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This chapter describes how to configure connections through X.25 networks, including Link Access Procedure, Balanced (LAPB) connections. LAPB procedures are presented first for those users who only want to configure a simple, reliable serial encapsulation method. For a complete description of the commands mentioned in this chapter, refer to in the "X.25 and LAPB Commands" chapter of the Router Products Command Reference publication. For historical background and a technical overview of X.25 and LAPB, see the Internetworking Technology Overview publication.
A group of specifications published by the CCITT (French acronym translating to International Telegraph and Telephone Consultative Committee) defines Recommendations, or specifications, for X.25. Although it is most commonly referred to as the CCITT, this international standards body recently adopted the name International Telecommunications Union, so you may also see references to ITU Recommendations.The CCITT specifications basically specify connections between data terminal equipment (DTE) and data communications equipment (DCE) for remote terminal access and computer communications. The X.25 specifications include LAPB as the data link layer protocol and X.25 as the network layer protocol, or packet layer protocol (PLP) as it is also known. The CCITT updates its specifications every four years, and the specifications dated 1980 and 1984 are the most common versions currently in use. Additionally, the International Standards Organization (ISO) has published ISO 7776:1986 as an equivalent to the LAPB standard, and ISO 8208:1989 as an equivalent to the CCITT 1984 X.25 Recommendation packet layer. Our X.25 software follows the CCITT 1984 X.25 Recommendation, except for its Defense Data Network (DDN) and Blacker Front-End Encryption (BFE) encapsulation operation, which follows the CCITT 1980 X.25 Recommendation.
In addition to providing remote terminal access, our X.25 software provides transport for a growing list of LAN protocols--IP, DECnet, XNS, ISO CLNS, AppleTalk, Novell IPX, Banyan VINES, and Apollo Domain--and bridging. For information about these protocols, refer to the specific protocol chapters in this publication.
Briefly, the Cisco Systems X.25 software provides the following capabilities:
Our X.25 implementation does not support fast switching.
The router either originates or accepts encapsulation virtual circuits (VCs) in order 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/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 will be 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 will be lowered if the lower layer (LAPB) cannot support the sizes proposed. A reverse-charged CALL will be CLEARed if neither the interface nor the map allows it. A CALL that specifies a Network User Identification (NUID) will be CLEARed if the user authentication fails.
Routed X.25 traffic may have facilities added, deleted, or modified by the router.
The following subsections describe how standard (1984 X.25) facilities are treated when routing a switched virtual circuit (SVC).
Our routed X.25 implementation requires that the flow control parameters be negotiated end-to-end and actively intervenes in the negotiation process using the CALL/CALL ACCEPTED packet exchange to ensure this. This behavior is important because routed X.25 traffic is simply passed between the two interfaces (local or remote) without fragmentation, reassembly, or local flow control handling. As part of the flow control negotiation process, the following activities take place:
An incoming Throughput facility will be forwarded.
A basic format Closed User Group selection facility will be forwarded; any other format of Closed User Group selection (extended format, CUG with outgoing access or Bilateral CUG) will be stripped.
An incoming Reverse Charging facility will be forwarded.
An incoming Fast Select facility will be forwarded.
An incoming NUID facility on a CALL packet will be forwarded; an NUID facility on a CALL ACCEPTED packet will be stripped.
Any Charging Information or Request will be stripped.
Any RPOA Selection will be stripped.
A Called Line Address Modified notification will be forwarded.
A Call Redirection notification will be stripped.
An incoming Transit Delay facility will be forwarded.
The following subsections describe how CCITT-specified marker facilities are treated when routing an SVC.
An incoming Calling Address Extension facility will be forwarded.
An incoming Called Address Extension facility will be forwarded.
Any of the Quality of Service facilities will be stripped.
An Expedited Data Negotiation facility will be stripped.
The following subsections describe how local marker facilities are treated when routing an SVC.
An incoming DDN Service Type facility will be stripped from a CALL, but inserted if a forwarded CALL ACCEPTED packet specifies a DDN precedence facility.
An incoming DDN Precedence facility will be forwarded through the router. However, both the input and output interfaces need to be configured for DDN X.25 encapsulation.
An incoming BFE Emergency Mode Addressing facility will be stripped.
It is possible to use only LAPB as a serial encapsulation method. This can be done using a leased serial line. You must use one of the X.25 packet-level encapsulations when attaching to an X.25 network.
The LAPB standards distinguish between two types of hosts: data terminal equipment (DTE), and data circuit-terminating equipment (DCE). At Level 2, or the data link layer in the OSI model, LAPB allows for orderly and reliable exchange of data between a DTE and a DCE. A router using LAPB encapsulation can act as a DTE or DCE device at the protocol level, which is distinct from the hardware DTE or DCE level.
Using LAPB under noisy conditions can result in greater throughput than HDLC encapsulation. When LAPB detects a missing frame, the router retransmits the frame instead of waiting for the higher layers to recover the lost information. This behavior is good only if the host timers are relatively slow. In the case of quickly expiring host timers, however, you will discover that LAPB is spending much of its time transmitting host retransmissions. If the line is not noisy, the lower overhead of HDLC encapsulation is more efficient than LAPB. When using long delay satellite links, for example, the lock-step behavior of LAPB makes HDLC encapsulation the better choice.
Perform the following tasks to configure LAPB:
The following sections describe the LAPB configuration tasks. See the section "Monitor and Maintain LAPB and X.25" for the LAPB administrative tasks. Examples of LAPB configurations appear at the end of this chapter.
Set the appropriate LAPB encapsulation to run datagrams over a serial interface. One end of the link must be DTE and the other must be DCE.
Select an encapsulation and the protocol if using a single protocol, or select the multiple protocol operation, according to the table that follows. Perform these tasks in interface configuration mode:
For an example of configuring LAPB DCE operation, see the section "Typical LAPB Configuration Example" later in this chapter.
X.25 Level 2 or LAPB operates at the data link layer of the OSI reference model. LAPB specifies methods for exchanging data (in units called frames), detecting out-of-sequence or missing frames, retransmitting frames, and acknowledging frames.
When connecting to an X.25 network, use the N1 parameter value set by the network administrator. This value is the maximum number of bits in a LAPB frame, which is determined from the maximum size of an X.25 packet. When using LAPB over leased lines, the N1 parameter should be eight times the hardware maximum transmission unit (MTU) size. The other frame parameters are determined by the network configuration; see Table 1-1 for the default values.
The retransmission timer determines how long a transmitted frame can remain unacknowledged before the router polls for an acknowledgment. For X.25 networks, the router retransmission timer setting should match that of the network.
For leased-line circuits, the retransmission timer setting is critical. The timer setting must be large enough to permit a maximum-sized frame to complete one round trip on the link. If the timer setting is too small, the router will poll before the acknowledgment frame can return, which may result in duplicated frames and severe protocol problems; the design of LAPB assumes that a frame has been lost if it is not acknowledged within period T1. If the timer setting is too large, the router waits longer than necessary before requesting an acknowledgment, which reduces bandwidth.
Table 1-1 summarizes the LAPB parameters you can set in interface configuration mode.
For an example of configuring the LAPB T1 timer, see the section "Typical LAPB Configuration Example" later in this chapter.
You can define the maximum number of packets to be held while the other side is not accepting data. To do so, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Define the size of a packet hold queue. | lapb hold-queue queue-size |
To configure X.25, you can choose from the following tasks, depending upon the X.25 application or task required for your network:
Note that all X.25 applications except DDN and BFE X.25 operation and routing require that an encapsulation method and an X.121 address be set; see the sections "Configure X.25 DTE or DCE Operation" and "Set the X.25 Interface Address" for the tasks to set them.
Default parameters have been provided for X.25 operation; however, you can change the settings to meet the needs of your X.25 network or as defined by your X.25 service supplier. We also provide special and custom configuration settings to further optimize your X.25 network. See the section "Configure X.25 Level 3 Parameters and Special Features" later in this chapter for more information.
The following sections describe how to implement the supported configuration types and special configuration tasks. See the "X.25 and LAPB Commands" chapter of the Router Products Command Reference publication for information about the commands in the tasks. See the end of this chapter for examples of configuring X.25.
X.25 support is most commonly configured as a transport for datagrams across an X.25 network. This is accomplished by first establishing a mapping between protocol addresses (for example, IP or DECnet) and the X.121 addresses of the X.25 network. When datagrams for a particular destination are routed for the first time, a virtual circuit is set up to the appropriate X.121 address. The Call User Data portion of the initial Call Request identifies the protocol of the datagrams being carried by a particular virtual circuit. If multiple protocols are in use, multiple virtual circuits will be opened.
Figure 1-1 illustrates two routers sending data across an X.25 public data network (PDN).
You must perform the following tasks to configure your router as a transport for X.25 datagrams (note that these tasks are required by all X.25 applications except DDN and BFE X.25):
Perform the following tasks, as necessary, to complete the X.25 configuration for your network needs:
The following sections describe how to perform these configuration tasks. Configuring the X.25 parameters and special features, including TCP header compression and X.25 bridging, are described in the section "Configure X.25 Level 3 Parameters and Special Features" later in this chapter.
A router using X.25 Level 3 encapsulation can act as a DTE or DCE protocol device. Default serial encapsulation is HDLC. Configure one of these encapsulation types, according to the needs of your X.25 service supplier. To do so, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set X.25 DTE operation. | encapsulation x25 |
| Set X.25 DCE operation. | encapsulation x25-dce |
Typically a public data network will require attachment as a DTE. (This is distinct from the hardware interface DTE and DCE assignments.)
For an example of configuring X.25 DTE operation, see the section "Typical X.25 Configuration Example" later in this chapter.
Set the X.121 address of the network interface. The X.121 address is assigned by the X.25 network service provider. To set the X.121 address, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set the X.121 address. | x25 address X.121-address |
For an example of configuring the X.25 interface address, see the section "Typical X.25 Configuration Example" later in this chapter.
The routers can use several methods to map protocol addresses to X.121 addresses used by X.25 as follows:
A router set up for DDN or BFE service uses a dynamic mapping technique to convert between Internet and X.121 addresses. This technique has two severe limitations: BFE mapping applies only to Class A Internet addresses, and it ignores the third octet of the Internet address. This technique works well for the DDN and blacker connections, but not for other networks. The BFE service is a variant of DDN service.
You can establish the X.121 address of a particular network interface using the x25 address interface subcommand. A DDN or BFE interface will have a default address generated from the interface IP address
You also can set up explicit protocol-to-X.121 address mapping for a host. Because no defined protocol can dynamically determine such mappings, you must enter a mapping for each host with which the router will exchange traffic.
To establish a map, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Establish a protocol-to-X.121 address map. | x25 map protocol-keyword protocol-address X.121-address [options] |
As an example, if you are running IP over X.25, you must define an IP address for the X.25 interface as well as map the IP address to the X.121 address. These tasks are described in the chapter "Configuring IP," later in this publication.
When setting up the address map, you can include features such as enabling broadcasts and number of virtual circuits allowed as well as the user facility settings as part of the options. The following list summarizes the supported protocol keywords for map definitions.
For specific information about how to establish a protocol to run over X.25, refer to each of the protocol chapters in this publication.
The configuration for the Open Shortest Path First (OSPF) protocol can be greatly simplified by adding the optional broadcast keyword when doing this task. See the x25-map command description in the "X.25 and LAPB Commands" chapter of the Router Products Command Reference publication for more information.
Permanent virtual circuits (PVCs) are the X.25 equivalent of leased lines; they are never disconnected. Specify the required network protocol-to-X.121 address map before you set up a PVC. To establish a PVC, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set a PVC. | x25 pvc circuit protocol-keyword protocol-address [options] |
See the previous section for a list of protocols; all but OSI and TCP header compression are supported by this task. For an example of configuring a PVC, see the section "Example of Using a PVC to Exchange IP Traffic" later in this chapter.
The Call Request packet that sets up a virtual circuit (VC) might contain a field called the Call User Data (CUD) field. Typically, the software uses the first byte of Call User Data to distinguish which high-level protocol will be carried by a particular virtual circuit.
Table 1-2 lists the hexadecimal values of the Call User Data and its corresponding network level protocol. The use of 0x81 for ISO 8473 (CLNS) is an ISO standard. The use of 0xCC for Department of Defense IP is defined by RFC 877. The use of C0 00 80 C4 is defined by Banyan. The other values are meaningful only to our X.25 software. Most of the single-byte identifiers are padded to four bytes with three bytes of 0x00. BFE IP encapsulation requires that only one byte be used. CLNS may use one or five bytes as defined in ISO 8473.
| Protocol | Initial CUD Byte |
|---|---|
| ISO CLNS | 0x81 |
| DOD IP | 0xCC |
| DECnet | 0xD0 |
| XNS | 0xD1 |
| AppleTalk | 0xD2 |
| Novell IPX | 0xD3 |
| Apollo Domain | 0xD4 |
| Banyan VINES | 0xC0 00 80 C4 |
| Bridging | 0xD5 |
| TCP Header Compression (THC) | 0xD8 |
You can specify the protocol assumed by the router to interpret calls with unknown Call User Data (CUD) or with no CUD by performing the following task in interface configuration mode:
| Task | Command |
|---|---|
| Establish the default protocol. | x25 default protocol |
The X.25 software implementation allows virtual circuits to be routed from one X.25 interface to another and from one router to another. The routing behavior can be controlled with switching and tunneling commands, based on a locally built table.
Encapsulated LAN protocols can share an X.25 serial interface with the X.25 switching support. Switching or forwarding X.25 virtual circuits can be done two ways:
Running X.25 over TCP/IP provides a number of benefits. The datagram containing the X.25 packet can be switched by other routers using their high-speed switching abilities. X.25 connections can be sent over networks running only the TCP/IP protocols. The TCP/IP protocol suite runs over many different networking technologies, including Ethernet, Token Ring, T1 serial, and FDDI. Thus X.25 data can be forwarded over these media to another router, where it can be output to an X.25 interface.
When the connection is made locally, the switching configuration is used; when the connection is across a LAN, the tunneling configuration is used. The basic function is the same for both types of connections, but different configuration commands are required for the two types of connections.
The X.25 switching subsystem supports the following facilities and parameters:
The tasks for configuring these facilities are described in the section "X.25 Level3 Parameters and Special Features."
To configure X.25 tunneling on your router, perform the following tasks:
You also need to configure the X.25 parameters and special features, as required for your network. Each task is described in a following section.
You can enable local switching, or tunneling, (used for remote switching), allowing you to route X.25 traffic through a LAN. To enable local X.25 routing, perform the following task in global configuration mode:
| Task | Command |
|---|---|
| Enable X.25 routing. | x25 routing [USE-TCP-IF-DEFS] |
The [USE-TCP-IF-DEFS] flag is used by some routers that receive remote routed calls from older software versions; it may be needed if the originating router cannot be migrated to a new software release.
For an example of configuring local X.25 routing, see the sections "X.25 Route Address Pattern Matching Example" and "X.25 Routing Example" later in this chapter.
The X.25 routing table is consulted when an incoming call is received that should be forwarded to its destination. Two fields are used to determine the route: the called X.121 network interface address (or destination host address) and the X.25 packet's Called User Data (CUD) field. When the destination address and the CUD of the incoming packet fit the X.121 and CUD patterns in the routing table, the call is forwarded. Additionally, when local switching, you can translate called and calling addresses. To construct the X.25 routing table, you must perform one of the following tasks in global configuration mode:
Entries have as a destination an interface (local switching) or IP address (remote switching). Up to six remote IP addresses can be entered as alternates routes that will be tried in turn to get a successful connection.
For an example of constructing the routing table, see the section "X.25 Routing Example" later in this chapter.
When interconnecting two separate X.25 networks, it is sometimes necessary to provide for address translation. Your X.25 switch supports translation of X.25 called and calling addresses for local switching. To translate addresses, perform the following task in global configuration mode:
Note that address substitution is only performed on routes to an interface. When running X.25 over IP, address substitution can be performed on the destination IP system if the destination system is configured with the appropriate X.25 routing commands.
Some X.25 calls, when forwarded by the X.25 switching support, need the calling (source) X.121 address updated to that of the outgoing interface. This is necessary when forwarding calls from private data networks to public data networks; other networks might require either address to be suppressed. To update or suppress the X.121 address, perform one of the following tasks in interface configuration mode:
You can configure an X.25 Permanent Virtual Circuit (PVC) in the X.25 switching software. This means that DTEs that require permanent circuits can be connected to a router acting as an X.25 switch and have a properly functioning connection. X.25 RESETs will be sent to indicate when the circuit comes up or goes down. Both interfaces must define complementary locally switched PVCs. To configure a local switched PVC, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure a switched PVC. | x25 pvc pvc-number1 interface interface-name pvc pvc-number2 [options] |
For an example of configuring a local switched PVC, see the section "Example of Switching a PVC on the Same Router" later in this chapter.
A PVC can be forwarded to another router over a LAN using the TCP/IP protocols. When the interfaces come up, a TCP stream connection is established to the router that is acting as the switch for the destination. All X.25 packets will be sent and received over this reliable data stream. Flow control is maintained from end-to-end. This is known as remote PVC switching or tunneling.
Running X.25 over TCP/IP provides a number of benefits. Other routers can switch IP datagrams containing the X.25 packets using the router's high-speed switching abilities. X.25 data can be sent over networks running only TCP/IP protocols. The TCP/IP protocol suite runs over many different networking technologies, including Ethernet, Token Ring, T1 serial, and FDDI. Thus X.25 data can be forwarded over these media to another of our routers where it can be output to an X.25 interface. Both interfaces must define complementary tunneled PVCs.
To connect two PVCs across a TCP/IP LAN, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Connect two PVCs across a TCP/IP LAN. | x25 pvc pvc-number1 tunnel IP-address interface serial string pvc pvc-number2 [options] |
For an example of configuring a remote tunneled PVC, see the section "Example of Establishing a Connection between Two PVCs" later in this chapter.
The Connection-Mode Network Service (CMNS) provides a mechanism through which local X.25 switching can be extended to nonserial media by using OSI-based NSAP addresses. This implementation runs packet-level X.25 over frame-level LLC2.
In addition, our CMNS implementation allows LAN-based OSI resources, such as a DTE host and a Sun workstation, to be interconnected to each other via the router's LAN interfaces and to a remote OSI-based DTE through a WAN interface (using, for example, an X.25 PSN).
All local mapping is performed by statically mapping MAC addresses and X.121 addresses to NSAP addresses.
Implementing CMNS routing involves the following basic steps:
See the following sections for more specific information about these tasks.
Enable CMNS on a nonserial interface by performing the following task in interface configuration mode:
| Task | Command |
|---|---|
| Enable CMNS. | cmns enable |
For an example of enabling CMNS on an interface, see the section "Example of Enabling CMNS for X.121 and MAC Addresses" later in this chapter.
After enabling CMNS on a nonserial interface (or specifying X.25 encapsulation on a serial interface), you must map NSAP addresses to either MAC-layer addresses or X.121 addresses, depending on the application.
For CMNS support over dedicated serial links (such as leased lines), an X.121 address is not needed, but can be optionally included. You must specify the X.121 address for CMNS connections over a packet-switched network, and you must specify a MAC address for CMNS connections over a nonserial media (Ethernet, FDDI, or Token Ring). Map the NSAP addresses to either a MAC address or X.121 address by performing one of the following tasks in interface configuration mode:
| Task | Command |
|---|---|
| Statically map NSAP addresses to either MAC-layer or X.121 addresses. | x25 map cmns NSAP MAC-address x25 map cmns NSAP [X.121-address] |
For an example of configuring a CMNS static map of addresses, see the section "Example of Enabling CMNS for X.121 and MAC Addresses" later in this chapter.
The DDN X.25 protocol has two versions: Basic Service and Standard Service. Our X.25 implementation only supports the Standard Service. DDN X.25 Standard Service requires that the X.25 data packets carry IP datagrams. The DDN Packet Switch Nodes (PSNs) can extract the IP packet from within the X.25 packet and pass data to another Standard Service host.
The DDN X.25 Standard is the required protocol for use with DDN PSNs. The Defense Communications Agency (DCA) has certified our DDN X.25 Standard implementation for attachment to the Defense Data Network. As part of the certification, our software is required to provide a scheme for dynamically mapping Internet addresses to X.121 addresses. See the section "DDN X.25 Dynamic Mapping" that follows for details on that scheme.
Complete the following tasks to enable DDN X.25 service:
Perform the following task, as necessary, to complete the configuration.
The DDN X.25 standard implementation includes a scheme for dynamically mapping all classes of Internet addresses to X.121 addresses without a table. This scheme requires that the Internet addresses conform to the formats shown in Table 1-3. These formats segment the Internet addresses into network (N), host (H), logical address (L), and PSN (P) portions. For the BFE encapsulation, the Internet address is segmented into Port (P), Domain (D), and BFE ID number (B). The DDN algorithm requires that the host value be less than 64.
The DDN conversion scheme uses the host and PSN portions of an Internet address to create the corresponding X.121 address. Strictly speaking, the DDN conversion mechanism is limited to
Class A Internet addresses. However, the router can convert Class B and Class C addresses as well. As indicated, this method uses the last two octets of a Class B address as the host and PSN identifiers, and the upper and lower four bits in the last octet of a Class C address as the host and PSN identifiers, respectively. The BFE conversion scheme requires a Class A IP address.
The DDN conversion scheme uses a physical address mapping if the host identifier is numerically less than 64. (This limit derives from the fact that a PSN cannot support more than 64 nodes.) If the host identifier is numerically larger than 64, the resulting X.121 address is called a logical address. The DDN does not use logical addresses.
The format of physical DDN X.25/X.121 addresses is ZZZZFIIIHHZZ(SS), where each character represents a digit. ZZZZ represents four zeros, F is zero to indicate a physical address, III represents the PSN octet from the Internet address padded with leading zeros, HH is the host octet from the Internet address padded with leading zeros, and ZZ represents two zeros. (SS) represents the optional and unused subaddress.
The physical and logical mappings of the DDN conversion scheme always generate a
12-digit X.121 address. Subaddresses are optional; when added to this scheme, the result is a
14-digit X.121 address. The DDN does not use subaddressing.
Packets using routing and other protocols that require broadcast support can successfully traverse X.25 networks, including the DDN. This traversal requires the use of network protocol-to-X.121 maps, because the router must know explicitly where to deliver broadcast datagrams. (X.25 does not support broadcasts.) You can mark network protocol-to-X.121 map entries to accept broadcast packets; the router then sends broadcast packets to hosts with marked entries. If you do not specify the address for an interface configured for DDN X.25, the router uses the DDN mapping technique to obtain the X.121 address of an interface.
Both DCE and DTE operation cause the router to specify the Standard Service facility in the Call Request packet, which notifies the PSNs to use Standard Service.
To enable DDN X.25, perform one of the following tasks in interface configuration mode, as appropriate for your network:
| Task | Command |
|---|---|
| Set DDN X.25 DTE operation. | encapsulation ddnx25 |
| Set DDN X.25 DCE operation. | encapsulation ddnx25-dce |
For an example of enabling DDN X.25, see the section "DDN X.25 Configuration Example" later in this chapter.
Using Standard Service, the DDN can be configured to provide separate service for datagrams with high precedence values. If the router receives an Internet packet with a nonzero Internet precedence field, it uses a different virtual circuit which requested the DDN-specified precedence mapping in the Call Request packet. Different virtual circuits are maintained based on the precedence mapping values and the number of virtual circuits permitted.
By default, the DDN X.25 software opens one virtual circuit for all types of service values. You can enable the precedence sensitivity feature by performing the following task in interface configuration mode:
| Task | Command |
|---|---|
| Allow a new virtual circuit based on the type of service (TOS) field. | x25 ip-precedence |
Some hosts send nonstandard data in the TOS field, thereby causing multiple, wasteful virtual circuits to be created.
For environments that require a high level of security, your router software supports the Defense Data Network Blacker Front-End Encryption (BFE) and Blacker Emergency Mode.
Blacker Emergency Mode allows your BFE device and your router to function in emergency situations. When the router is configured to participate in emergency mode and the BFE device is in emergency mode, the router sends address translation information to the BFE device to assist it in sending information.
Our implementation of Blacker Emergency Mode adheres to the specifications outlined in the DCA Blacker Interface Control document, published March 21, 1989.
Your BFE device is configured to be in one of three possible modes as follows:
Perform these tasks to configure Blacker Emergency Mode:
The following tables describe these tasks.
BFE encapsulation operates to map between Class A IP addresses and the X.121 addresses expected by the BFE encryption device. To set BFE encapsulation, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set BFE encapsulation on the router attached to a BFE device. | encapsulation bfex25 |
For an example of enabling BFE Emergency mode, see the section "Example of Setting BFE Emergency Mode" later in this chapter.
You must set up a table that provides the address translation information the router sends to the BFE when the BFE is in emergency mode. To do so, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set up the table that lists the BFE nodes (host or gateways) to which the router will send packets. | x25 remote-red host-ip-address remote-black Blacker-Internet- address |
For an example of configuring setting up the table, see the section "Example of Setting BFE Emergency Mode" later in this chapter.
You can define the circumstances under which the router participates in emergency mode and how it will participate in emergency mode. To do so, perform the following tasks in interface configuration mode:
For an example of configuring the circumstances under which the router participates in emergency mode, see the section "Example of Setting BFE Emergency Mode" later in this chapter.
To set the router to participate in emergency mode or to end participation in emergency mode when your system is so configured, perform the following task in EXEC mode:
| Task | Command |
|---|---|
| Set router to participate in emergency mode. | bfe {enter | leave} interface-type number |
For an example of configuring the router to participate in emergency mode, see the section "Example of Setting BFE Emergency Mode" later in this chapter. See the previous task for more information about configuring the router to participate in emergency mode.
The router software allows you to configure the standard Level 2 and Level 3 X.25 parameters and user facilities.
This section describes the X.25 parameters, user facilities, and special features you can configure. Which tasks you perform depends upon the structure of your network and the requirements of the service provider. These parameters must be adjusted to match the values used by the X.25 network. It is common for networks to require values different from the our defaults.
Perform the following tasks to configure the optional parameters, user facilities, and special features:
The following sections describe these tasks.
The X.25 protocol maintains multiple connections over one physical link between a DTE and a DCE. These connections are called virtual circuits (VCs) or logical channels (LCs). X.25 can maintain up to 4095 VCs numbered 1 through 4095. An individual VC is identified by giving its logical channel identifier (LCI) or virtual circuit number (VCN). Many documents use the terms VC and LC, and VCN, LCN, and LCI interchangeably. Each of these terms refer to the virtual circuit number.
An important part of X.25 operation is the range of virtual circuit numbers. Virtual circuit numbers are broken into four ranges (listed here in numerically increasing order):
The incoming-only, two-way, and outgoing-only ranges define the VC numbers over which a switched virtual circuit (SVC) can be established by placing an X.25 call, much like a telephone network establishes a switched voice circuit when a call is placed.
The rules about DCE and DTE devices initiating calls are as follows:
There is no difference in the operation of the SVCs except the restrictions on which a device can initiate a call. These ranges can be used to prevent one side from monopolizing the virtual circuits, which can be useful for X.25 interfaces with a small total number of SVCs available.
Six X.25 parameters define the upper and lower limit of each of the three SVC ranges. A permanent virtual circuit (PVC) must be assigned a number less than the numbers assigned to the SVC ranges. An SVC range is not allowed to overlap another range.
Table 1-4 lists the virtual circuit range types and the corresponding commands used to configure the ranges. Note that the values for these parameters must be the same on both ends of an X.25 link. For connection to a public data network (PDN), these values must be set to the values assigned by the network. An SVC range is unused if its lower and upper limits are set to 0; other than this use for marking unused ranges, VC 0 is not available.
To set the X.25 virtual circuit parameters, perform any of the following tasks in interface configuration mode:
For an example of configuring virtual circuit ranges, see the section "Example of Setting Virtual Circuit Ranges" later in this chapter.
The router can clear a datagram transport switched virtual circuit (SVC) after a set period of inactivity. Routed SVCs are not timed for inactivity. You can set this time by performing the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set the idle time before an SVC is cleared. | x25 idle minutes |
For an example of configuring the SVC idle timer, see the section "Typical X.25 Configuration Example" later in this chapter. See the section "Monitor and Maintain LAPB and X.25" later in this chapter for additional commands that clear virtual circuits.
For X.25 datagram transport, you can establish up to eight switched virtual circuits to a host for each protocol. Perform one or both of the following tasks in interface configuration mode to increase the number of VCs allowed:
For an example of increasing the number of virtual circuits allowed, see the sections "Typical X.25 Configuration Example" and "DDN X.25 Configuration Example" later in this chapter.
Upon receiving a Clear for an outstanding datagram transportCall Request, the X.25 support code immediately tries another Call Request if it has more traffic to send. This action can overrun some X.25 switches. You can define the number of minutes it takes to prevent calls from going to a previously failed destination by performing the following task in interface configuration mode (Incoming calls will still be accepted):
| Task | Command |
|---|---|
| Configure the ignore destination timer. | x25 hold-vc-timer minutes |
X.25 networks have a default input and output window size that is defined by the network administrator. You must set the router default input and output window sizes to match those of the network; see the note following the next section "Set Default Packet Sizes." To set the default window sizes, perform the following tasks in interface configuration mode:
For an example of setting the default window sizes, see the sections "Typical X.25 Configuration Example" and "DDN X.25 Configuration Example" later in this chapter.
X.25 networks have a default maximum input and output packet size that is defined by the network administrator. You must set the router default input and output maximum packet sizes to match those of the network by performing the following tasks in interface configuration mode:
| Task | Default | Command |
|---|---|---|
| Set the default input maximum packet size. | 128 | x25 ips bytes |
| Set the default output maximum packet size. | 128 | x25 ops bytes |
To send a packet larger than the agreed X.25 packet size over an X.25 virtual circuit, a router must break the packet into two or more X.25 packets with the M-bit ("more data" bit) set. The receiving device collects all packets in the M-bit sequence and reassembles them.
For an example of setting the default maximum packet sizes, see the sections "Typical X.25 Configuration Example" and "DDN X.25 Configuration Example" later in this chapter.
You can instruct the router to send an acknowledgment packet when it has received a threshold of data packets it has not acknowledged, instead of waiting until its input window is full. A value of 1 will send an acknowledgment for each data packet received if it cannot be acknowledged in an outgoing data packet. A value of 0 restores the default behavior of waiting until the input window is full. This approach improves line responsiveness at the expense of bandwidth. To establish the acknowledgment threshold, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Establish the threshold at which to acknowledge data packets. | x25 th delay-count |
The X.25 Level 3 retransmission timers determine how long the router must wait before retransmitting various packets. You can set these timers independently using the commands listed in Table 1-5. Each keyword requires a time value in seconds as its argument. The last column shows the default timer values, in seconds. Four of the timers apply to DTE devices, and the other four apply to DCE devices. To set the retransmission timers, perform any of the tasks listed in Table 7-5 in interface configuration mode.
For an example of setting the Retransmission timers, see the section "DDN X.25 Configuration Example" later in this chapter.
We support RFC 1144 TCP/IP header compression on serial lines using HDLC and X.25 encapsulation. The implementation of compressed TCP over X.25 uses one virtual circuit (VC) to pass the compressed packets. Any noncompressed packets are carried over separate IP encapsulations. To set X.25 TCP header compression, perform the following tasks in interface configuration mode:
| Task | Command |
|---|---|
| Implement header compression on IP packets. | ip tcp header-compression [passive] |
| Allow a separate VC for compressed packets. | x25 map compressedtcp IP-address X.121-address [options] |
To define symbolic host names, perform the following task in global configuration mode:
| Task | Command |
|---|---|
| Define a symbolic host name. | x25 host name X.121-address [cud call-user-data] |
To instruct the router how to interpret unknown Call User Data, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Interpret incoming calls with unknown Call User Data. | x25 default protocol |
The X.25 software provides commands to support the X.25 user facilities--options specified by the creators of the X.25 Recommendation that allow the network planner to implement features such as accounting, user identification, and flow control negotiation. The facilities configured by the x25 map commands are on a per-peer basis; all other facility commands specify the values sent for calls originated by the interface.
To set the supported X.25 user facilities, perform any of the following tasks in interface configuration mode:
Additionally, the D-bit is supported and passed through transparently. Both restricted and unrestricted fast select are also supported and are transparently handled by the software. No configuration is required for use of the D-bit or fast select facilities.
Our implementation of X.25 supports the extended packet sequence numbering. To use the extended packet sequence, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set the packet numbering modulo | x25 modulo modulus |
You can omit the calling (source) address in outgoing calls. This option is required for networks that expect only subaddresses in the calling address field. To suppress the calling address, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Omit the calling (source) X.121 address in Call Request packets. | x25 suppress-calling-address |
You can omit the called (destination) address in outgoing calls. This option is required for networks that expect only subaddresses in the called address field. To suppress the called address, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Omit the called (destination) X.121 address in Call Request packets. | x25 suppress-called-address |
By default, a packet-level Restart is performed when the link level is reset. This behavior can be disabled for networks that do not allow it, but disabling this behavior can cause anomalous packet layer behavior. To disable packet-level restarts, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Disable packet-level restarts. | no x25 linkrestart |
You can define the maximum number of packets that can be held while a virtual circuit is unable to send data. To define the size of the packet hold queue, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Define the sVC packet hold queue size. | x25 hold-queue queue-size |
You can supply alias X.121 addresses for an interface. This allows the interface to accept data encapsulation calls to an address other than the address defined by the x25 address command.To configure an alias, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Supply an alias for other X.121 addresses that will be accepted on the interface. | x25 route [# position] x121-pattern [cud pattern] alias interface number |
Out transparent bridging software supports bridging over X.25 VCs. To enable the X.25 bridging capability, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Define bridging of X.25 frames. | x25 map bridge x.121-address broadcast [options] |
To monitor and maintain X.25 and LAPB, perform any of the following tasks in EXEC mode:
Use the examples in this section to help you understand how to configure LAPB and X.25 for your network. The examples provided are as follows:
In the following example, the frame size (N1), window size (k), and maximum retransmission (N2) parameters retain their default values. The encapsulation subcommand sets DCE operation for IP packets only, and the lapb t1 subcommand sets the retransmission timer to 4,000 milliseconds (4 seconds) for a link with a long delay or slow connecting DTE device.
interface serial 3
encapsulation lapb-dce
lapb t1 4000
The following example shows the complete configuration for a serial interface connected to a commercial X.25 PDN for routing the IP protocol. The IP subnetwork address 131.108.9.0 has been assigned for the X.25 network.
interface serial 2
ip address 131.108.9.1 255.255.255.0
!
encapsulation X25
!
! The "bandwidth" command is not part of the X.25
! configuration; it's especially important to understand that it doesn't
! have any connection with the X.25 entity of the same name.
!"bandwidth" commands are used by IP routing processes (currently only IGRP),
! to determine which lines are the best choices for traffic.
! Since the default is 1544, and X.25 service at that rate isn't generally
! available, most X.25 interfaces that are being used with IGRP in a
! real environment will have "bandwidth" settings.
!
! This is a 9.6 Kbaud line:
!
bandwidth 10
!
! These Level 3 parameters are default flow control values; they need to
! match the PDN defaults. The values used by an SVC are negotiable on a per-call basis:
!
x25 win 7
x25 wout 7
x25 ips 512
x25 ops 512
!
! You must specify an X.121 address to be assigned to the X.25
! interface by the PDN.
!
x25 address 31370054065
!
! The following Level 3 parameters have been set to match the network.
! You generally need to change some Level 3 parameters, most often
! those listed below. You may not need to change any Level 2
! parameters, however.
!
x25 htc 32
x25 idle 5
x25 nvc 2
!
! The following commands configure the X.25 map. If you want to exchange
! routing updates with any of the routers, they would need
! "broadcast" flags.
! If the X.25 network is the only path to them, static routes are
! generally used to save on packet charges. If there is a redundant
! path, it might be desirable to run a dynamic routing protocol.
!
x25 map IP 131.108.9.3 31370019134 ACCEPT-REVERSE
! (ACCEPT-REVERSE allows collect calls)
x25 map IP 131.108.9.2 31370053087
!
! If the PDN cannot handle fast back-to-back frames, use the
!"transmitter-delay" command to slow down the interface:
!
transmitter-delay 1000
!
The following example sets the following VC ranges: 5 to 20 dedicated to incoming calls only (from the DCE to the DTE), 25 to 1024 for either incoming or outgoing calls, no VC range dedicated to outgoing calls (from the DTE to the DCE). Up to four permanent virtual circuits can be defined on VCs 1 through 4.
x25 lic 5
x25 hic 20
x25 ltc 25
In the following example, there is a PVC connected between two serial interfaces on the same router. In this type of interconnection configuration, the destination interface must be specified along with the PVC number on that interface. To make a working PVC connection, two commands must be specified, each pointing to the other.
interface serial 0
encapsulation x25
x25 ltc 5
!
x25 pvc 1 interface serial 1 pvc 4
interface serial 1
encapsulation x25
x25 ltc 5
x25 pvc 4 interface serial 0 pvc 1
The following example shows how to indicate that X.25 calls to addresses whose first four Data Network Identification Code (DNIC) digits are 1111 should be routed through interface serial 3, but that the DNIC field in the addresses presented to the equipment connected to that interface should be changed to 2222. The \1 in the rewrite pattern indicates the portion of the original address matched by the digits following the 1111 DNIC.
x25 route ^1111(.*) substitute-dest 2222\1 interface serial 3
The following example shows a more contrived command intended to illustrate the power of the rewriting scheme.
It causes all X.25 calls with 14-digit called addresses to be routed through interface serial 0. The incoming DNIC field would be moved to the end of the address. The fifth, sixth, ninth, and tenth digits would be deleted, and the thirteenth and fourteenth would be moved before the eleventh and twelfth.
The following examples illustrate how to enable an X.25 switch, how to enable call forwarding, and how to configure a router on a Tymnet/PAD switch to accept and forward calls.
This first example shows how to enable X.25 switching, as well as how to enter routes into the X.25 routing table:
!
! Enable X.25 forwarding
x25 routing
!
! Enter routes into the table. Without a positional parameter, entries
! are appended to the end of the table
x25 route ^100$ interface serial 0
x25 route 100 cud ^pad$ interface serial 2
x25 route 100 interface serial 1
x25 route ^3306 interface serial 3
x25 route .* ip 10.2.0.2
!
The routing table forwards calls for X.121 address 100 out interface serial 0. Otherwise, if the X.121 address contains 100 anywhere within it and contains no Call User Data, or the Call User Data is not the string pad, it is forwarded onto serial 1. If the X.121 address contains the digits 100 and the Call User Data is the string "pad", the call is forwarded onto serial 2. All X.121 addresses that do not match the first three routes are checked for a DNIC of 3306 as the first four digits. If they do match, they are forwarded over serial 3. All other X.121 addresses will match the fifth entry, which is a match-all pattern and will have a TCP connection established to the IP address 10.2.0.2. The router at 10.2.0.2 will then route the call according to its X.25 routing table.
This second example configures a router that sits on a Tymnet PAD/switch to accept calls and have them forwarded to a DEC VAX system. This feature permits running X.25 network over a generalized, existing IP network, thereby making it unnecessary to get another physical line for one protocol. The router positioned next to the DEC VAX system is configured with X.25 routes, as follows:
x25 route vax-x121-address interface serial 0
x25 route .* ip cisco-on-tymnet-ipaddress
This routes all calls to the DEC VAX X.121 address out to serial 0, where the VAX is connected running PSI. All other X.121 addresses are forwarded to the cisco-on-tymnet address using its IP address. This takes all outgoing calls from the VAX and sends them to cisco-on-tymnet for further processing.
On the router named cisco-on-tymnet, you would enter these commands:
x25 route vax-x121-address ip cisco-on-vax
x25 route .* interface serial 0
This forces all calls with the VAX X.121 address to be sent to the router with the VAX connected to it. All other calls with X.121 addresses are forwarded out to Tymnet. If Tymnet can route them, a CALL ACCEPTED packet is returned, and everything proceeds normally. If Tymnet cannot handle it, it clears the call, and the CLEAR REQUEST packet is forwarded back toward the VAX.
The following example, illustrated in Figure 1-2, demonstrates how to use the PVC to exchange IP traffic between Router X and Router Y.
interface serial 2
ip address 131.108.1.3 255.255.255.0
x25 map ip 131.108.1.4 0
x25 pvc 3 ip 131.108.1.4
interface serial 3
ip address 131.108.1.4 255.255.255.0
x25 map ip 131.108.1.3 0
x25 pvc 4 ip 131.108.1.3
In this example, the PDN has established a PVC through its network connecting PVC number 3 of access point A to PVC number 4 of access point B. On Router X a connection is established between Router X and Router Y's IP address, 131.108.1.4. On Router Y a connection is established between Router Y and Router X's IP address, 131.108.1.3.
In the following example, a connection is established between two PVCs across a LAN. Because the connection is remote (across the LAN), the tunneling command is used. This example establishes a PVC between Router X, Serial 0, PVC1 and Router Y, Serial 1, PVC 2. Figure 1-3 provides a visual representation of the configuration.
interface serial 0
x25 pvc 1 tunnel 131.108.1.2 interface serial 1 pvc 2
interface serial 1
x25 pvc 2 tunnel 131.108.1.1 interface serial 0 pvc 1
In Figure 1-4 the connection between points A and B is switched, and the connections between points A or B and C are tunneled.
interface ethernet 0
ip address 131.108.1.1 255.255.255.0
!
interface serial 0
x25 ltc 5
x25 pvc 1 interface serial 1 pvc 1
x25 pvc 2 tunnel 131.108.1.2 interface serial 0 pvc 1
!
interface serial 1
x25 ltc 5
x25 pvc 1 interface serial 0 pvc 1
x25 pvc 2 tunnel 131.108.1.2 interface serial 0 pvc 2
interface ethernet 0
ip address 131.108.1.2 255.255.255.0
!
interface serial 0
x25 ltc 5
x25 pvc 1 tunnel 131.108.1.1 interface serial 0 pvc 2
x25 pvc 2 tunnel 131.108.1.1 interface serial 1 pvc 2
The following examples illustrate enabling CMNS and configuring X.121 and MAC address mappings:
interface ethernet 0
cmns enable
x25 map cmns 38.8261.1000.0150.1000.17 0000.0c00.ff89
! Above maps NSAP to MAC-address on Ethernet0
!
interface serial 0
encapsulation x25
x25 map cmns 38.8261.1000.0150.1000.18 3110451
! Above maps NSAP to X.121-address on Serial0
! assuming the link is over a PDN
!
interface serial 1
encapsulation x25
x25 map cmns 38.8261.1000.0150.1000.20
! Above specifies cmns support for Serial1
! assuming that the link is over a leased line
The following example depicts switching CMNS over a packet-switched public data network (PDN). Figure 1-5 illustrates the general network topology for a CMNS switching application where calls are being made between resources on opposite sides of a remote link to Host A (on an Ethernet) and Host B (on a Token Ring), with a PDN providing the connection.
The following configuration listing allows resources on either side of the PDN to call
Host A or Host B. This configuration allows traffic intended for the remote NSAP address specified in the x25 map cmns commands (for the serial ports) to be switched through the serial interface for which CMNS is configured.
The CMNS-related configuration for Router C2 in Figure 1-5 would be as follows:
! This configuration specifies that any traffic from any other
! interface intended for any NSAP address with NSAP prefix 38.8261.17
! will be switched to MAC address 0800.4e02.1f9f
! through Token Ring 0
!
interface token 0
cmns enable
x25 map cmns 38.8261.17 0800.4e02.1f9f
!
! This configuration specifies that traffic from any other interface
! on Cisco Router C2 that is intended for any NSAP address with
! NSAP-prefix 38.8261.18 will be switched to
! X.121 address 2095551000 through Serial 0
!
interface serial 0
encapsulation x25
x25 address 4085551234
x25 map cmns 38.8261.18 2095551000
The CMNS-related configuration for Router C1 in Figure 1-5 would be as follows:
! This configuration specifies that any traffic from any other
! interface intended for any NSAP address with NSAP 38.8261.18
! will be switched to MAC address 0800.4e02.2abc through Ethernet 0
!
interface ethernet 0
cmns enable
x25 map cmns 38.8261.18 0800.4e02.2abc
!
! This configuration specifies that traffic from any other interface
! on Cisco Router C1 that is intended for any NSAP address with
! NSAP-prefix 38.8261.17 will be switched to X.121 address
! 4085551234 through Serial 1
!
interface serial 1
encapsulation x25
x25 address 2095551000
x25 map cmns 38.8261.17 4085551234
The following example illustrates switching CMNS over a leased line. Figure 1-6 illustrates the general network topology for a CMNS switching application where calls are being made by resources on the opposite sides of a remote link to Host C (on an Ethernet) and Host D (on a Token Ring), with a dedicated leased line providing the connection.
The following configuration listing allows resources on either side of the leased line to call Host C or Host D. This configuration allows traffic intended for the remote NSAP address specified in the x25 map cmns commands (for the serial ports) to be switched through the serial interface for which CMNS is configured.
A key difference for this configuration compared with the previous example is that with no PDN, the specification of an X.121 address in the x25 map cmns command is not necessary. The specification of an X.25 address also is not needed, but is included for symmetry with the previous example.
The CMNS-related configuration for Router C4 in Figure 1-6 would be as follows:
! This configuration specifies that any traffic from any other
! interface intended for any NSAP address with NSAP 38.8261.21
! will be switched to MAC address 0800.4e02.bcd0 through Token Ring 0
!
interface token 0
cmns enable
x25 map cmns 38.8261.21 0800.4e02.bcd0
!
! This configuration specifies that traffic from any other interface
! on Cisco Router C4 that is intended for any NSAP address with
! NSAP-prefix 38.8261.20 will be switched through Serial 0
!
interface serial 0
encapsulation x25
x25 address 00002
x25 map cmns 38.8261.20
The CMNS-related configuration for Router C3 in Figure 1-6 would be as follows:
! This configuration specifies that any traffic from any other
! interface intended for any NSAP address with NSAP 38.8261.20
¡ will be switched to MAC address 0800.4e02.123f through Ethernet 0
!
interface ethernet 0
cmns enable
x25 map cmns 38.8261.20 0800.4e02.123f
!
! This configuration specifies that traffic from any other interface
! on Cisco Router C3 that is intended for any NSAP address with
! NSAP-prefix 38.8261.21 will be switched through Serial 1
!
interface serial 1
encapsulation x25
x25 address 00001
x25 map cmns 38.8261.21
The following example illustrates how to configure a communication server interface to run
DDN X.25:
interface serial 0
ip address 192.31.7.50 255.255.255.240
encapsulation DDNX25
x25 win 6
x25 wout 6
x25 ips 1024
x25 ops 1024
x25 t20 10
x25 t21 10
x25 t22 10
x25 t23 10
x25 nvc 2
x25 map IP 192.31.7.49 000000010300 BROADCAST
In the following example, interface Serial 0 is configured to require an EXEC command from the administrator before it participates in emergency mode. The host IP address is 21.0.0.12, and the address of the remote BFE unit is 21.0.0.1. When the BFE enters emergency mode, the router will prompt the administrator for the EXEC command bfe enter to direct the router to participate in emergency mode.
interface serial 0
ip address 21.0.0.2 255.0.0.0
encapsulation bfex25
x25 bfe-emergency decision
x25 remote-red 21.0.0.12 remote-black 21.0.0.1
x25 bfe-decision ask
For ping commands to work in an X.25 environment (when load sharing over multiple serial lines), you must include entries for all adjacent interface IP addresses in the x25 map command for each serial interface. The following example illustrates this point.
Consider two routers, Router A and Router B, communicating with each other over two serial lines via an X.25 PDN (see Figure 1-7) or over leased lines. In either case, all serial lines must be configured for the same IP subnet address space. In order to allow for successful ping commands, the configuration might be as in the lists that follow. In any event, a similar configuration is required for the same subnet IP addresses to work across X.25.
!
interface serial 1
ip 131.108.170.1 255.255.255.0
x25 address 31370054068
x25 map ip 131.108.170.3 31370054065
x25 map ip 131.108.170.4 31370054065
!
interface serial 2
ip 131.108.170.2 255.255.255.0
x25 address 31370054069
x25 map ip 131.108.170.4 31370054067
x25 map ip 131.108.170.3 31370054067
! allow either source address
x25 31370054068 alias serial2
x25 31370054069 alias serial1
!
interface serial 0
ip 131.108.170.3 255.255.255.0
x25 address 31370054065
x25 map ip 131.108.170.1 31370054068
x25 map ip 131.108.170.2 31370054068
!
interface serial 3
ip 131.108.170.4 255.255.255.0
x25 address 31370054067
x25 map ip 131.108.170.2 31370054069
x25 map ip 131.108.170.1 31370054069
! allow either source address
x25 31370054065 alias serial3
x25 31370054067 alias serial0
When netbooting over X.25, you cannot netboot via a broadcast. You must netboot from a specific host. Also, an x25 map command must exist for the host that you netboot from.The x25 map command is used to map an IP address into an X.121 address. There must be an x25 map command that matches the IP address given on the boot system command line. The following is an example of such a configuration:
boot system gs3-bfx.83-2.0 131.108.13.111
!
interface Serial 1
ip address 131.108.126.200 255.255.255.0
encapsulation X25
x25 address 10004
x25 map IP 131.108.13.111 10002 broadcast
lapb n1 12040
clockrate 56000
In this case, 10002 is the X.121 address of the remote router that can get to host 131.108.13.111.
The remote router must have the following x25 map entry:
x25 map IP 131.108.126.200 10004 broadcast
This entry allows the remote router to return a boot image (from the netboot host) to the router netbooting over X.25.
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