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Table Of Contents
Related Features and Technologies
ISDN LAPB-TA
Feature Overview
To carry asynchronous traffic over ISDN, your system must be able to convert that traffic and forward it over synchronous connections. This is normally implemented by the V.120 protocol, which carries asynchronous traffic over ISDN. (For more information on the V.120 protocol, see "Configuring V.120 Access" in the Cisco IOS Release 12.0 Dial Solutions Configuration Guide.)
However, several countries in Europe (Germany, Switzerland, and some Eastern European countries) use Link Access Procedure, Balanced (LAPB) as the protocol to forward their asynchronous traffic over synchronous connections.
Your system, therefore, must be able to recognize and accept calls from these asynchronous/synchronous conversion devices. Link Access Procedure, Balanced-Terminal Adapter (LAPB-TA) peforms that function. (LAPB is sometimes referred to as "X.75," because LAPB is the link layer specified in the ITU-T X.75 recommendation for carrying asynchronous traffic over ISDN.)
LAPB-TA allows a system with an ISDN terminal adapter supporting asynchronous traffic over LAPB to call into a router and establish an asynchronous Point to Point Protocol (PPP) session. LAPB supports both local Challenge Handshake Authentication Protocol (CHAP) authentication and external RADIUS authorization on the Authentication, Authorization and Accounting (AAA) server.
Benefits
LAPB-TA allows devices that use LAPB instead of the V.120 protocol to communicate with routers on the Cisco 3600 and 5300 series.
Restrictions
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LAPB-TA does not currently support the ability to set a maximum frame size per user.
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Related Features and Technologies
The LAPB-TA feature is closely related to the existing V.120 feature, which is documented in the chapter "Configuring V.120 Access" in the Cisco IOS Release 12.0 Dial Solutions Configuration Guide.
Related Documents
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Supported Platforms
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Prerequisites
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Supported MIBs and RFCs
MIBs
None.
RFCs
None.
For descriptions of supported MIBs and how to use MIBs, see the Cisco MIB web site on CCO at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.
List of Terms and Acronyms
AAA—Authentication, authorization, and accounting.
CHAP—Challenge Handshake Authentication Protocol. Security feature supported on lines using PPP encapsulation that prevents unauthorized access. CHAP does not itself prevent unauthorized access, it merely identifies the remote end. The router or access server then determines whether that user is allowed access.
DDR—dial-on-demand routing. Technique whereby a router can automatically initiate and close a circuit-switched session as transmitting stations demand. The router spoofs keepalives so that end stations treat the session as active. DDR permits routing over ISDN or telephone lines using an external ISDN terminal adaptor or modem.
ISDN—Integrated Services Digital Network. Communication protocol that permits telephone networks to carry data, voice, and other source traffic.
LAPB—Link Access Procedure, Balanced. Data link layer protocol in X.25.
PPP—Point-to-Point Protocol. Successor to SLIP that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP works with several network layer protocols. PPP also has built-in security mechanisms, such as CHAP and PAP.
RADIUS—Database for authenticating modem and ISDN connections and for tracking connection time.
Configuration Tasks
If an interface is already configured for V.120, only two additional configuration commands are required on the interface because V.120 and LAPB-TA sessions are configured in a similar way:
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Perform the following tasks to configure LAPB-TA:
Configuring LAPB-TA
For more information about virtual templates and sample template configurations, see the section "Protocol Translation and Virtual Asynchronous Device Commands" in the Cisco IOS Release 12.0 Dial Solutions Command Reference.
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Creates a virtual asynchronous interface.
2
Applies virtual template to the virtual asynchronous interface.
3
Creates a virtual interface template, and enters interface configuration mode.
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Assigns an IP address to the virtual interface template.
5
Enables encapsulation on the virtual interface template.
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Disables an IP address from a pool to the device connecting to the virtual access interface
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Enables the CHAP protocol for PPP authentication.
8
Exits to global configuration mode.
9
Specifies CHAP password to be used to authenticate calls from caller "user1."
10
Enters interface configuration mode for a D-channel serial interface1 .
11
Configures PPP encapsulation as the default.
12
Specifies the dialer group belonging to the interface.
13
Enables the CHAP protocol for PPP authentication.
14
Enables autodetect encapsulation for LAPB-TA protocols.
15
Configures a range of 32 virtual terminal (vty) lines starting with vty0.
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Defines which protocol to use to connect to a specific line of the access server.
1 The D channel is the signalling channel.
Verifying LAPB-TA
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Router# show running configurationBuilding configuration...Current configuration:!version 12.0service timestamps debug datetime msec localtimeservice timestamps log datetime msec localtimeno service password-encryptionservice udp-small-serversservice tcp-small-servers!hostname Router...(output omitted)interface Serial0:23description ENG PBX BRI num.:81063no ip addressno ip directed-broadcastencapsulation pppno ip route-cachedialer pool-member 1autodetect encapsulation ppp lapb-taisdn switch-type primary-5essno peer default ip addressno fair-queueno cdp enableppp authentication chap...(output omitted)!endConfiguration Examples
The following example configures a virtual-template LAPB-TA connection capable of running PPP. It assumes you have already configured usernames and passwords for PPP authentication:
Router(config)# vty-asyncRouter(config)# vty-async virtual-template 1Router(config)# interface virtual-template 1Router(config-if)# ip unnumbered Ethernet0Router(config-if)# encapsulation pppRouter(config-if)# no peer default ip addressRouter(config-if)# ppp authentication chapRouter(config-if)# exitRouter(config)# interface Serial0:23Router(config-if)# autodetect encapsulation lapb-taThe following configuration example treats the LAPB-TA and V.120 calls identically, by immediately starting a PPP session without asking for username and password, and relying on PPP authentication to identify the caller:
Router(config)# vty-asyncRouter(config)# vty-async virtual-template 1Router(config)# interface Loopback0Router(config-if)# ip address 10.2.2.1 255.255.255.0Router(config-if)# exitRouter(config)# interface BRI3/0Router(config-if)# encapsulation pppRouter(config-if)# autodetect encapsulation ppp lapb-ta v120Router(config-if)# exitRouter(config)# interface Virtual-Template1Router(config-if)# ip unnumbered Loopback0Router(config)-if# ppp authentication chapRouter(config-if)# exitRouter(config)# ip local pool default 10.2.2.64 10.2.2.127Router(config)# line vty 0 2Router(config-line)# password <removed>Router(config-line)# loginRouter(config-line)# transport input telnetRouter(config-line)# exitRouter(config)# line vty 3 4Router(config-line)# no loginRouter(config-line)# transport input lapb-ta v120Router(config-line)# autocommand ppp negRouter(config-line)# exitRouter(config)# endCommand Reference
This section documents modified commands. All other commands used with this feature are documented in the Cisco IOS Release 12.0 command references.
autodetect encapsulation
To enable automatic detection of the encapsulation types operating over a point-to-point link to a specified serial or ISDN interface, use the autodetect encapsulation interface configuration command. To disable automatic dynamic detection of the encapsulation types on a link, use the no form of this command.
autodetect encapsulation {lapb-ta | ppp | v120}
no autodetect encapsulation {lapb-ta | ppp | v120}Syntax Description
lapb-ta
Automatically detects LAPB for an ISDN terminal adapter.
ppp
Automatically detects PPP encapsulation on the interface.
v120
Automatically detects V.120 encapsulation on B channels.
Defaults
No default behavior or values.
Command Modes
Interface configuration.
Command History
11.2
This command was first introduced.
12.0(4)T
lapb-ta was added as a keyword.
Usage Guidelines
At least one encapsulation type is required in the command, but you can specify additional encapsulation types.
Use this command to enable the specified serial or ISDN interface to accept calls and dynamically change the encapsulation in effect on the interface when the remote device does not signal the call type. For example, if an ISDN call does not identify the call type in the Lower Layer Compatibility fields and is using an encapsulation that is different from the one configured on the interface, the interface can change its encapsulation type dynamically.
This command enables interoperation with ISDN terminal adapters that use V.120 encapsulation but do not signal V.120 in the call setup message. An ISDN interface that by default answers a call as synchronous serial with PPP encapsulation can change its encapsulation and answer such calls.
Autodetection of LAPB traffic on an ISDN terminal adapter is now possible, by adding the keyword lapb-ta to the command line. This allows recognition of incoming LAPB-TA calls.
Automatic detection is attempted for the first 10 seconds after the link is established or the first five packets exchanged over the link, whichever is first.
Examples
The following example enables the LAPB-TA and V.120 protocols for autodetection on interface serial0:23 after you have configured the virtual terminals to handle asynchronous traffic:
Router(config)# vty-asyncRouter(config)# interface serial0:23Router(config-if)# autodetect encapsulation lapb-ta v120Related Commands
transport input
To define which protocols to use when connecting to a specific line on the router, use the transport input line configuration command. To disable all protocols when connecting to a specific line on the router, use the no form of this command.
transport input {all | lapb-ta | lat | mop | nasi | none | pad | rlogin | telnet | v120}
no transport input {all | lapb-ta | lat | mop | nasi | none | pad | rlogin | telnet | v120}
Syntax Description
Default
The default is none.
Command Mode
Line configuration.
Command History
10.0
This command was first introduced.
11.1
lat, pad, all, and v120 keywords first appeared, and none became the default.
12.0(4)T
lapb-ta keyword was added.
Usage Guidelines
CautionCisco routers do not accept incoming network connections to asynchronous ports (TTY lines) by default. You must specify an incoming transport protocol, or use the transport input all command before the line will accept incoming connections. For example, if you are using your router as a terminal server to make console-port connections to routers or other devices, you will not be able to establish a Telnet to these devices. You will receive the message "Connection Refused." This behavior was introduced in Cisco IOS Release 11.1. Previous to this release, the default was all. If you are upgrading to Cisco IOS Release 11.1(1) or later from Cisco IOS Release 11.0 or earlier, you must add the transport input {protocol | all} command, or you will be locked out of your router.
You can specify one protocol, multiple protocols, all protocols, or no protocols. To specify multiple protocols, enter the keyword for each protocol, separated by a space.
This command can be useful in distributing resources among different types of users, or making certain that only specific hosts can access a particular port. When using two-step protocol translation, the transport input command is useful in controlling exactly which protocols can be translated to other protocols.
Access lists for each individual protocol can be defined in addition to the allowances created by the transport input command. Any settings made with the transport input command override settings made with the transport preferred command.
Adding the keyword lapb-ta to the command line allows any incoming LAPB-TA call to be connected to the specified virtual terminal.
Example
The following example sets the incoming protocol to Telnet and LAPB-TA for virtual terminals 0 through 32:
Router(config)# line vty 0 32Router(config-line)# transport input telnet lapb-taRelated Commands
Debug Commands
This section documents the new debug lapb-ta command related to the LAPB-TA feature.
debug lapb-ta
To display debug messages for LAPB-TA, use the debug lapb-ta privileged EXEC command. Use the no form of the command to disable debugging output.
debug lapb-ta [error | event | traffic]
no debug lapb-ta [error | event | traffic]
Syntax Description
error
(Optional) Displays LAPB-TA errors.
event
(Optional) Displays LAPB-TA normal events.
traffic
(Optional) Displays LAPB-TA in/out traffic data.
Defaults
Debugging for LAPB-TA is not enabled.
Command History
Examples
The following is sample output from the debug lapb-ta command with the error, event, and traffic keywords activated:
Router# debug lapb-ta errorLAPB-TA error debugging is onRouter# debug lapb-ta eventLAPB-TA event debugging is onRouter# debug lapb-ta trafficLAPB-TA traffic debugging is onMar 9 12:11:36.464:LAPB-TA:Autodetect trying to detect LAPB onBR3/0:1Mar 9 12:11:36.464: sampled pkt: 2 bytes: 1 3F.. matchMar 9 12:11:36.468:LAPBTA:get_ll_config:BRI3/0:1Mar 9 12:11:36.468:LAPBTA:line 130 allocated for BR3/0:1Mar 9 12:11:36.468:LAPBTA:process 79Mar 9 12:11:36.468:BR3/0:1:LAPB-TA startedMar 9 12:11:36.468:LAPBTA:service change:LAPB physical layer up,context 6183E144 interface up, protocol downMar 9 12:11:36.468:LAPBTA:service change:, context 6183E144 upMar 9 12:11:36.468:LAPB-TA:BR3/0:1, 44 sent2d14h:%LINEPROTO-5-UPDOWN:Line protocol on Interface BRI3/0:1, changed state to up2d14h:%ISDN-6-CONNECT:Interface BRI3/0:1 is now connected to 60213Mar 9 12:11:44.508:LAPB-TA:BR3/0:1, 1 rcvdMar 9 12:11:44.508:LAPB-TA:BR3/0:1, 3 sentMar 9 12:11:44.700:LAPB-TA:BR3/0:1, 1 rcvdMar 9 12:11:44.700:LAPB-TA:BR3/0:1, 3 sentMar 9 12:11:44.840:LAPB-TA:BR3/0:1, 1 rcvdMar 9 12:11:44.840:LAPB-TA:BR3/0:1, 14 sentMar 9 12:11:45.852:LAPB-TA:BR3/0:1, 1 rcvdMar 9 12:11:46.160:LAPB-TA:BR3/0:1, 2 rcvdMar 9 12:11:47.016:LAPB-TA:BR3/0:1, 1 rcvdMar 9 12:11:47.016:LAPB-TA:BR3/0:1, 10 sent
