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X.25 Facility Handling

X.25 Facility Handling

This appendix provides reference material describing how X.25 facilities are handled by the Cisco IOS software.

Facility Handling in Encapsulated X.25 Virtual Circuits

A router either originates or accepts encapsulation switched virtual circuits (SVCs) to transport LAN traffic through an X.25 network.

When the router originates a call for LAN traffic encapsulation, the facilities in the call are controlled by the facilities configured for the interface and the map statement that specifies the LAN and X.25 encapsulation. Because a router can be attached to a public data network (PDN), the interface and map configurations allow a number of facilities to be specified in outgoing calls. These facilities are specified in all originated calls relating to the given interface and map with one exception--the incoming and outgoing maximum packet sizes proposed are lowered if the lower layer (LAPB) cannot support the specified data packet size.

When the router accepts an encapsulation call, many facilities are simply ignored. The maximum packet sizes are lowered if the lower layer (LAPB) cannot support the sizes proposed. A reverse-charge call is cleared if neither the interface nor the map allows it. A call that specifies a network user identification (NUID) is cleared if the user authentication fails.

Facility Handling in Routed X.25 Virtual Circuits

Routed X.25 traffic might have facilities added, deleted, or modified.

Standard (1984 X.25) Facilities

Table 69 describes how standard (1984 X.25) facilities are treated when routing a switched virtual circuit (SVC). To configure these facilities, refer to the "Configure X.25 User Facilities" section in the "Configuring X.25 and LAPB" chapter of the Wide Area Networking Configuration Guide.


Table  69: Treatment of Standard X.25 Facilities
Facility Treatment
Flow Control Negotiation (negotiation of window size and maximum packet size) The Cisco IOS software adds, removes, or changes flow control parameter values to match the values on both interfaces, as described in the following cases:

  • Requested flow control values do not match the outgoing interface's defaults.

  • Inserts flow control parameters into the outgoing switched call.
  • Requested values match the outgoing interface's defaults.
  • Strips parameter values from the outgoing switched call.
  • Requested maximum packet size exceeds the capability of either interface.
  • Lowers the packet size to the largest value that can be supported by the two interfaces.
  • Call is routed from a modulo 128 interface to a modulo 8 interface.
  • Lowers the larger requested window size to 7.
  • Call is remotely routed over a TCP connection.
  • Ensures that both proposed maximum packet sizes and proposed window sizes for a call are present.
  • Call is received from an X.25 over a TCP connection without one or more flow control parameter values.
  • By default, forces the call to use the maximum packet sizes (128/128) and window sizes (2/2). If the x25 routing use-tcp-if-defs command and keyword are specified, the router supplies the call with the default values of the outgoing serial interface. In either case, the Call Confirm packet sent back over the X.25-over-TCP (XOT) connection indicates the final flow control values negotiated for the connection.
  • Accepted flow control parameter values are different, for any reason, from the values proposed for the incoming call.
  • Sends an outgoing Call Accepted packet that indicates the accepted flow control values.

Throughput Negotiation

Forwards the incoming Throughput facility.
Closed User Group Selection Forwards a basic format Closed User Group (CUG) selection facility; any other format of CUG selection (extended format, CUG with outgoing access or Bilateral CUG) will be stripped.
Reverse Charging Forwards an incoming Reverse Charging facility.
Fast Select Forwards an incoming Fast Select facility.
Network User Identification (NUID) Forwards an incoming NUID facility on a Call packet; an NUID facility on a Call Accepted packet is stripped.
Charging Information Strips any Charging Information or Request.
RPOA Selection Strips any RPOA Selection.
Called Line Address Modified Notification Forwards a Called Line Address Modified Notification.
Call Redirection Notification Strips a Call Redirection Notification.
Transit Delay Selection Forwards an incoming Transit Delay facility.

The implementation of X.25 prior to Release 9.1(4.1) software did not insert flow control parameter values into Call packets sent over X.25-over-TCP (XOT) connections. When such an XOT call is received by Release 9.1(4.1) or later, the call is forced to use the standard flow control values. This use may cause migration problems when the router is connecting X.25 equipment that is not capable of negotiating flow control parameters; you can use the optional use-tcp-if-defs keyword of the x25 routing command if you encounter this problem.

ITU-T-Specified Marker Facilities

Table 70 describes how CCITT/ITU-T-specified marker facilities are treated when routing an SVC.


Table  70: Default Treatment of ITU-T-Specified Marker Facilities
Facility Treatment
Calling Address Extension Forwards an incoming Calling Address Extension facility.
Called Address Extension Forwards an incoming Called Address Extension facility.
Quality of Service Negotiation Strips any of the Quality of Service facilities.
Expedited Data Negotiation Strips an Expedited Data Negotiation facility.

The router requires the Calling Address Extension facility to route to a CMNS host.

The encoding of any CCITT/ITU-T facilities is preceded by a marker, as displayed in the output of the debug x25 command.

Local Marker Facilities Specified for DDN or BFE X.25

Table 71 describes how local marker facilities are treated when routing an SVC.


Table  71: Default Treatment of Local Marker Facilities Specified for DDN or BFE X.25
Facility Treatment
DDN Service Type Strips an incoming DDN Service Type facility from a call, but inserts DDN Service Type if a forwarded Call Accepted packet specifies a DDN precedence facility.
DDN Precedence Forwards an incoming DDN Precedence facility. However, both the input and output interfaces need to be configured for DDN X.25 encapsulation. To configure treatment of this facility, see the "Define IP Precedence Handling" section in the "Configuring X.25 and LAPB" chapter of the Wide-Area Networking Configuration Guide.
BFE Emergency Mode Addressing Strips an incoming BFE Emergency Mode Addressing facility. To configure treatment of this facility, see the "Configure Blacker Front End (BFE) X.25" section in the "Configuring X.25 and LAPB" chapter of the Wide-Area Networking Configuration Guide

The Cisco IOS software supports DDN Standard service but not DDN Basic service. Consequently, the DDN Service Type facility does not have to be configured.

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