Module Overview
35.1 ISDN
Concepts
35.1.1
Introducing ISDN
35.1.2 ISDN
standards and access methods
35.1.3 ISDN
3-layer model and protocols
35.1.4 ISDN
functions
35.1.5 ISDN
reference points
35.1.6
Determining the router ISDN interface
35.1.7 ISDN
switch types
35.2 ISDN
Configuration
35.2.1
Configuring ISDN BRI
35.2.2
Configuring ISDN PRI
35.2.3 Verifying
ISDN configuration
35.2.4
Troubleshooting the ISDN configuration
35.3 DDR
Configuration
35.3.1 DDR
operation
35.3.2
Configuring legacy DDR
35.3.3 Defining
static routes for DDR
35.3.4 Specifying
interesting traffic for DDR
35.3.5
Configuring DDR dialer information
35.3.6 Dialer
profiles
35.3.7
Configuring dialer profiles
35.3.8 Verifying
DDR configuration
35.3.9
Troubleshooting the DDR configuration
Module: Summary
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
35.1
ISDN Concepts
35.1.1
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.
35.1
ISDN Concepts
35.1.2
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.
35.1
ISDN Concepts
35.1.3
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:
- ITU-T Q.920
- ITU-T Q.921
- ITU-T Q.922
- 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, circuit-switched, 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.
35.1
ISDN Concepts
35.1.4
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 end-to-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:
- The D channel is used to send
the called number to the local ISDN switch.
- The local switch uses the SS7
signaling protocol to set up a path and pass the called number to the
remote ISDN switch.
- The remote ISDN switch signals
the destination over the D channel.
- The destination ISDN NT-1
device sends the remote ISDN switch a call-connect message.
- The remote ISDN switch uses SS7
to send a call-connect message to the local switch.
- 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.
35.1
ISDN Concepts
35.1.5
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 RS-232
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.
35.1
ISDN Concepts
35.1.6
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:
- 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).
- 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.
- 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 fixed-configuration, or
non-modular router, then it must use an existing serial interface. With
non-native ISDN interfaces such as serial interfaces, an external TA 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.
CAUTION:
A router with a U
interface should never be connected to an NT1 as it will damage the interface.
35.1 ISDN Concepts
35.1.7
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.
35.2
ISDN Configuration
35.2.1
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-typeswitch-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
spid1spid-number [ldn ]
Router(config-if)#isdn
spid2spid-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
brislot/port
Router(config)#interface
bri0/0
Router(config-if)#isdn
spid1 51055540000001 5554000
Router(config-if)#isdn
spid2 51055540010001 5554001
35.2
ISDN Configuration
35.2.2
Configuring ISDN PRI
ISDN PRI is delivered over a
leased T1 or E1 line. The main PRI configuration tasks are as follows:
- Specify the correct PRI switch
type that the router interfaces with at the CO of the ISDN provider.
- Specify the T1/E1 controller,
framing type, and line coding for the facility of the ISDN provider.
- 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
[timeslotsrange]
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
35.2 ISDN Configuration
35.2.3
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).
35.2
ISDN Configuration
35.2.4
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.
35.3 DDR Configuration
35.3.1
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:
- The router receives traffic,
performs a routing table lookup to determine if there is a route to the
destination, and identifies the outbound interface.
- If the outbound interface is
configured for DDR, the router does a lookup to determine if the traffic
is interesting.
- The router identifies the
dialing information necessary to make the call using a dialer map to
access the next-hop router.
- 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.
- After the link is enabled, the
router transmits both interesting and uninteresting traffic. Uninteresting
traffic can include data and routing updates.
- 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.
35.3 DDR Configuration
35.3.2
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
35.3
DDR Configuration
35.3.3
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 non-local 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.
35.3
DDR Configuration
35.3.4
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-listdialer-group-num
protocolprotocol-name {permit | deny | listaccess-list-number }
Thedialer-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.
35.3
DDR Configuration
35.3.5
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
group-number 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-listgroup-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-timeoutseconds 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.
35.3
DDR Configuration
35.3.6
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.
35.3
DDR Configuration
35.3.7
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:
- 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
- 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.
- 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
35.3
DDR Configuration
35.3.8
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.
35.3
DDR Configuration
35.3.9
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 indicates a call setup
message
- 0x02 indicates a call
proceeding message
- 0x07 indicates a call connect
message
- 0x0F indicates a connect
acknowledgment (ack) message
The debug isdn
q931 command is useful for viewing Layer 2 ISDN call 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.
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, high-speed, 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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