X.25 is a legacy WAN communication protocol established by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) in the 1970s. Similar to Frame Relay, X.25 is designed to interconnect user devices and network devices across geographically dispersed areas. X.25 is designed to operate effectively over any transmission medium regardless of the quality of the line. The X.25 protocol has a built-in error recovery mechanism, which allows it to work over poor-quality transmission media.
This chapter discusses the functionalities of Annex G, a feature that allows X.25 protocol to be transported over a Frame Relay network. The Cisco IOS software supports the Annex G feature. Annex G allows X.25 data packets to be encapsulated in a Frame Relay frame for delivery over a Frame Relay network. This feature is useful for owners of legacy X.25 networks who want to migrate to newer technology, such as Frame Relay.
This chapter aims to provide an overview of the Annex G feature in the context of a Frame Relay environment. This chapter looks at the Annex G feature's special ability to allow service providers to migrate X.25 networks smoothly over to Frame Relay networks. This chapter introduces users to the basic configuration tasks and commands for configuring X.25 and the Annex G feature on a Cisco router. Readers will also find useful configuration examples of the Annex G feature used in a Frame Relay/X.25 network on Cisco routers.
An extensive study of the X.25 protocol is outside the scope of this book. It is assumed that the reader has at least some basic understanding of X.25 protocol and its terminologies. Please refer to Cisco Connection Online (CCO) for the full configuration guide on configuring X.25 protocol on Cisco devices (http://www.cisco.com/en/US/products/sw/iosswrel/ps1818/products_configuration_guide_chapter09186a0080087858.html).
The topics and questions that this chapter addresses include the following:
Understanding the use of the Annex G feature for Frame Relay/X.25 Interworking
Configuring basic X.25 on a Cisco router
Overview of X.121 Addressing
Configuring the Annex G feature on a Cisco router
Monitoring and maintaining the Annex G feature on a Cisco router
After completing this chapter, readers will recognize the performance issues commonly associated with the legacy X.25 protocols. The need for a feature to migrate X.25 networks to the newer technologies such as Frame Relay, Asynchronous Transfer Mode (ATM), and IP will become apparent. Readers will be able to understand the operation of the Annex G feature to allow X.25 packets to be carried over a Frame Relay network. Then readers will learn how to perform the basic configuration tasks for setting up X.25 on a Cisco router. Finally, readers will be able to configure the Annex G feature on a Cisco router with Cisco IOS configuration commands and to maintain and troubleshoot Annex G configuration using Cisco IOS show and debug commands.
The X.25 protocol has a great deal of operating overhead because it is designed to support legacy data networks with a high error rate. The high error rate on the legacy network can be because of physical interference or poor line condition. Therefore, in most X.25 implementations, X.25 can only support data rates up to 64 kbps.
Frame Relay is designed to operate over a reliable and clean digital transmission medium. Frame Relay does not support the error correction functions in the X.25 protocol, and therefore, Frame Relay does not incur the operating overhead of X.25. Frame Relay eliminates much of the X.25 overhead because Frame Relay only supports an error checking function in its headers. It leaves the error correction and flow control functions to upper-layer protocols such as TCP/IP. Besides the ability to increase performance, Frame Relay has the ability to dynamically allocate bandwidth. Frame Relay has become a very cost-effective solution with improved network performance and response time. To some degree, Frame Relay has superceded X.25.
The use of X.25 protocol on network backbones is fast becoming obsolete. Modern networks are implementing newer technologies such as Frame Relay, ATM, and IP on their backbones. Nonetheless, X.25 is ubiquitous; the banking industry and parts of Europe and Asia still rely on X.25 networks. For network operators and customers who have considerable investments in X.25 networks and equipment, a feature to protect these investments, by allowing a gradual migration to newer technologies, is considered necessary. The basic issue is to allow X.25 traffic to be transported over other newer technologies, such as Frame Relay, which is seeing widespread popularity.
Figure 12-1 shows a network diagram illustrating an X.25 network interworking with a Frame Relay backbone. In the figure, PAD refers to Packet Assembly and Disassembly. User sessions are carried across an X.25 network using the PAD protocols defined by the ITU-T Recommendations X.3 and X.29.
Annex G is a migration strategy for X.25 to Frame Relay. The Annex G feature facilitates the migration from an X.25 backbone to a Frame Relay backbone by allowing the encapsulation of CCITT X.25/X.75 traffic within a Frame Relay connection. Annex G was initially developed to accommodate the many Cisco customers in certain parts of Europe and Asia where IP is not as common. In these regions, X.25 technology continues to be a very popular protocol. Today, many service providers still offer X.25 services over their public data networks (PDNs). There is also an increasing need for service providers to offer newer services as an alternative to X.25, such as Frame Relay. With Annex G, the process of transporting X.25 over Frame Relay has been simplified, by allowing direct X.25 encapsulation over a Frame Relay network.
The X.25 over TCP (XOT) feature, defined in RFC 1613, allows X.25 packets to be transported over a TCP/IP network rather than over a Link Access Procedure, Balanced (LAPB) link.
Annex G uses direct encapsulation as opposed to XOT, which uses TCP encapsulations. Annex G offers greater interoperability among different vendors and is supported by many Frame Relay equipment manufacturers.
NOTE
XOT is an X.25 feature and is not be covered in this text. Information and configuration examples on XOT can be found at http://www.cisco.com/en/US/tech/tk713/tk730/technologies_configuration_example09186a0080093c0e.shtml.
The Annex G function, commonly known as Frame Relay/X.25 Interworking, describes a procedure for encapsulating X.25 traffic within a Frame Relay ANSI T1.618 frame. ANSI T1.618 describes the core aspects of Frame Relay protocol for use with Frame Relay Bearer Service.
NOTE
The details of the Annex G or Frame Relay/X.25 Interworking procedures can be found in both the ANSI T1.617 Annex G and the CCITT Recommendation I.555 documents. The ANSI public web site is found at http://www.ansi.org, and the ITU public web site is located at http://www.itu.int/home/index.html.
Figure 12-2 shows a model of the Annex G Frame Relay/X.25 Interworking function. Two X.25 data terminal equipment (DTE) routers are connected over Frame Relay permanent virtual circuit (PVC) in a Frame Relay network. In this diagram, the Annex G function is implemented on a X.25 DTE device and also on a X.25 data circuit-terminating equipment (DCE) device. The Annex G function can be implemented on both DTE and DCE X.25 devices. The Frame Relay/X.25 Interworking function encapsulates and de-encapsulates the X.25 traffic in a Frame Relay ANSI T1.618 frame. It also provides termination for the LAPB node-to-node link layer protocol used for X.25. With Annex G, the X.25 traffic is “tunneled” through a Frame Relay network.
LAPB is the data link layer protocol in X.25 protocol stack. It is responsible for ensuring reliable transport of data in an X.25 network. X.25PLP refers to the X.25 Packet Level Protocol, which is a network layer protocol responsible for network routing functions in the X.25 network.
Using Annex G, a stream of LAPB/X.25 packets is encapsulated and transmitted transparently over Frame Relay. The X.25 client data is encoded in an X.25 data packet in an LAPB frame. The LAPB frame and its payload are both encapsulated by the Frame Relay's DLCI header (2 to 4 bytes) and the 2 bytes of trailers that comprises of the cyclic redundancy check (CRC). The Annex G function can only support one upper-layer protocol (such as IP, IPX, and DECNET) for every single Frame Relay DLCI connection.
The Cisco implementation of the Annex G feature supports the Annex G specifications described in ANSI T1.617. It provides the following services:
Support of X.25 profiles
Multiple logical X.25 switched virtual circuits (SVCs) per Annex G link
X.25 switched and PAD services over Annex G
Modulo 8 or 128 packet sequence numbering
The Cisco Annex G feature uses X.25 profiles, which were created to streamline X.25 and LAPB configurations. X.25 profiles can contain existing X.25 and LAPB commands. Once created and named, X.25 profiles can be simultaneously associated with more than one DLCI connection by using the profile name.
The X.25 Layers 2 and 3 are transparently supported over Annex G. LAPB treats the Frame Relay network as an X.25 network link and passes all of the data and control messages over the Frame Relay network.
With modulo 8, the basic operating modulo is selected as the LAPB operating modulo. Modulo 128 specifies LAPB's extended mode.
When the Cisco Annex G feature is implemented in a Frame Relay/X.25 environment, it provides the following benefits and advantages to its users:
It allows transparent support of X.25 encapsulation over a Frame Relay network.
The X.25 profiles allow direct X.25 and LAPB configurations on a per-DLCI basis.
The X.25 profiles allow specification of X.25 and LAPB configurations without having to allocate hardware interface data block (IDB) information.
The X.25 profiles consist of bundled X.25 and LAPB commands, which eliminate the need to enter the same X.25 or LAPB commands for each DLCI configured.
It has low memory requirements; only the memory necessary to hold the X.25 or LAPB configuration data structure is required.
It offers multiple Annex G DLCIs per X.25 profile.
Both modulo 8 and 128 packet sequence numbering are supported.
Before you enable the Annex G feature on a Cisco router, note the following limitations and restrictions imposed on it. The following list outlines the restrictions that apply to Annex G configured on a Cisco router:
Annex G is supported only on Frame Relay physical interfaces. Frame Relay logical subinterfaces are not supported at this stage.
Annex G supports only Frame Relay PVC. Frame Relay SVC is not supported.
Each Frame Relay DLCI can be configured for only one Frame Relay service at a time. Hence, if the DLCI is using Annex G, it cannot be configured for another Frame Relay service.
Only X.25 SVC connections over Annex G are supported. X.25 PVC connections are not supported.
X.25 profiles do not support IP encapsulation.
Before X.25 Annex G can be configured on the serial interface of a Cisco router, the serial interface must be configured to process X.25 packets. The next section describes the basic configuration tasks required to enable X.25 on the serial interface of a Cisco router, including setting up X.25 encapsulation.
This section provides an overview of the configuration tasks and Cisco IOS commands required to enable X.25 processing on a serial interface of a Cisco router. This section also explains the configuration commands required to establish an X.25 PVC connection through an X.25 network. The Cisco IOS commands introduced in this section serve to familiarize readers with the basic X.25 configuration commands in the Cisco IOS software. Take note that you need at least a Cisco IP IOS image to support X.25 protocol on a Cisco router.
To enable X.25 processing on a serial interface, you are required to explicitly configure X.25 encapsulation on a serial interface. Enabling X.25 encapsulation on the serial interface overrides the default High-Level Data Link Control (HDLC) encapsulation. To enable X.25 encapsulation on a serial interface, use the encapsulation x25 interface configuration command. The encapsulation x25 command allows the user to optionally specify the mode of operation and the encapsulation type to use on the X.25 interface. Your X.25 service provider normally specifies the additional settings you need to configure on your serial interface. The default mode of operation is dte and default encapsulation used is ietf. Airline X.25 (AX25), Blacker Front End (BFE), and Defense Data Network (DDN) are among the supported encapsulation options. These options must be explicitly specified if they are required by your provider.
In the examples shown in this section, the default operation mode (dte) and encapsulation type (ietf) are used. Example 12-1 shows the Cisco IOS configuration commands as well as the configuration options for configuring X.25 on a serial interface.
NOTE
Take note that X.25 is a non-broadcast multi-access (NBMA) protocol similar to Frame Relay and ATM. As such, an X.25 interface is susceptible to the issues related to split-horizon when configuring dynamic routing protocols on a Cisco router.
Example 12-1. Configuring X.25 Encapsulation on a Serial Interface
Router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router(config)#interface serial4/0 Router(config-if)#encapsulation x25 ? ax25 Default to IATA's Airline X.25 bfe Blacker Front End attachment dce DCE operation ddn Defense Data Network attachment dte DTE operation ietf Default to IETF's RFC-1356 encapsulation profile Use a defined X.25 profile configuration <cr> Router(config-if)#encapsulation x25 dte ? ax25 Default to IATA's Airline X.25 bfe Blacker Front End attachment ddn Defense Data Network attachment ietf Default to IETF's RFC-1356 encapsulation <cr>
Example 12-2 shows the output of the show interface serial command on a Cisco router after encapsulation x25 is configured on the serial interface 4/0 from the previous example. Take note that the serial interface is set up as an X.25 DTE interface.
Example 12-2. Output of the show interface serial Command after encapsulation x25 Is Configured
Router#show interface serial4/0 Serial4/0 is up, line protocol is up Hardware is M4T MTU 1500 bytes, BW 2048 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation X25, crc 16, loopback not set X.25 DTE, address <none>, state R1, modulo 8, timer 0 Defaults: idle VC timeout 0 cisco encapsulation input/output window sizes 2/2, packet sizes 128/128 Timers: T20 180, T21 200, T22 180, T23 180 Channels: Incoming-only none, Two-way 1-1024, Outgoing-only none RESTARTs 0/0 CALLs 0+0/0+0/0+0 DIAGs 0/0 LAPB DTE, state CONNECT, modulo 8, k 7, N1 12056, N2 20 T1 3000, T2 0, interface outage (partial T3) 0, T4 0 VS 1, VR 1, tx NR 1, Remote VR 1, Retransmissions 0 Queues: U/S frames 0, I frames 0, unack. 0, reTx 0 IFRAMEs 1/1 RNRs 0/0 REJs 0/0 SABM/Es 1/0 FRMRs 0/0 DISCs 0/0 Last input 00:34:22, output 00:19:12, output hang never Last clearing of "show interface" counters 00:19:12 Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 3 packets input, 11 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 2 packets output, 7 bytes, 0 underruns 0 output errors, 0 collisions, 1 interface resets 0 output buffer failures, 0 output buffers swapped out 1 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up
The next step requires you to specify the range of virtual circuit numbers (VCNs) for X.25 operation. In X.25, multiple connections are maintained over a physical link between the DTE and DCE devices. These connections are known as virtual circuits or logical channels. Up to 4095 logical channels can be maintained by a X.25 network. The logical channel identifier (LCI) or VCN is used to identify individual circuits. In addition, X.25 defines a range of VCNs that are an important part of X.25 operation. There are four ranges used as follows:
PVC
Incoming-only circuits
Two-way circuits
Outgoing-only circuits
The definition and use of the PVC is obvious. The incoming-only, two-way, and outgoing-only ranges define the VCNs over which an SVC can be established. There are six X.25 commands that can be used to define the upper and lower limit of each of the three SVC ranges, as shown in Table 12-1.
Table 12-1. Setting the Parameters for an X.25 SVC Connection
Description | Command | Default Value |
|---|---|---|
Set the lowest incoming-only circuit number. | X25 lic limit | 0 |
Set the highest incoming-only circuit number. | X25 hic limit | 0 |
Set the lowest two-way circuit number. | X25 ltc limit | 1 |
Set the highest two-way circuit number. | X25 htc limit | 1024 (serial) |
Set the lowest outgoing-only circuit number. | X25 loc limit | 0 |
Set the highest outgoing-only circuit number. | X25 hoc limit | 0 |
For PVC, the circuit number assigned to it must be lower than the number assigned to a SVC circuit. The circuit numbers must match on both ends of the X.25 connection.
When connecting a router to your service provider's X.25 network, you are required to set up the X.121 addressing on the interface. X.121 is an ITU-T recommendation for representing the numbering plans for International Data Numbers (IDNs). The numbering plan, or Data Network Identification Codes (DNICs), allows for a number of PDNs in a country, the identification of a country, and also a specific PDN within the country. A DNIC is comprised of four digits, whereby the first three digits represent the International Data Country Codes (DCCs) and the fourth digit may indicate up to 10 different networks within the country. For example, the list of numbers from 310 to 316 represents the X.121 Data Country Codes (DCC) for the United States. Singapore is represented by the DCC of 525.
Both X.25 point-to-point and multipoint subinterfaces are supported on Cisco routers. Point-to-point subinterfaces only accept a single encapsulation command for a given protocol. Only one destination host for the protocol can be specified for a point-to-point subinterface. For physical interfaces or multipoint subinterfaces, one or more destination hosts can be configured. There is no restriction on the number of encapsulation commands that can be configured on a multipoint subinterface. When a routing process forwards a packet to a multipoint interface, the X.25 encapsulation process must be able to map the packet's destination address to a configured encapsulation command. Otherwise, the encapsulation fails and the packet is discarded.
After the interface has been configured, use the x25 pvc interface configuration command to establish an encapsulation PVC for the interface. Using the x25 pvc command is very similar to using the frame-relay interface-dlci command on a Frame Relay point-to-point subinterface. You can use the x25 map command to perform network layer address to local X.121 address mapping. Repeat the command to perform the mapping for each remote destination host. The x25 map is similar to the frame-relay map command on Frame Relay multipoint interfaces.
On X.25 serial interfaces, split horizon is enabled by default. This is contrary to Frame Relay, where split horizon is automatically disabled on the physical interfaces. Take note of this issue when configuring routing protocols on an X.25 network over a hub-and-spoke topology.
The basic configuration tasks for configuring X.25 on an interface are summed up in the following list. X.25 configuration tasks should be performed as listed below:
In the interface configuration mode, configure the serial interface for X.25 mode of operation and encapsulation with the encapsulation x25 [dte | dce] [[ddn | bfe] | [ietf]] command.
Set up the virtual circuit ranges using the commands listed in Table 12-1. The VCN you use should be assigned to you by your service provider.
Set up the X.121 addressing for your X.25 circuit on your X.25 serial interface. The X.121 address numbering used is dependent on your country region, and your X.25 service provider should assign it to you. Use the x25 address x.121-address interface configuration command to set up the assigned X.121 address.
If PVC is used, establish the PVC using the x25 pvc circuit protocol address x.121-address [option] command. Otherwise, set up the x.25 address mapping on the interface using the x25 map protocol address x.121-address [option] command. For both commands, multiple protocols and addresses can be specified in a single command.
The basic X.25 network illustrated in Figure 12-3 is used to demonstrate the X.25 configuration examples in this section. In Figure 12-3, for simplicity, two Cisco routers are connected with a back-to-back serial cable without connecting to any X.25 switch. Router R2, which is connected to the DCE end of the serial cable, acts as the X.25 DCE device. Its serial interface is configured with the clock rate command to supply clocking signals to the far end DTE. In addition, Figure 12-3 demonstrates that a Cisco router can be set up as a X.25 DCE device to perform similar functions as a traditional X.25 switch.
The configuration examples of R1 and R2 are shown in Example 12-3.
Example 12-3. Configuration Examples for Basic X.25
! R1 <text omitted> ip routing ! interface Serial0 ip address 172.16.1.1 255.255.255.0 encapsulation x25 x25 address 123456 x25 ltc 3 x25 pvc 1 ip 172.16.1.2 654321 ! R2 <test omitted> ip routing ! interface Serial0 ip address 172.16.1.2 255.255.255.0 encapsulation x25 dce x25 address 654321 x25 ltc 3 x25 pvc 1 ip 172.16.1.1 123456 clockrate 64000
The show interface command can be used to verify the status and LAPB state of the interface. Example 12-4 demonstrates the sample outputs at both R1 and R2.
Example 12-4. Output of show interface Command at R1 and R2
R1#show interface serial0
Serial0 is up, line protocol is up
Hardware is cxBus Serial
Internet address is 172.16.1.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation X25, crc 16, loopback not set
Restart-Delay is 0 secs
X.25 DTE, address 123456, state R1, modulo 8, timer 0
Defaults: idle VC timeout 0
cisco encapsulation
input/output window sizes 2/2, packet sizes 128/128
Timers: T20 180, T21 200, T22 180, T23 180
Channels: Incoming-only none, Two-way 3-1024, Outgoing-only none
RESTARTs 1/0 CALLs 0+0/0+0/0+0 DIAGs 0/0
LAPB DTE, state CONNECT, modulo 8, k 7, N1 12056, N2 20
T1 3000, T2 0, interface outage (partial T3) 0, T4 0
VS 6, VR 6, tx NR 6, Remote VR 6, Retransmissions 0
Queues: U/S frames 0, I frames 0, unack. 0, reTx 0
IFRAMEs 6/6 RNRs 0/0 REJs 0/0 SABM/Es 1/1 FRMRs 0/0 DISCs 0/0
Last input 00:03:04, output 00:03:04, output hang never
Last clearing of "show interface" counters 00:05:17
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
10 packets input, 540 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
9 packets output, 538 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 output buffer failures, 0 output buffers swapped out
1 carrier transitions
RTS up, CTS up, DTR up, DCD up, DSR up
R2#show interface serial0
Serial0 is up, line protocol is up
Hardware is M8T-V.35
Internet address is 172.16.1.2/24
MTU 1500 bytes, BW 2048 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation X25, crc 16, loopback not set
X.25 DCE, address 654321, state R1, modulo 8, timer 0
Defaults: idle VC timeout 0
cisco encapsulation
input/output window sizes 2/2, packet sizes 128/128
Timers: T10 60, T11 180, T12 60, T13 60
Channels: Incoming-only none, Two-way 3-1024, Outgoing-only none
RESTARTs 1/0 CALLs 0+0/0+0/0+0 DIAGs 0/0
LAPB DCE, state CONNECT, modulo 8, k 7, N1 12056, N2 20
T1 3000, T2 0, interface outage (partial T3) 0, T4 0
VS 6, VR 6, tx NR 6, Remote VR 6, Retransmissions 0
Queues: U/S frames 0, I frames 0, unack. 0, reTx 0
IFRAMEs 6/6 RNRs 0/0 REJs 0/0 SABM/Es 1/1 FRMRs 0/0 DISCs 0/0
Last input 00:05:09, output 00:05:09, output hang never
Last clearing of "show interface" counters 00:05:54
Queueing strategy: fifo
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
9 packets input, 538 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
10 packets output, 540 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets
0 output buffer failures, 0 output buffers swapped out
1 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up
The show x25 map command, similar to the show frame-relay map command, can be used to observe the Layer 3 to X.121 addressing mapping on the local router. Example 12-5 shows the output of the show x25 map command at R1 and R2.
Example 12-5. Output of show x25 map Command at R1 and R2
R1#show x25 map Serial0: X.121 654321 <-> ip 172.16.1.2 PVC, 1 VC: 1/P R2#show x25 map Serial0: X.121 654321 <-> ip 172.16.1.2 PVC, 1 VC: 1/P
An extended ping from R1 to the network layer address 172.16.1.2/24 at R2 is performed. Example 12-6 shows the outcome.
Example 12-6. Performing a Ping from R1 to R2 over the X.25 Connection
R1#ping Protocol [ip]: Target IP address: 172.16.1.2 Repeat count [5]: 20 Datagram size [100]: Timeout in seconds [2]: Extended commands [n]: Sweep range of sizes [n]: Type escape sequence to abort. Sending 20, 100-byte ICMP Echos to 172.16.1.2, timeout is 2 seconds: !!!!!!!!!!!!!!!!!!!! Success rate is 100 percent (20/20), round-trip min/avg/max = 28/30/32 ms
Take note that the X.25 configuration commands discussed so far are only the minimum configurations required for setting up a basic X.25 circuit. There are many more X.25 configuration options and features available that are not covered here. You can make use of the Cisco Feature Navigator tool on the CCO (login is required) to find out other useful X.25 features for your network.
NOTE
The Cisco Annex G feature is released in Cisco IOS Release 12.0(3)T or later. The Cisco IOS T-train Release introduces new features into the Cisco IOS software and also provides fixes to defects. It is supported on major Cisco router platforms, including Cisco 1600, 2600, 3600, 7200, and 7500 series.
The next section presents a case study on the use of the Annex G feature. The scenario in the case study illustrates the use of the Annex G feature to transport the traffic from two private X.25 networks across a Frame Relay network.
This chapter presented a basic introduction to the X.25 protocol. X.25 is a popular WAN technology that is slowly becoming obsolete as a result of the widespread adoption of newer technologies, such as Frame Relay, ATM, and IP. Nevertheless, X.25 is not completely gone. It is ubiquitous in some industries and certain parts of the world today. For this reason, a feature to allow existing X.25 backbones to migrate to the newer technologies is needed.
The Cisco IOS software supports the Frame Relay Annex G feature, which allows X.25 backbone to Frame Relay migration by encapsulating X.25 traffic in a Frame Relay connection. The Annex G feature is very useful to service provider customers who have invested considerably in X.25 equipment and infrastructure but would like to migrate to newer technologies, such as Frame Relay. The Annex G feature facilitates a smooth migration strategy toward Frame Relay.
This chapter covered the basic configuration tasks required to enable X.25 on a Cisco router. The chapter also explained the operations of the Annex G feature and presented the configuration tasks for configuring Annex G on Cisco routers. The end of this chapter presents a case study to demonstrate the Cisco IOS show and debug commands for monitoring and maintaining Annex G configurations on a Cisco router. The next chapter covers the Cisco Frame Relay Enhanced LMI feature.
Annex G: ANSI T1.617a documentation (1993): http://www.ansi.org
Cisco IOS 12.0T WAN Configuration Guide: http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t3/x25anxg.htm
This section presents a case study on the use of the Annex G feature. This section also covers the configuration tasks and commands required to set up Annex G on Cisco routers. Refer to the network diagram in Figure 12-4 as the reference for the configuration examples presented in this case study. The topology depicted in Figure 12-4 allows an X.25 customer to make use of the higher-speed Frame Relay for transit.
NOTE
The Cisco IGX 8400 switch is used for provisioning the Frame Relay PVCs in this Frame Relay network. Alternatively, a Cisco router supporting the Frame Relay switching feature could be used to provision the PVCs.
Before moving to the configuration tasks for Annex G, first verify the status of the Frame Relay PVCs provisioned at the FR_R1 and FR_R2 routers. Example 12-7 indicates the status of the show frame-relay pvc command at both FR_R1 and FR_R2.
Example 12-7. Verify that the Frame Relay Circuits Are Properly Provisioned Before Configuring Annex G
! FR_R1: FR_R1#show frame-relay pvc PVC Statistics for interface Serial3/2 (Frame Relay DTE) Active Inactive Deleted Static Local 0 0 0 0 Switched 0 0 0 0 Unused 1 0 0 0 DLCI = 100, DLCI USAGE = UNUSED, PVC STATUS = ACTIVE, INTERFACE = Serial3/2 input pkts 0 output pkts 0 in bytes 0 out bytes 0 dropped pkts 0 in pkts dropped 0 out pkts dropped 0 out bytes dropped 0 in FECN pkts 0 in BECN pkts 0 out FECN pkts 0 out BECN pkts 0 in DE pkts 0 out DE pkts 0 out bcast pkts 0 out bcast bytes 0 switched pkts 0 Detailed packet drop counters: no out intf 0 out intf down 0 no out PVC 0 in PVC down 0 out PVC down 0 pkt too big 0 shaping Q full 0 pkt above DE 0 policing drop 0 pvc create time 00:00:23, last time pvc status changed 00:00:03 ! FR_R2: FR_R2#show frame-relay pvc PVC Statistics for interface Serial3/2 (Frame Relay DTE) Active Inactive Deleted Static Local 0 0 0 0 Switched 0 0 0 0 Unused 1 0 0 0 DLCI = 100, DLCI USAGE = UNUSED, PVC STATUS = ACTIVE, INTERFACE = Serial3/2 input pkts 0 output pkts 0 in bytes 0 out bytes 0 dropped pkts 0 in pkts dropped 0 out pkts dropped 0 out bytes dropped 0 in FECN pkts 0 in BECN pkts 0 out FECN pkts 0 out BECN pkts 0 in DE pkts 0 out DE pkts 0 out bcast pkts 0 out bcast bytes 0 switched pkts 0 Detailed packet drop counters: no out intf 0 out intf down 0 no out PVC 0 in PVC down 0 out PVC down 0 pkt too big 0 shaping Q full 0 pkt above DE 0 policing drop 0 pvc create time 00:00:23, last time pvc status changed 00:00:03
The Annex G feature is configured on both FR_R1 and FR_R2. X25_R1 and X25_R2 are strictly X.25 routers unaware of the “X.25 tunnel” configured through the Frame Relay network. To configure Annex G on the Frame Relay routers, you need to first set up X.25 profiles using the x25 profile profile-name global configuration command.
X.25 profiles allow specification of an X.25 configuration and LAPB configuration on an interface. Under the X.25 profile subconfiguration command, you can set up the X.25-specific configurations within. Example 12-8 lists the complete X.25 configuration commands available within the X.25 profile.
Example 12-8. X.25 Configuration Options Inside an X.25 Profile
FR_R1(config)#x25 profile Annex_G FR_R1(config-x25)#x25 ? accept-reverse Accept all reverse charged calls address Set interface X.121 address alias Define an alias address pattern aodi Enable AODI (Always On/Direct ISDN) Service default Set protocol for calls with unknown Call User Data facility Set explicit facilities for originated calls fail-over Set fail-over interface and delay time hic Set highest incoming channel hoc Set highest outgoing channel hold-queue Set limit on packets queued per circuit hold-vc-timer Set time to prevent calls to a failed destination htc Set highest two-way channel idle Set inactivity time before clearing SVC ip-precedence Open one virtual circuit for each IP TOS ips Set default maximum input packet size lic Set lowest incoming channel linkrestart Restart when LAPB resets loc Set lowest outgoing channel ltc Set lowest two-way channel map Map protocol addresses to X.121 address modulo Set operating standard nonzero-dte-cause Allow non-zero DTE cause codes nvc Set maximum VCs simultaneously open to one host per protocol ops Set default maximum output packet size subscribe Subscribe to a supported behavior suppress-called-address Omit destination address in outgoing calls suppress-calling-address Omit source address in outgoing calls t20 Set DTE Restart Request retransmission timer t21 Set DTE Call Request retransmission timer t22 Set DTE Reset Request retransmission timer t23 Set DTE Clear Request retransmission timer threshold Set packet count acknowledgement threshold use-source-address Use local source address for forwarded calls win Set default input window (maximum unacknowledged packets) wout Set default output window (maximum unacknowledged packets)
The frame-relay interface-dlci command or the encapsulation x25 command can reference a configured x.25 profile. Multiple Annex G Frame Relay DLCIs can also use the same profile. A Frame Relay DLCI configured for Annex G is logically equivalent to a single X.25/LAPB interface. Annex G service is only supported on the Frame Relay main interface.
After this step is done, the x25 route command must be used to switch X.25 traffic from the inside X.25 network over Annex G. Both the X.25 serial interface and the Frame Relay DLCI number must be specified.
The configuration tasks for configuring Annex G is summed up in the list below. The configurations are applied to each router connected between the X.25 and Frame relay network (FR_R1 and FR_R2 in Figure 12-4).
Set up the X.25 profile for Annex G using the global x25 profile profile-name [dte|dce|dxe] configuration command. The default mode is DTE.
Activate Frame Relay encapsulation on the interface to be used for Annex G. This interface should be a Frame Relay main interface connected to a Frame Relay switch/network.
Under the main interface, configure the Frame Relay DLCI with the frame-relay interface-dlci command.
Under the Frame Relay interface DLCI configuration mode, assign the X.25 profile configured in Step 1 to the DLCI using the x25-profile profile-name command.
Enable X.25 routing of outgoing calls with x25 routing command. This is required for Step 6.
Use the x25 route number interface serial-interface dlci number command to assign an X.25 route for the DLCI on that interface. This is needed if the router is to accept switched calls and to place outgoing calls.
The running configurations for the routers in Figure 12-4 are shown in Example 12-9. Note that some text output from the complete configuration file is omitted, and only the configurations related specifically to Annex G are shown.
Example 12-9. Running Configurations of X25_R1, FR_R1, FR_R2, and X25_R2 in Figure 12-4
! X25_R1: <text omitted> ! ip routing ! interface Serial1 ip address 172.16.1.1 255.255.255.0 encapsulation x25 x25 address 123456 x25 ltc 128 x25 nvc 4 x25 map ip 172.16.1.2 654321 broadcast ! FR_R1: <text omitted> ! x25 profile FR_ANNEX_G dte x25 accept-reverse x25 routing ! interface Serial3/0 no ip address encapsulation x25 dce x25 ltc 128 x25 nvc 4 clock rate 64000 ! interface Serial3/2 no ip address encapsulation frame-relay no fair-queue frame-relay interface-dlci 100 x25-profile FR_ANNEX_G ! x25 route 123456 interface Serial3/0 x25 route 654321 interface Serial3/2 dlci 100 ! FR_R2: <text omitted> ! x25 profile FR_ANNEX_G dce x25 accept-reverse x25 routing ! interface Serial1/0 no ip address encapsulation x25 dce x25 nvc 4 clockrate 64000 ! interface Serial1/3 no ip address encapsulation frame-relay frame-relay interface-dlci 200 x25-profile FR_ANNEX_G ! x25 route 654321 interface Serial1/0 x25 route 123456 interface Serial1/3 dlci 200 ! X25_R2: <text omitted> ip routing ! interface Serial3 ip address 172.16.1.2 255.255.255.0 encapsulation x25 x25 address 654321 x25 nvc 4 x25 map ip 172.16.1.1 123456 broadcast
Take note that one of the X25 profiles configured on FR_R1 and FR_R2 is in the DTE mode, whereas the other is set to DCE mode. The configurations at both Annex G routers must be compatible, otherwise the Annex G circuit will fail to function properly.
The show x25 interface serial/number dlci dlci_number privileged EXEC command can be used to observe the operating configuration status of a particular Annex G connection, as shown in Example 12-10.
Example 12-10. Using the show x25 interface serial/number dlci dlci_number Command
FR_R1#show x25 interface s3/2 dlci 100 SVC 1, State: D1, Interface: Se3/2 DLCI 100 Started 00:06:08, last input 00:06:08, output 00:06:08 Connects 654321 <--> 123456 to Serial3/0 SVC 128 Window size input: 2, output: 2 Packet size input: 128, output: 128 PS: 5 PR: 5 ACK: 5 Remote PR: 4 RCNT: 0 RNR: no P/D state timeouts: 0 timer (secs): 0 data bytes 500/500 packets 5/5 Resets 0/0 RNRs 0/0 REJs 0/0 INTs 0/0
The state is D1, which indicates Flow Control Ready. Table 12-2 lists the other X.25 states that may be encountered.
Table 12-2. X.25 Connection State Definitions
State | Definition |
|---|---|
D1 | Flow control ready |
D2 | DTE reset request |
D3 | DCE reset indication |
P1 | Idle |
P2 | DTE waiting for DCE to connect call |
P3 | DCE waiting for DTE to accept call |
P4 | Data transfer |
P5 | Call collision |
P6 | DTE clear request |
P7 | DCE clear indication |
R1 | Packet level ready |
R2 | DTE restart request |
R3 | DCE restart indication |
X1 | Nonstandard state for a virtual circuit in hold-down |
The show x25 profile command can also be used to verify the parameters of the X25 profiles configured on the local router. Example 12-11 shows an example of the use of the command.
Example 12-11. Output of show x25 profile Command
FR_R1#show x25 profile
X.25 profile name: FR_ANNEX_G
Number of references: 1
In use by:
Annex G: Serial3/2 DLCI 100
PROFILE DTE, address <none>, state R/Inactive, modulo 8, timer 0
Defaults: idle VC timeout 0
input/output window sizes 2/2, packet sizes 128/128
Timers: T20 180, T21 200, T22 180, T23 180
Channels: Incoming-only none, Two-way 1-1024, Outgoing-only none
LAPB DTE, modulo 8, k 7, N1 default, N2 20
T1 3000, T2 0, interface outage (partial T3) 0, T4 0
The output of the command shows the number of Annex G DLCIs that are currently referencing this particular X25 profile. More than one Frame Relay Annex G DLCI can use an X25 profile. Next, the show x25 vc command can be used to display the information about an X.25 VC connected to an Annex G service. An example is given in Example 12-12.
Example 12-12. Output of show x25 vc Command
FR_R1#show x25 vc SVC 128, State: D1, Interface: Serial3/0 Started 00:18:40, last input 00:18:39, output 00:18:39 Connects 654321 <--> 123456 from Serial3/2 DLCI 100 Window size input: 2, output: 2 Packet size input: 128, output: 128 PS: 5 PR: 5 ACK: 4 Remote PR: 5 RCNT: 1 RNR: no P/D state timeouts: 0 timer (secs): 0 data bytes 500/500 packets 5/5 Resets 0/0 RNRs 0/0 REJs 0/0 INTs 0/0 SVC 1, State: D1, Interface: Se3/2 DLCI 100 Started 00:18:40, last input 00:18:39, output 00:18:39 Connects 654321 <--> 123456 to Serial3/0 SVC 128 Window size input: 2, output: 2 Packet size input: 128, output: 128 PS: 5 PR: 5 ACK: 5 Remote PR: 4 RCNT: 0 RNR: no P/D state timeouts: 0 timer (secs): 0 data bytes 500/500 packets 5/5 Resets 0/0 RNRs 0/0 REJs 0/0 INTs 0/0
Finally, verify the connectivity provided by the Annex G service by performing an extended ping from X25_R1 to X25_R2. The outcome of the ping test is shown in Example 12-13.
Example 12-13. Performing a Ping from X25_R1 to X25_R2 over the Annex G Connection
X25_R1#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.1.0 is directly connected, Serial1
X25_R1#ping
Protocol [ip]:
Target IP address: 172.16.1.2
Repeat count [5]: 20
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 20, 100-byte ICMP Echos to 172.16.1.2, timeout is 2 seconds:
!!!!!!!!!!!!!!!!!!!!
Success rate is 100 percent (20/20), round-trip min/avg/max = 116/118/120 ms
This example verifies proper operation of the Annex G connection. X.25 data packets from X25_R1 are transported transparently over the Frame Relay network to the remote X.25 destination, X25_R2. Using Annex G, the local X25 host has no knowledge that the connection service is provided by a Frame Relay network.
Two IOS commands are available for troubleshooting the Annex G connection on a Cisco router. The clear x25 serial [number] [dlci_number] command allows an Annex G DLCI link to be restarted. The user can specify to either clear all X.25 circuits on that link or to clear a specific X.25 logical circuit number. The example that follows clears all the X.25 connections configured on serial 3/0.
FR_R1#clear x25 serial 3/0 Force Restart [confirm]
The debug x25 annexg command can be used to display relevant X.25 debugging information for a Frame Relay Annex G connection. Example 12-14 shows a sample output observed on FR_R1 after the command was enabled and X.25 data packets were received from X25_R1, routed to X25_R2.
Example 12-14. Sample Output of debug x25 annexg Command
FR_R1#debug x25 annexg X.25 over FR (Annex-G) debugging is on 21:25:06: annexg_restart_tx: sending pak to Serial3/2 21:25:06: annexg_restart_tx: sending pak to Serial3/2 21:25:06: annexg_restart_tx: sending pak to Serial3/2 21:25:06: annexg_restart_tx: sending pak to Serial3/2 21:25:06: annexg_restart_tx: sending pak to Serial3/2 21:25:06: annexg_restart_tx: sending pak to Serial3/2 21:25:06: annexg_restart_tx: sending pak to Serial3/2 21:25:07: annexg_restart_tx: sending pak to Serial3/2 21:25:07: annexg_restart_tx: sending pak to Serial3/2 21:25:07: annexg_restart_tx: sending pak to Serial3/2
The debug messages indicate that X.25 packets are rerouted out to the Frame Relay Annex G connection on Serial 3/2.