To enable automatic detection of the encapsulation types operating over a point-to-point link to a specified serial or ISDN interface or dialer interface under Media Gateway Control Protocol (MGCP) network access server (NAS) packages, use the autodetect encapsulation command in interface configuration mode. 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 Link Access Procedure, Balanced (LAPB) for an ISDN terminal adapter.
ppp PPP encapsulation on the interface.
v120 V.120 encapsulation on B channels.

Command Default

No default behavior or values.

Command Modes

Interface configuration

Command History

Release Modification
11.2 This command was introduced.
12.0(4)T The lapb-ta keyword was added.
12.3(7)YB Support was added for MGCP NAS packages.
12.4(6)T Support for MGCP NAS packages was integrated into Cisco IOS Release 12.4(6)T.

Usage Guidelines

At least one encapsulation type must be specified, but you can specify multiple encapsulation types. Encapsulation types can be specified in any order.

Use this command to enable the specified serial or ISDN interface or dialer interface under an MGCP NAS package 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 possible by issuing the lapb-ta keyword. This allows recognition of incoming LAPB-terminal adapter (TA) calls.

Automatic detection is attempted for 10 seconds after the link is established or the first five packets exchanged over the link, whichever is first.


The following example configures BRI 0 to call and receive calls from two sites, use PPP encapsulation on outgoing calls, and use Challenge Handshake Authentication Protocol (CHAP) authentication on incoming calls. This example also enables BRI 0 to configure itself dynamically to answer calls that use V.120 but that do not signal V.120.

interface bri 0
 encapsulation ppp
 autodetect encapsulation v120
 no keepalive
 dialer map ip name EB1 234
 dialer map ip name EB2 456
 dialer-group 1
 isdn spid1 0146334600
 isdn spid2 0146334610
 isdn T200 1000
 ppp authentication chap

The following example enables the LAPB-TA and V.120 protocols for autodetection on the serial interface after you have configured the virtual terminals to handle asynchronous traffic:

interface serial0:23
 autodetect encapsulation lapb-ta v120

The following example enables PPP encapsulation and LAPB-TA and V.120 protocols for autodetection on the dialer interface under an MGCP NAS package:

interface Dialer1
 ip unnumbered Loopback1
 encapsulation ppp
 dialer in-band
 dialer idle-timeout 240
 dialer extsig
 dialer-group 1
 autodetect encapsulation ppp v120 lapb-ta
 ppp authentication chap

Related Commands

Command Description
encapsulation Sets the encapsulation method used by the interface.

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