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Contents
This appendix provides reference material describing how X.25 facilities are handled by the Cisco IOS software.
A router either originates or accepts datagram transport (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 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 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.
If an interface is configured as a DCE that is subscribed to closed user group (CUG) services, datagram encapsulation calls that originate and terminate on the interface will be subject to the requirements of CUG security.
As a general rule, the X.25 switch services will forward facilities encoded in Call, Call Confirm, Clear, and Clear Confirm packets. This handling, however, is subject to the following restrictions:
The table below describes how X.25 standard facilities are treated when a switched virtual circuit (SVC) is routed. If the facility was introduced in a recommendation later than the 1980 X.25 recommendation, the recommendation in which the facility was introduced is listed in parentheses after the facility name. By default, Cisco IOS software supports the 1984 recommendation.
The table below describes how CCITT/ITU-T-specified marker facilities are treated when an SVC is routed.
Facility |
Treatment When Switched by Cisco IOS Software |
---|---|
Calling Address Extension (1984) |
Forwards the incoming Calling Address Extension facility. |
Called Address Extension (1984) |
Forwards the incoming Called Address Extension facility. |
Quality of Service (QoS) Negotiation |
Forwards the QoS facilities and their associated values. |
Expedited Data Negotiation (1984) |
Forwards the Expedited Data Negotiation facility. |
CMNS hosts commonly use the Called Address Extension facility, which can be used to make X.25 routing decisions.
The encoding of any CCITT/ITU-T facilities is preceded by a marker, as displayed in the output of the debug x25 command.