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224 CHAPTER 16: CONFIGURING LAPB AND X.25
[Router-Serial0]lapb module 128
[Router-Serial0]lapb window-size 127
2 Configure Router B:
a Select interface
[Router]configure
[Router]interface serial 1
b Specify IP address for this interface
[Router-Serial1]ip address 202.38.160.2 255.255.255.0
c Configure the link layer protocol of the interface to LAPB and specify its
working mode as DCE
[Router-Serial1]link-protocol lapb dce
d Configure other LAPB parameters (if the link quality is good, and a higher rate
is required, the flow control parameter modulo can be increased to 128, k to
127, but they must be the same for both ends in the direct connection)
[Router-Serial1]lapb modulo 128
[Router-Serial1]lapb window-size 127
Typical X.25
Configuration
Example
Back to Back Direct
Connection of Two
Routers via Serial
Interface
I. Networking Requirement
As shown in the diagram below, two routers are to be directly connected back to
back; the X.25 protocol is used between the serial ports for IP data packet
transmission.
II. Networking Diagram
Figure 68 Direct connection of two routers via serial ports
III. Configuration Procedure
1 Configure Router A:
a Select interface
[Router]interface serial 0
b Specify IP address for this interface
[Router-Serial0]ip address 202.38.160.1 255.255.255.0
c Configure the link layer protocol of the interface to X.25 and specify its
working mode as DTE
[Router-Serial0]link-protocol x25 dte
d Specify X.121 address of this interface
[Router-Serial0]x25 x121-address 20112451
V.24/V.35 cable
Router A
Serial 1
Serial 0
Router B