I've been reading and playing with ISDN for the last few weeks and I
thought I would share my notes. This might help someone just starting
on this material; create a new topic of discussion; and lastly, perhaps
some people will notice errors I can correct, or areas I did not think
to cover. It starts with the most boring stuff, so if you want to read
up on snapshot routing or dialer profiles, skip a bit. If you have any
feedback on making this better, please let me know...
--- Dennis
-----
ISDN
Provides digital service that runs over existing telephone networks.
Normally used to support applications requiring high-speed voice, video,
and data communications for home users, remote offices, etc.
Protocols standards
� E specifies ISDN on existing telephone technology.
� I specify concepts, terminology and services.
� Q specifies switching and signaling.
TE1---|---S/T-----NT1---U---LT---V---ET
|
TE2---R---TA--|
Note: U is two wire, S/T is four wire. The NT1 provides this
conversion.
Equipment
� TA - Terminal adapter converts from RS-232, V.35, and other
signals into BRI.
� TE1|2 - Terminal equipment 1 (integrated TA, understands ISDN)
or 2 (needs TA, predates ISDN).
� NT1 - Network termination type 1 - equipment that connects the
subscription 4 wires to the 2 wire local loop.
� NT2 - Network termination type 1 - equipment that performs
protocol functions of the data link and network layers.
� LT - Local Termination - portion of the local exchange that
terminates the local loop.
� ET - Exchange Termination - portion of the exchange that
communicates with the ISDN components.
Reference Points
� R reference points define the hand-off from non-ISDN equipment
and the TA.
� S reference points define hand-off from user terminals to an
NT2.
� T reference points define hand-off between NT1 and NT2.
� U reference points define hand-off between NT1 and
line-termination equipment in a carrier network. (Only US/Japan, where
NT1 not provided by carrier).
Channels
� B(earer) channel: Used for data transfer (voice or data).
� D(ata) channel: Used for control/signaling information using
LAPD.
Types of Equipment
� CPE - Customer Premise Equipment (on-site).
� DCE - Data Communications Equipment. The devices and
connections that make up the network end of the user-to-network
interface.
� DTE - Data Terminal Equipment. The devices and connections that
make up the user end of the user-to-network interface. Terminals, PCs,
and routers would be examples.
Flavors of ISDN
� BRI - 2B /1D (B=64kb / D = 16kb)
� PRI (T1) - 23B / 1D (B=64kb / D = 64kb)
� E1 (Europe) - 30B / 1D (B=64kb / D = 64kb)
ISDN Switch Types
� basic-5ess - AT&T basic rate switches (USA)
� basic-dms100 - NT DMS-100 (North America)
� basic-nil - National ISDN-1 (North America)
� basic-Itr6 - German ITR6 ISDN switches
� basic-nwnet3 - Norwegian Net3 switches
� basic-nznet3 - New Zealand Net3 switches
� basic-ts013 - Australian TS013 switches
� basic-net3 - Switch type for Net3 in UK and Europe
� ntt - NTT ISDN switch (Japan)
� vn3 - French VN3 ISDN switches
� none - No specific switch specified
What is passed at different OSI levels
Layers D Channel B Channel
Data-Link Layer Q.921 PPP, HDLC
Network Layer Q.931 IP, IPX
Q.931 as the network layer protocol that provides messages for ISDN call
setup and tear down on the D Channel. It uses Q.921, a derivative of
HDLC, as a data-link layer transport.
The B Channel (or Bearer Channel) transports user data.
ISDN Configuration Commands
backup delay {enable-delay | never} {disable-delay | never} - This
interface command defines the number of seconds between the line failure
and the initialization of the backup interface. This command also
defines the number of seconds between when the line comes back up and
the traffic is redirected back to the primary interface.
backup interface type number - This interface command defines the
interface to be used in the event the primary link goes down. The backup
interface remains in standby mode unit activated. The command is put in
the interface configuration of the primary link.
controller t1 number - This interface command specifies a T1 controller
dialer-group group-number - This interface command associates an
interface to a dialer-group in order to control access.
dialer idle-timeout seconds - This interface command defines the time to
wait without interesting traffic before the DDR connection is broken
down. This defines a timer that is reset whenever defined interesting
traffic is seen; when the timer expires, the line is broken down.
dialer-list dialer-group protocol protocol-name {permit | deny | list
access-list-number | access-group} - This global command applies an
access list to define interesting traffic for an interface.
dialer load-threshold load [outbound | inbound | either] - This
interface command defines the bandwidth threshold before an additional B
Channel is initialized to share the traffic. The Load argument is used
to determine whether to initiate another call or to drop a link to the
destination. This argument represents a utilization percentage; it is a
number between 1 and 255, where 255 is 100%.
dialer map protocol next-hop-address [name hostname] [spc] [speed 56 |
64] [broadcast] dial-string [:isdn-subaddress] - This interface command
defines the next hop for an ISDN link to a far-end device.
dialer pool number - This interface command is only used on dialer
interfaces and specifies which dialer pool to use when connecting to a
specific destination.
dialer pool-member number [priority priority] [min-link minimum]
[max-link maximum] - This interface command causes a physical interface
to become a member of a dialer pool.
dialer remote-name username - This interface command is only used on
dialer interfaces and specifies the authentication name of a remote
router.
dialer string dial-string[:isdn-subaddress] - This legacy DDR interface
command specifies a destination string (phone number) to be used when
making an ISDN call.
interface dialer number - This global command defines a dialer rotary
group; which is then used to apply a single interface configuration a
set of physical interfaces. This allows a group of interfaces to be used
as a resource pool for calling many destinations.
isdn spid1 spid-number [ldn] - This interface command defines the
Service Profile Identifier (SPID) number assigned by the service
provider to Channel B1. The optional Local directory number (LDN)
argument is a 7-digit number assigned by the service provider.
isdn spid2 spid-number [ldn] - This interface command defines the
Service Profile Identifier (SPID) number assigned by the service
provider to Channel B2
isdn switch-type switch-type - This global or interface (to override the
global setting) command defines the D Channel signaling protocol used on
an ISDN interface.
ppp multilink - This interface command causes the interface to negotiate
a multilink PPP data-link protocol.
pri-group [timeslots range]- This controller configuration command
defines a channelized T1 interface to be used as a PRI circuit. The
timeslots argument specified a single range of values from 1 to 23.
dialer map snapshot sequence-number dial-string - This interface
command defines a dialer map for snapshot routing on a client router
connected to a DDR interface. Sequence-number is the dialer map
identifier, and can have a value of between 1 and 254. The dial-string
is the remote snapshot servers DN.
snapshot client active-time quiet-time [suppress-statechange-updates]
[dialer] - This interface command defines a router as the snapshot
client, who initiates communication with the snapshot server when the
quiet-time period expires.
snapshot server active-time [dialer] - This interface command defines a
router as a snapshot router, which provides routing updates to snapshot
clients when requeted.
ip ospf demand-circuit - This interface command configures OSPF to treat
the interface as an OSPF demand circuit, causing OSPF adjacencies to be
formed and suppress the passing of hello packets
ISDN Diagnostic Commands
debug isdn q921 - Shows Layer 2 (data link layer) D Channel traffic
debug isdn q931 - Shows Layer 3 (network layer) D Channel call setup and
break down messages
debug isdn events - Shows ISDN state changes at Layer 3 (network layer)
To determine ISDN activity occurring on the user side of the ISDN
interface
debug dialer - Shows Layer 3 (network layer) activity to determine
dial-on-demand routing information
debug ppp negotiation - provides messages regarding PPP traffic passing
across a link, and reveals if bridging is being negotiated
debug ppp authentication - provides messages telling if a client is
passing authentication. For version before release 11.2, use the debug
ppp chap command instead.
debug ppp error - displays protocol errors and error statistics
associated with PPP connection negotiation and operation
show dialer - Shows current Layer 3 (network layer) status of any dialup
adaptors (ISDN or analog)
show interfaces bri number[[:bchannel] | [first] [last]] [accounting] -
Shows current status of the physical ISDN BRI B and D channels interface
or when the more restrictive arguments are used, information on the
specified B channel(s).
show isdn service - displays the current state and the service status of
each ISDN channel
show isdn {active | history | memory | status [dsl | interface-type
number] | timers}
� Active - Displays current call information; including called
number, the time until the call is disconnected, AOC charging units used
during the call, and whether the AOC information is provided during
calls or at end of calls.
� History - Displays historic and current call information;
including the called number, the time until the call is disconnected,
AOC charging time units used during the call, and whether the AOC
information is provided during calls or at the end of calls.
� Status [dsl | interface-type number] - Displays the status of
all ISDN interfaces or, optionally, a specific digital signal link (DSL)
or a specific ISDN interface. The dsl range can vary, depending on the
hardware platform. Interface-type can be bri or serial.
� Timers - Displays the values of Layer 2 and Layer 3 timers.
show ppp multilink - Shows the current bundle information for the
Multilink PPP bundles
show dialer map - Shows all dynamically and statically configured dialer
maps
show controllers bri number - check the status of the Layer 1 (physical
layer) of a BRI link.
show controller t1 - shows the current hardware controller status, which
is useful for diagnostic purposes
show snapshot [type number] - shows the snapshot routing parameters
associated with an interface
debug dialer packets - provides debugging information about the packets
received on a dialer interface and shows the interface type, the
protocol of the packet, the source and destination addresses, and the
size of the packet
debug dialer events - displays debugging information about the cause of
any call received on a dialer interface when DDR is enabled on the
interface
Emutel Lite ISDN BRI Simulator Configuration
Port B-channel DN* SPID
1 1 384000 3840000001
1 2 384010 3840000002
2 1 384020 3840200001
2 2 384030 3840200002
* The DN (Directory Number) is similar to a phone number used to
identify a desired connection.
The ISDN switch-type for the Emutel Lite is basic-dms100.
When connecting this device to Cisco 2503's you will need one NT1 per
connection. An NT1 is a piece of equipment that connects the
subscription 4 wires to the 2 wire local loop. They don't usually need
to be configured, but the ports must be accurate; U goes to the
simulator, ST to the router. These units, by various manufactures, are
available new from $150 to $200, or used on e-bay for about $50. A very
common configuration for a home CCIE study lab would be:
Cisco 2503 ------ NT1 ------ ISDN Simulator ------ NT1 ------- Cisco
2503
ISDN Setup Steps
Basic BRI configuration
There are only two commands necessary to get a ISDN working at the most
basic level:
Router(config)# isdn switchtype xxx
Router(config-if)# no shutdown
Once these commands are issues, a quick "show isdn status" will show the
interface is up. Since there isn't much that can be done with this most
basic configuration, let's immediately go on to a useful configuration...
Setup for BRI as DDR (and variations)
Global Level:
o Setup username for opposite router (optional, for CHAP)
o Setup ISDN switch-type
o Setup dialer-list
Interface Level:
o IP address
o Encapsulation
o Idle-timeout (optional)
o Map
o Load-threshold (optional)
o Associate dialer-group
o Setup Spids
o Authentication (optional, for CHAP)
o Multilink
o Turn on the interface
Special:
o If working as a Backup Interface set the primary interface
configuration for "backup delay" and "backup interface".
o If working as with a Floating Static Route use the global
command to create a static route with a higher administrative distance
then the dynamic protocols (for instance, ip route 0.0.0.0 0.0.0.0
196.1.1.2 121 (RIP has an AS of 120)).
Point-to-Point Protocol (PPP)
PPP is a standard method of transporting multi-protocol datagrams over
point-to-point links. PPP only runs over the B Channels, while
Q.921/Q.931 protocols run only over the D Channel. PPP provides:
� A means of encapsulating multi-protocol datagrams
� A Link Control Protocol (LCP) for establishing, configuring and
testing the data-link connection
� A set of Network Control Protocols (NCPs) for establishing and
configuring network layer protocols
PPP provides two methods of authentication, PAP and CHAP. CHAP is
preferred because PAP transmits passwords in clear text over the
network.
CHAP and PAP authentication requires the router to have a user/password
database that is created by issuing the global username command. For
example, this command would add the username RouterA, password
ciscorocks to the local database (remember that everything is case
sensitive):
Example: Router (config)# username RouterA password ciscorocks
The host name is normally the destination router's hostname, but can be
configured to use a different username by using this command on the
destination interface (the new username is ciscoland):
Example: Router (config-if)# ppp chap hostname ciscoland
Dialer Profiles
Imagine you had a phone in your office that was preprogrammed to dial
one number exclusively. Every time you picked up the phone you would be
immediately and reliably connected to the most important person you
know, but it could not be used to reach anyone else.
That's the situation with the dialup links we've developed so far. The
specific interface is programmed to quickly and reliably dial one other
interface. With IOS 11.3 Cisco introduced an incredible level of
flexibility through the use of Dialer Profiles. This technique takes
the static configuration we've been using up till now and rips it into
interchangeable modules of code that allow much greater granularity to
our control of the interface. The previous configuration has now been
compartmentalized to the following pieces:
Physical Interface: This is the actual physical dial-up adaptor with
the minimum programming necessary to make it work.
Dialer interface: The major part of the configuration necessary for a
link is developed under a specific virtual interface and is only
associate with the physical port when a call is initiated, and then
removed when the call is broken down. This allows different
configurations to be applied depending on the requirements of the
specific destination.
Dialer-pool: If the router has several identical physical interfaces,
they can be defined as a dialer-pool. Similar to a hunt-group in the
telecom world, when a call is being set-up it selects the first
available member of the dialer pool to complete the call; if the first
interface is already in use, the next one is used until the call has
been connected, or until it is determined that no interfaces are
available. This allows a small number of interfaces to provide balanced
coverage for a large number of potential destinations.
Map-Class: An optional element of the Dialer Profiles configuration
that allows the reparative elements in the Dialer Interface
configurations to be defined once and used over and over again. The
idle-timeout and fast-idle settings would be examples of commands that
could be off-loaded to the Map-Class.
An example of Dialer Profiles in the real world would be a central
office that needs to periodically push information to twenty remote
sites. If they had a router with three BRI ports and lines the router
could be configured as follows:
� The respective elements of the configuration could be programmed
into a Map-Class.
� Dialer Interfaces could be developed with the appropriate
settings for each destination, and using the predefined Map-Class to
simplify programming.
� The Physical Interfaces could be configured, and then defined as
a dialer-pool.
Whenever there was a request to push information to one of the satellite
facilities the router would associate a Dialer Interface with the first
available Physical Interface, the call would be setup, the data
transferred, the call broken down, and the interfaces released for the
next call.
Snapshot Routing
Distance Vector routing protocols can keep an ISDN dial-up link active
through the periodic passing of updates (30 seconds for IP RIP, 60 for
IPX RIP and 90 for IGRP). Snapshot routing does this by defining
"quiet" and "active" periods; when the timer tracking the quiet period
expires the link is brought up, the routing tables are passed during the
active period, and the link is brought down to begin a new quiet period.
During the quiet period routing updates are suppressed, keeping the
known routes in the routing table active.
Snapshot routing is implemented in two parts; a hub-site is configured
as the server and the spoke site as a client. The clients will initiate
a call to the server when their quiet time has expired. The commands to
configure the client side of the relationship are "dialer map snapshot"
and "snapshot client", and to configure the server side the "snapshot
server" command is used. All snapshot commands are interface commands.
To verify the proper function of Snapshot routing the "debug snapshot",
"debug dialer packet" and "debug dialer event" commands are used.
OSPF and ISDN
OSPF can keep an ISDN dial-up link active through the periodic passing
of hello packets. Applying the "ip ospf demand-circuit" interface
command on either side of a BRI connection will cause OSPF adjacencies
to be formed and suppress the passing of hello packets.
Things to Know and Common Problems
� Encapsulation can be PPP, HDLC or LAPD, with the default
encapsulation method being HDLC. CHAP and PAP authentication are
associated with PPP.
� A SPID is similar to a telephone number in that it is a unique
line identifier provided by the LEC (Local Exchange Carrier).
� The major advantage of Floating Static Routes over Backup
Interfaces is that you can test the ISDN interface by pinging the far
end interface without disrupting the client network. For a Backup
Interface you must disable the primary link to test the redundant link.
� You cannot enable both a "dialer string" and "dialer map" on the
same interface. They are mutually exclusive and the router will not
allow the commands to be entered together.
� You cannot enable both a legacy DDR connection and a dialer
profile configuration on the same interface. They are mutually
exclusive and the router will not allow the commands to be entered
together.
� Always configure the DDR with an open definition of the access
list, adding the restrictions after the interface is completely
configured and tested (for example, "dialer-list 1 protocol ip permit".
This is a basic case of configuring one thing at a time.
� A common problem is incorrectly configuring the local and remote
identifiers (numbers or names) on configuration command arguments, such
as reversing the SPIDs or configuring the wrong DN on a map statement.
� Defining the right traffic as interesting in the dialer-list
configuration is another area where problems are commonly found.
� Determining if PPP authentication (CHAP or PAP) is negotiating a
connection properly important.
Gotcha's
� Many Cisco routers with built-in ISDN interfaces have an ST
interface. In order to convert the U interface circuit from the carrier
to an ST interface circuit that the router can handle, an external
Network Terminating Unit (NT1) is needed. This is an external piece of
electronics that you might see in the lab. There might be two of these
units sitting between the BRI ports on the ISDN simulator and the
routers. These units usually do not need to be configured, but the
ports must be accurate, U goes to the simulator, ST to the router.
� The CHAP authentication configuration must be correct.
� Routing protocols that send out full routing tables at fixed
intervals (Distance Vector), or which send out hello packets (Link
State) at fixed intervals, can keep an ISDN dial-up link up permanently.
The solutions to this are:
� Use static routes.
� Use SNAPSHOT routing for Distance Vector routing protocols.
� For OSFP use the interface command "ip ospf demand-circuit",
which, when implemented on either side of an BRI connection OSPF
adjacencies will be formed and the passing of hello packets will be
suppressed.
Useful links:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/isdn.htm
http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2011.htm
http://www.cisco.com/warp/public/129/27.html
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/cs008.htm
http://www.cisco.com/warp/public/793/access_dial/ddr_dialer_profile.html
http://www.cisco.com/warp/public/129/21.html
http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1917.htm
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