open the LCP state, then authenticate, and finally negotiate NCP. * The debug ppp error command displays protocol errors and error statistics associated with PPP connection negotiation and operation. Use the debug ppp commands to troubleshoot a Layer 2 problem if the show isdn status command does not indicate an ISDN problem.
[4.3] [4.3.1]
DDR Configuration DDR operation
Dial-on-demand routing (DDR) is triggered when traffic that matches a predefined set of criteria is queued to be sent out a DDR-enabled interface. The traffic that causes a DDR call to be placed is referred to as interesting traffic. Once the router 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 all traffic from a specific protocol to bring up a DDR link, or they can query an access list to see what specific types of traffic should bring up the link. Dialer lists do not filter traffic on an interface. Even traffic that is not interesting will be forwarded if the 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 if there 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 to determine if the traffic is interesting. 3. The router identifies the dialing information necessary to make the call using a 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 is currently connected to the desired remote destination, the traffic is sent. If the interface is not currently connected to the remote destination, the router sends call-setup information through the BRI using the D channel. 5. After the link is enabled, the router transmits both interesting and uninteresting 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 timer configuration.
The idle timer setting specifies the length of time the router should remain connected if no interesting traffic has been sent. Once a DDR connection is established, any traffic to that destination will be permitted. However, only interesting traffic resets the idle timer.
[4.3.2]
Configuring legacy DDR
Legacy DDR is a term used to define a very basic DDR configuration in which a single set of dialer parameters is applied to an interface. If multiple unique dialer configurations 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 remote site for every routing update or hello message if these packets are defined as interesting 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 Central LAN. If the network attached to the Home router is a stub network, then all nonlocal traffic should be sent to Central. A default route is a better choice for the Home 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 route because of its lower administrative distance. Without additional configuration, a dynamic route to a network will be ignored if a static route is present in the routing table for the same network.
* To reduce the number of static route entries, define a summarized or default static route.
[4.3.4]
Specifying interesting traffic for DDR
DDR calls are triggered by interesting traffic. This traffic can be defined as any of the 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 is as 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 list to the router. The command dialer-list 1 protocol ip permit will allow all IP traffic to trigger a call. Instead of permitting all IP traffic, a dialer list can point to an access list in order to specify exactly what types of traffic should bring up the link. The reference to access list 101 in dialer list 2 prevents FTP and Telnet traffic from activating the DDR link. Any other IP packet is considered interesting, and will therefore initiate the DDR link.
[4.3.5]
Configuring DDR dialer information
There are several steps involved in configuring the DDR interface. PPP is configured on the dialer interface using the same commands that enable PPP on a serial interface. HDLC is the default encapsulation for an ISDN interface on a Cisco router, but most networks employ PPP for circuit-switched connections. Because of its robustness, interoperability, and additional features such as authentication, PPP is the data link protocol in use on the B channels of most routers. To configure PPP on the DDR interface use the following commands:
Home(config)#username Central password cisco
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 groupnumber command:
Home(config-if)#dialer-group 1
In the command, group-number specifies the number of the dialer group to which the interface belongs. The group number can be an integer from 1 to 10. This number must match the dialer-list group-number . Each interface can have only one dialer group. However, the same dialer list can be assigned to multiple interfaces with the dialer-group command.
The correct dialing information for the remote DDR interface needs to be specified. This is done using the dialer map command.
The dialer map command maps the remote protocol address to a telephone number. 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 always dials the one phone number regardless of the traffic destination. This step is unique to legacy DDR. Although the information is always required, the steps to configure destination information are different when using dialer profiles instead of legacy DDR.
The dialer idle-timeout seconds command may be used to specify the number of idle seconds before a call is disconnected. The seconds represent the number of seconds until a call is disconnected after the 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 a physical interface. Since the IP address is applied directly to the interface, then only DDR interfaces configured in that specific subnet can establish a DDR connection with that interface. This means that there is a one-to-one correspondence between the two DDR interfaces at each end of the link.
Dialer profiles remove the configuration from the interface receiving or making calls and only bind the configuration to the interface on a per-call basis. Dialer profiles allow physical interfaces to dynamically take on different characteristics based on incoming or outgoing call requirements. Dialer profiles can do all of the 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 scalable circuit-switched internetworks by implementing a more scalable DDR model in Cisco routers and access servers. Dialer profiles separate the logical portion of DDR, such as the network layer, encapsulation, and dialer parameters, from the physical interface that places or receives calls.
Using dialer profiles, the following tasks may be performed:
* 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 ISDN interface. * Eliminate the waste of ISDN B channels by letting ISDN BRIs belong to 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 group of one or more physical interfaces associated with a dialer profile. * Physical interfaces — Interfaces in a dialer pool are configured for encapsulation parameters and to identify the dialer pools to which the interface belongs. PPP authentication, encapsulation type, and multilink PPP are all configured on the physical interface.
Like legacy DDR, dialer profiles activate when interesting traffic is queued to be sent out a DDR interface. First, an interesting packet is routed to a remote DDR IP address. The router then checks the configured dialer interfaces for one that shares the same subnet as the remote DDR IP address. If one exists, the router looks for an unused physical DDR interface in the dialer pool. The configuration from the dialer profile is then applied to the interface and the router attempts to create the DDR connection. When the connection is terminated, the interface is returned to the dialer pool for the next call.
[4.3.7]
Configuring dialer profiles
Multiple dialer interfaces may be configured on a router. Each dialer interface is the complete configuration for a destination. The interface dialer command creates 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 router name and phone number to dial it. The dialer pool associates this logical interface with a pool of physical interfaces. 3. Configure the physical interfaces and assign them to a dialer pool using the dialer pool-member command.
An interface can be assigned to multiple dialer pools by using multiple dialer pool-member commands. If more than one physical interface exists in the pool, use the priority option of the dialer pool-member command to set the priority of the interface within a dialer pool. If multiple calls need to be placed and only one interface is available, then the dialer pool with the highest 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 format as the legacy DDR statistics on incoming and outgoing calls. The message "Dialer state is data link layer up" suggests that the dialer came up properly and interface BRI 0/0:1 is bound to the profile dialer1. The show isdn active command displays information about the current active ISDN 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 the BRI interface. In this output, ISDN Layer 1 is active, ISDN Layer 2 is established with SPID1 and SPID2 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 it should, or it is constantly dialing when it should not. Several debug commands can be used to help troubleshoot problems with a DDR configuration. In the following lines, the seventh and eighth most significant hexadecimal numbers indicate the type of message. * * * *
0x05 0x02 0x07 0x0F
indicates indicates indicates indicates
a a a a
call setup message call proceeding message call connect message connect acknowledgment (ack) message
The debug isdn q931 command is useful for viewing Layer 2 ISDN call setup exchanges for both outgoing and incoming calls. The “i =” field in the Q.921 payload field is the hexadecimal value of a Q.931 message. The debug dialer [events | packets] command is useful for troubleshooting DDR connectivity. The debug dialer events command sends a message to the
console indicating when a DDR link has connected and what traffic caused it to connect. If a router is not configured correctly for DDR, then the output of the command will usually indicate the source of the problem. If there is no debug output, then the router is not aware of any interesting traffic. An incorrectly configured dialer or access list may be the cause. Not all DDR problems result in an interface failing to dial. Routing protocols can cause an interface to continuously dial, even if there is no user data to send. An interface that is constantly going up and down is said to be flapping. The debug dialer packet command sends a message to the console every time a packet is sent out a DDR interface. Use this debug command to see exactly what traffic is responsible for a flapping DDR interface. If a router is not connecting when it should, then it is possible that an ISDN problem is the cause, as opposed to a DDR problem. The remote router may be incorrectly configured, or there could be a problem with the ISDN carrier network. Use the isdn call interface command to force the local router to attempt to dial into the remote router. If the routers cannot communicate using this command, then the lack of connectivity is an ISDN problem, not a DDR problem. However, if the routers can communicate, then both the toll network and the ISDN configurations on the routers are working properly. In this case, the problem is most likely an error in the DDR configuration on either router. In some cases it is useful to reset the connection between the router and the local ISDN switch. The clear interface bri command clears currently established connections on the interface and resets the interface with the ISDN switch. This command forces the router to renegotiate its SPIDs with the ISDN switch, and is sometimes necessary after making changes to the isdn spid1 and isdn spid2 commands on an interface.
Module Summary ISDN refers to a set of communication protocols proposed by telephone companies to permit telephone networks to carry integrated voice, video, and data services. ISDN permits communication over high-quality, highspeed, digital communication channels. DDR is used in order to save the costs of a dedicated WAN line for organizations and companies that do not need a permanent connection. It can also be used as a backup by organizations that use the dedicated line for critical applications. 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
* * * * * *
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
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