Configuration Procedure - 3Com MSR 50 Series Configuration Manual

308 C 14: X.25
HAPTER

Configuration Procedure

LAPB C
AND ONFIGURATION
At present, since IP network is used widely, it is necessary, in practice, to carry X.25
data and implement the interconnection between X.25 networks via IP network.
The traditional X.25 protocol belongs to layer 3 (network layer) of OSI 7-layer
model, and it can obtain the reliable data transmission link via LAPB protocol.
Since TCP has such mechanisms as error retransmission and window flow control
to ensure the reliability of the link, it can be used by X.25 protocol. XOT
establishes a TCP tunnel connection between X.25 networks at both ends, and
X.25 packet, as the data of application layer, is carried over TCP, i.e. TCP serves as
the "link layer" protocol of X.25. Router B, Router C and IP network in the middle
can be looked upon as a big "X.25 switch", and the data sent by Router A is
directly switched to Router D via this "switch".
XOT characteristics conform to the RFC1613 standard, which features as follows:
Supporting SVC application. The routers at both ends can dynamically establish
■
an SVC by sending call packet, and this SVC will be automatically cleared when
no data is transmitted.
Supporting PVC application. After being configured with a PVC, the routers at
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both ends directly enter the data transmission status without establishing a call.
Moreover, this PVC will not be dynamically deleted when no data is
transmitted.
Supporting Keepalive attribute of TCP. If Keepalive is not configured, TCP
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connection will still not be cleared or cleared after a long time even if the
connection is interrupted. However, after Keepalive is configured, TCP will
timely detect the availability of the link. If TCP does not receive the response
from the peer for many times, it will initiatively clear its connection.
XOT implementation principle (taking SVC as an example):
As shown in the above figure, when transmitting data, Router A first sends a call
request packet to establish VC. After receiving this call packet and judging it as
XOT application, Router B will establish a TCP connection with Router C, then add
XOT header to X.25 call packet and encapsulate it into TCP, finally transmit it to
Router C. After deleting TCP and XOT header, Router C transfers the call request
packet to Router D via X.25 local switching. After receiving it, Router D will give
out call acknowledgement until the link is completely established to transmit data.
The whole process for establishment and application of TCP connection is
transparent for Router A and Router D that do not care whether data is forwarded
via IP network or X.25 network.
Configure XOT
To configure XOT, use the following commands:
To do...
Enter system view
Enable X.25 switching
Enter interface view
Use the command...
system-view
x25 switching
interface interface-type
interface-number
Remarks
-
Required
Not enabled by default
-