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X.25 335

TIP

X.25 operates at Layer 3 of the OSI model, and LAPB operates at Layer 2.

Figure 10-5 shows two IP routers communicating over an X.25 link. Notice in the figure that
the window size is not end-to-end but rather only between the router and the local X.25 switch.
This window size negotiation happens on both sides of the cloud independently in this example.
Flow control is also implemented locally, or between the router and the local X.25 switch. Flow
control and window size negotiation also occur between the X.25 switches in the cloud via the
X.75 protocol.

Figure 10-5

X.25 in the WAN

X.25 is packet-switched, so a packet goes in and gets switched between the X.25 switches
across the network, as shown in Figure 10-5. Notice that the two X.25 switches communicate
via the X.75 protocol. (Cisco does not support the X.75 protocol.)

Depending on your carrier, there might not be an X.25 switch in the carrier's network. The
carrier might actually encapsulate the X.25 packets into IP, a technique known as XOT (X.25
over TCP/IP [RFC 1613, first available in Cisco IOS version 9.21]), as shown in Figure 10-6.
In this example, X.25 packets are being encapsulated into TCP/IP by the routers. The IP
network is transparent to the X.25 devices.

11. x25 address 2222
12. x25 map ip 192.168.100.1 1111

Example 10-1

X25 Connection (Continued)

X.25 cloud

X.25 Switch left on left

X.25 Switch right on right

X.75 between the switches

< X.25 Flow control>

< window size >

Router left

Router right

X.25 device

87200333.book Page 335 Wednesday, August 22, 2001 2:53 PM