7.2 Configuring X.25
7.2.3 Configuring X.25 SVCs
To activate X.25 on an interface, you must enter the encapsulation x25 command to specify the encapsulation style to be used on the serial interface:

Router(config-if)#encapsulation x25 [dte | dce]

The router can be an X.25 DTE device, which is typically used when the X.25 PDN is used to transport various protocols. The router can also be configured as an X.25 DCE device, which is typically used when the router acts as an X.25 switch.

Configuring X.25 SVC Examples

Figure shows a sample network to be configured for X.25.

Example shows the central-site SVC configuration for the network in Figure .

Example shows the branch office SVC configuration for the network in Figure .

Table describes the commands that are used in this example.

If you change the interface configuration from an X.25 DCE to X.25 DTE or vice versa, the interface configuration will change to the default interface settings.

IP routing on CENTRAL SITE forwards datagrams destined for subnet 10.60.8.0 to interface serial 1. The interface map identifies the destination to the X.25 cloud. In this typical configuration, CENTRAL SIGHT tries to establish an SVC to BRANCH OFFICE using its X.121 source address and a destination X.121 address of 311082191234 when it sends packets to 10.60.8.2.

Upon receipt of the setup request, Cisco B identifies the remote IP address from the source X.121 address and accepts the connection. After the SVC is connected, each router uses it as a point-to-point data link for the identified destination.

The two X.25 attachments need complementary map configurations to establish the VC that will encapsulate IP datagrams.

Example illustrates the use of the broadcast option on the x25 map command. It is not typically used in cases where you do not want the link to continually come up to send broadcasts.

The sample network in Figure illustrates an X.25 network if multiple circuits are established from the central-site router. In Example , the separate x25 map statements are configured to create separate links to each branch office. In this case, the same network layer protocol, IP, is used.

Lab Activity    
  Because an X.25 network is not available, you will first configure a router to act as a X.25 switch. Once the switch is configured, you will configure two routers to interface with the X.25 network. Use X.25 SVCs to create a functional link between the two routers. The branch router is a stub network, so configure a default route on the branch router side and a static route on the central router side.