Software User's Guide Version 3.4

Using the X.25 Network Interface

The X.25 network interface connects a router to an X.25 virtual circuit switched network. The X.25 network interface software and hardware allows the router to communicate over a public X.25 network. The X.25 network interface complies with CCITT 1980, CCITT 1984, CCITT 1988 and ISO 8208 1990 specifications for X.25 interfaces offering multiplexed channels and reliable end-to-end data transfer across a wide area network.

This chapter includes the following sections:

• "Basic Configuration Procedures"
• "Null Encapsulation"
• "Understanding Closed User Groups"

For information on configuring X.25 Transport Protocol (XTP) for transporting X.25 traffic over TCP/IP, see Using XTP.

Basic Configuration Procedures

This section outlines the minimal configuration steps required to get the X.25 interface up and running. The X.25 parameters must be consistent with the X.25 network the interface on the router will connect to. For more information, refer to the configuration commands described in this chapter.

Note:

You must restart the router for the configuration changes to take effect.

1. At the OPCON prompt (*), type talk 6.

   The Config> prompt appears.

2. Type list devices to display a list of the interfaces from which you can select. Use the appropriate interface number in the following step.
3. Type set data-link x25.

   The Interface Number [0]? prompt appears.

4. Type the appropriate interface number.
5. Connect to the network by typing net # at the Config> prompt.

   The X.25 Config [#]> prompt appears.

6. At this prompt, type set address x.25-node-address.

   The X.25 address is a unique X.121 address that is used during call establishment. For DDN networks, use the add htf-addr and the set htf-addr commands to convert the protocol address associated with this interface to the X.121 address format required for DDN address translation. Failure to set the network address prevents the X.25 interface from joining the attached network.

7. Type set equipment-type and specify whether the frame and packet levels act as DCE or DTE. The default for this command is DTE.
8. Type set svc and define the lowest and highest SVCs that you are using. The default is for 1 SVC.
9. Type add protocol protocol_name to add the protocols that will be running over the X.25 interface. You will be prompted for window size, default packet size, maximum packet size, circuit idle time, and max VCs.

   Note:

   You need to add the protocols only once for all X.25 networks on the router.

10. Type add address protocol_name to add an address translation for each protocol's destination address reachable over this interface.
11. Type exit to return to the Config> prompt.
12. Press Ctrl-P to return to the OPCON prompt (*).
13. Type restart and respond yes to the prompt.

Setting the National Personality

Each public data network, such as GTE's Telenet or DDN's Defense Data Network, has its own standard configuration. The term National Personality specifies a group of variables used to define a public data network's characteristics. The configuration information in the National Personality provides the router with control information for packets being transferred over the link. The National Personality option defines 27 default parameters for each public data network.

To view the configuration values that are in your X.25 National Personality, execute the X.25 configuration list detailed command. Configure each public data network connected to the router by executing the X.25 configuration national-personality set command.

The National Personality is a generalized template for network configuration. If necessary, you can individually configure each frame and packet layer parameter.

Understanding the X.25 Defaults

The following tables list the defaults for the various parameters for the X.25 set, national set and national enable commands.

Table 57. Set Command

Parameter	Default
address ...	none
cable	none
calls-out ...	4
clocking ...	external
default-window-size ...	2
encoding	NRZ
equipment-type ...	DTE
htf addr ...	none
inter-frame-delay ...	0
mtu	1500
national-personality ...	GTE Telenet
pvc ...	low=0 high=0
speed	9600
svc	low inbound=0, high inbound=0 low 2-way=1, high 2-way=64 low outbound=0, high outbound=0
throughput-class ...	inbound=outbound=2400
vc-idle ...	30

Table 58. National Enable Parameters

Parameter	DDN Default	GTE Default
accept-reverse-charges	off	on
bi-cug	off	off
bi-cug-with-outgoing-access	off	off
cug	off	off
cug-deletion	off	off
cug-insertion	off	off
cug-with-incoming-access	off	off
cug-with-outgoing-access	off	off
cug-zero-override	off	off
flow-control-negotiation	on	on
frame-ext-seq-mode	off	off
packet-ext-seq-mode	off	off
request-reverse-charges	off	on
suppress-calling-addresses	off	off
throughput-class-negotiation	on	on
truncate-called-addresses	off	off

Table 59. National Set Parameters

Paramter	DDN Default	GTE Default
call-req	20 decaseconds	20 decaseconds
clear-req ...	retries=1 18 decaseconds	retries=1 18 decaseconds
disconnect-procedure ...	passive	passive
dly-recall-timer...	0	0
dp-timer	500 milliseconds	500 milliseconds
frame-window-size	7	7
n2-timeouts	20	20
packet-size ...	128, max=256	128, max=256
reset ...	retries=1 18 decaseconds	retries=1 18 decaseconds
restart ...	retries=1 18 decaseconds	retries=1 18 decaseconds
max-recall-retires ...	3	3
min-recall	10 seconds	10 seconds
min-connect	90 seconds	90 seconds
collision-timer	10 seconds	10 seconds
standard-version	1984	1984
t1-timer	4 seconds	4 seconds
t2-timer	0	0
truncate-called-addr-size	2	2

Null Encapsulation

Null Encapsulation allows the user to multiplex multiple network layer protocols over one X.25 circuit. This function may be used to avoid using an unreasonable number of virtual circuits.

Limitations

Null Encapsulation is not supported for QLLC. This function is supported for Switched Virtual Circuits (SVCs), but not for Permanent Virtual Circuits (PVCs).

Configuration Changes

The encapsulation option NULL has been added for the following T6 commands:

Under X25 config: add address IP (may input enc type = NULL)

Under X25 config: add address IPX (may input enc type = NULL)

Under X25 config: add address DNA (may input enc type = NULL)

Under X25 config: add address VINES (may input enc type = NULL)

Under X25 config: list addr will show active enc type = NULL if the priority 1 type is NULL.

T5 commands:

Under X25 int: List SVCS will include enc type = NULL

Configuring Null Encapsulation and Closed User Groups (CUG)

Since More than one Protocol can run over one virtual circuit while using Null Encapsulation, the CUG(s) defined for each protocol over that circuit must be the same. It is strongly suggested that the user configure multiple Protocols same destination as follows:

Configure CUG using the add address. The CUG(s) defined must be the same for each protocol defined at the same address.

If the CUG is defined at the add protocol level, The CUG(s) must be the same for all peers. (This method is more restrictive).

Configure CUG at the interface level. This insures all peers have the same CUG values. (This method is the most restrictive)

Any of the above methods may be used as long as any incoming call CUG definition must be valid for all protocols sharing that circuit. Valid means that the CUG was defined for the specific address or was defaulted to use either the protocol or interface circuit definition.

Figure 35. Closed User Group Null Encapsulation

Understanding Closed User Groups

A closed user group (CUG) is a group of X.25 DTEs allowed to establish connections with other specific DTEs. CUG numbers are defined by your network provider and you can only use the CUGs the provider assigns you. You can configure an address-specific CUG, a protocol-specific CUG, or an interface-specific CUG. If all of three types of CUG numbers are configured for a DTE, the closed user group facility uses the address-specific destination CUG in a call request when contacting another DTE. If only a protocol-specific and an interface-specific CUG are configured for a DTE, the closed user group facility uses the protocol-specific CUG in a call request when contacting another DTE.

A single DTE can belong to multiple CUGs. You must specify a preferred CUG for that DTE. The preferred CUG is used when the router initiates calls to other DTEs. A single DTE cannot have more than a total of 5 preferred or normal closed user groups.

Bilateral Closed User Groups

A bilateral closed user group (BCUG) is a closed user group consisting of only two DTEs. The DTEs within the BCUG can originate calls to members of the BCUG and any DTEs that are not members of any CUG or BCUG. A single DTE cannot have more than a total of 5 preferred or normal bilateral CUGs.

A DTE uses a BCUG to establish circuits in the same way the DTE uses CUGs to establish circuits (see Table 60), however, if both a BCUG and a CUG is defined for an interface, protocol, or address, the BCUG is used to establish the circuit.

Types of Extended Closed User Groups

The following extensions to closed user groups are supported:

CUG with Outgoing Access

The DTE can belong to one or more CUGs. The DTE can originate calls to members of the CUG and to any DTE belonging to other CUGs with Incoming Access.

CUG with Incoming Access

The DTE can belong to one or more CUGs. The DTE can receive calls from DTEs not belonging to any CUG or from DTEs belonging to other CUGs with Outgoing Access.

BCUG with Outgoing Access

The DTE can belong to one or more BCUGs. The DTE can originate calls to members of the BCUG and to any DTE not belonging to any BCUG.

Establishing X.25 Circuits with Closed User Groups on a Device

When you have enabled the closed user group facility, and a DTE receives a call request, it uses the CUG in the call request to determine whether to accept or reject the call from the DTE. If the CUG in the call request does not match a configured CUG on the interface, protocol, or on the destination associated with the calling DTE, the request is rejected. Table 60 summarizes how X.25 circuits are established based on CUGs, if the interface, protocol, and address CUG numbers are different and incoming access is not enabled.

Table 60. Establishing Incoming X.25 Circuits for Closed User Groups

Incoming Call Request Contains	Receiving DTE CUG Definition
	Interface CUG Only	Protocol CUG Only	Address Specific CUG	Interface and Protocol CUG	Interface and Address CUG	Protocol and Address CUG	All CUGs	No CUGs
No CUG	Reject	Reject	Reject	Reject	Reject	Reject	Reject	Accept
Interface CUG	Accept	Reject	Reject	Reject	Reject	Reject	Reject	Reject
Protocol CUG	Reject	Accept	Reject	Accept	Reject	Reject	Reject	Reject
Address Specific CUG	Reject	Reject	Accept	Reject	Accept	Accept	Accept	Reject

For outgoing calls on an interface, if you have enabled either the CUG or the BCUG facility, each call request will contain the configured preferred CUG (if any) for the destination or, if no address-specific CUG is configured, the CUG used is the CUG defined for the protocol, or if no protocol-specific CUG is configured, the CUG used is the CUG defined for the interface. If no CUG number has been configured, the CUG facility is not included in any outgoing call request.

Overriding Closed User Group Processing for CUG 0

You can configure the DTE such that it does not validate incoming calls with a CUG of 0 in the call request. This ability allows you to permit specific calls to complete even when you have not enabled incoming access. Using the national enable cug 0 override command forces the device to ignore the CUG facility if the CUG number is 0. The call request will not be compared with any configured CUG number.

Configuring X.25 Closed User Groups

To use closed user groups on X.25 interfaces:

1. Request CUG numbers from your network provider. You will need these numbers when configuring X.25.
2. Enable the closed user group facility using the national enable cug command and related commands.
3. Enable the bilateral closed user group facility, if desired, using the national enable bi-cug command and related commands.
4. Configure the appropriate CUG numbers for the DTEs. Specify the preferred CUG, CUG, preferred bilateral CUG, and bilateral CUG, as needed. This is done through the add address command.
5. Configure the appropriate CUG and bilateral CUG for the protocol, if required. This is done through the add protocol command.

   Note:

   You should only configure these CUGs if you are restricting all X.25 circuits established over the X.25 interface for this protocol to DTEs belonging to this set of unique CUGs or BCUGs unless you override it with an address-specific CUG.

6. Configure the appropriate CUG and bilateral CUG for the interface, if required. This is done through the add cug command.

   Note:

   You should only configure these CUGs if you are restricting all X.25 circuits established over the X.25 interface to DTEs belonging to this set of unique CUGs or BCUGs unless you override it with an address or protocol-specific CUG.