background image
234 Chapter 8: Establishing an X.25 Connection
Step 3
Map the appropriate next logical hop protocol address to its X.121
address. This function maps the L3 protocol address to the L2
X.121 address.
There are other interface configuration values that can be configured to control the flow of
information. These values can be set to specify packet sizes, window sizes, and modulo sizes.
The VC ranges and types need be configured as well.
Step 1: Setting the Interface Encapsulation, Specifying DCE or DTE
The encapsulation is set at the interface configuration mode prompt. The command is entered
as follows:
Router(config-if)#encapsulation x25 [dte | dce]
As stated previously, dte is the default setting. If the encapsulation is altered from dce to dte,
or vice versa, the interface settings are lost and reset to default values. The dce option is used
in environments in which the router might be used as a switch.
Step 2: Configuring the X.121 Address
The assignment of X.121 addresses is a simple process. The hardest part is usually getting the
address from the provider. Once the addresses are allocated to you by the service provider, use
the following syntax to assign it to the proper interface:
Router(config-if)#x25 address x.121-address
Step 3: Mapping the Appropriate Next Logical Hop Protocol Address to
its X.121 Address
Once the address has been assigned, it is necessary to associate the remote addresses. This is
done with a map statement. Again, this function is similar to dialer map and frame-relay map
statements used with ISDN and Frame Relay respectively. The syntax is as follows:
Router(config-if)#x25 map protocol address x.121 address [options]
The protocol keyword refers to the upper layer protocol being routed across the X.25 network
(for example, IP, IPX, AppleTalk, and so on). The address keyword is referring to the upper
layer protocol next logical hop (remote end) address. The x.121 address refers to the x.121
address assigned to the destination.
In essence the configuration informs the router that it can reach a specific next logical hop
address by utilizing the designated x.121 address to get across the X.25 network. The previous
syntax assumes a single protocol is going across a particular circuit. If the configuration