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The X.25 Suppression of Security Signaling Facilities feature allows the X.25 Call Redirection/Call Deflection Notification
(CRCDN) and Called Line Address Modified Notification (CLAMN) security signaling facilities to be disabled (suppressed) in
X.25 Call and Call Confirm packets (respectively) sent by an X.25-class service. This feature may be required when connecting
to equipment that implements a proprietary or nonstandard X.25 service that does not accept X.25 security signaling facilities.
Feature Specifications for the X.25 Suppression of Security Signaling Facilities
Determining Platform Support Through Cisco Feature Navigator
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Information About the X.25 Suppression of Security Signaling Facilities Feature
X.25 Security Facilities Suppression Scenarios
X.25 networks encode security facilities in X.25 Call, Call Confirm, and Clear packets to notify both stations participating
in the setup of a switched virtual circuit (SVC) of events that may result in a station connecting to an unexpected partner.
Note
This document refers to Call packets and Call Confirm packets. These names differ from those standardized by X.25. The standard
distinguishes between a Call packet sent by the DTE station (a Call Request) and one sent by the DCE station (an Incoming
Call), and similarly between a Call Confirm packet sent by the DTE (a Call Accepted) and one sent by the DCE (a Call Connected).
The packets are encoded identically and, in many cases, the processing that X.25 does is identical; however, there are cases
where the behavior is predicated on the station type receiving or sending the packet.
For example, when an X.25 Call is redistributed by a network through a hunt group, a standard implementation will encode a
CRCDN facility in the forwarded call. Thus, the receiver is notified that the Call packet was redistributed by a hunt group
and is notified of the original destination address. A standard network will also, if such a Call is accepted by a returned
Call Confirm packet, encode a CLAMN facility when forwarding the Call Confirm packet. This encoding notifies the originator
that the accepting destination was reached by distribution through a hunt group, and may also encode the destination address
of the accepting station. Both stations receive notification of what happened so each can decide to either proceed with the
SVC, if the resulting connection is permissible, or to clear the channel if not.
When Suppressing the Security Signaling Facilities Is Necessary
DANGER
X.25 security signaling facilities are used to explicitly notify the connecting stations of events that may raise security
issues if they were not signaled. Suppression of these facilities should only be configured when the attached equipment and
network configurations are sufficiently secure that the signaled information is unnecessary.
There are many X.25 implementations that will not operate as intended if presented with X.25 features or facilities beyond
a narrow set of those that occur most commonly. The security signaling facilities are less common, and there are a significant
number of X.25 implementations that will not proceed with an SVC that encodes them during Call setup. This can cause connection
failures when Cisco equipment is used to implement an X.25 hunt group. There are two security facilities that the Cisco hunt
group feature encodes: An X.25 Call packet forwarded out from a hunt group has the CRCDN facility encoded in the packet and,
when accepted, the returning X.25 Call Confirm packet has the CLAMN facility encoded in the packet.
Both the originator of the Call packet and the destination it reaches should be notified of the hunt group event, thus allowing
each side to clear the SVC if communication is not permitted by the station’s security policy. For this reason, the Cisco
implementation of hunt groups is designed to signal both stations participating in the Call setup using the X.25-designated
CRCDN and CLAMN facilities. The X.25 Suppression of Security Signaling Facilities feature allows this signaling to be suppressed
by the CRCDN facility in a Call packet. The
no x25 security crcdn command introduced in this feature provides this function, and there are no implications for correct protocol behavior by
using it.
X.25 operation can also be modified to suppress a CLAMN facility in X.25 Call Confirm packets when the
no x25 security clamn command is configured to disable that signaling. Configuring suppression of the CLAMN security signaling facility has an
implication for correct protocol behavior: The X.25 Recommendations specify that the CLAMN facility must be present in a Call
Confirm packet if that packet encodes a destination address that is not the null address and that differs from the address
encoded in the Call packet. When X.25 is configured to suppress the encoding of a CLAMN facility, it will also suppress the
encoding of the destination address. That is, when the address block is encoded in the Call Confirm packet, the destination
address will be encoded as the null address (zero digits) because no representation should be made as to what destination
was reached.
An X.25 profile may also be configured to suppress the X.25 security signaling facilities. This profile can be useful if
the network administrator wants to localize the suppression of these facilities. For example, a hunt group that switches a
connection using X.25 over TCP/IP (XOT) may be configured so that the security signaling facilities are not transmitted to
either hop participating in the Call setup.
As another example, some telephone company data communications networks (telco DCNs) use a nonstandard X.25 implementation
that blends elements of the 1980 and 1984 International Telecommunication Union Telecommunication Standardization Sector (ITU-T)
Recommendations. The figure below shows a portion of a telco DCN network where X.25 devices, also called CPE, are connected
to Cisco routers and the IP backbone network using serial links.
Figure 1. DCN Network Devices Connected to a Cisco IP Backbone Network
Early equipment in the telco DCN conformed to the ITU-T 1980 X.25 Recommendation, and Cisco provides support for this standard.
However, substantial ITU-T 1984 X.25 Recommendation elements, such as maximum packet sizes of 2048 and 4096 and X.25 Annex
G operation, have since been incorporated into the DCN. This mix of ITU-T 1980 and 1984 X.25 Recommendations in the telco
DCN has resulted in a design requirement that would allow the CPE to operate according to the ITU-T 1984 X.25 Recommendation,
but with a modification that would allow suppressing security signaling facilities encoded by the Cisco hunt group feature.
Because the ITU-T 1980 X.25 Recommendation does not define these security signaling facilities, the Cisco X.25 implementation
can now be configured to suppress them in the packets where they would otherwise be encoded.
How to Suppress the X.25 Security Signaling Facilities
Disabling the X.25 Security Signaling Facilities
To disable the X.25 CLAMN and CRCDN signaling facilities, perform the following steps:
SUMMARY STEPS
enable
configure {terminal | memory | network }
interface serial interface-number
encapsulation x25
no x25 security crcdn
no x25 security clamn
exit
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Router> enable
Enables higher privilege levels, such as privileged EXEC mode.
Enter your password if prompted.
Step 2
configure {terminal | memory | network }
Example:
Router# configure terminal
Enters global configuration mode.
Step 3
interface serial interface-number
Example:
Router(config)#
interface serial 0
Enters interface configuration mode.
Step 4
encapsulation x25
Example:
Router(config-if) encapsulation x25
Enables the default X.25 DTE operation mode.
Step 5
no x25 security crcdn
Example:
Router(config-if) no x25 security crcdn
Disables the CRCDN security signaling facility in X.25 Call packets transmitted.
Step 6
no x25 security clamn
Example:
Router(config-if) no x25 security clamn
Disables the CLAMN security signaling facility in X.25 Call Confirm packets and suppresses any destination address.
Step 7
exit
Example:
Router(config-if) exit
Ends interface configuration mode.
Enter the exit command once more to exit global configuration mode.
Troubleshooting Tips
Use the debug x25 EXEC command to determine when the X.25 facilities are present and when they are suppressed by the configured feature.
Configuration Example for Suppressing X.25 Security Signaling Facilities
The following example shows how to suppress both the CRCDN and CLAMN security signaling facilities:
interface serial 0
no ip address
encapsulation x25
no x25 security crcdn
no x25 security clamn
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