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336 Chapter 10: X.25/Frame Relay Topologies
Figure 10-6
Encapsulating the X.25 Packets into IP--XOT
X.25 governs how your switch talks to the carrier. What the carriers do is up to them. Another
possibility is using the STUN protocol to transport the X.25 traffic, which is a capability of the
Cisco routers. X.25 acts like an overengineered data link. It is very robust, but you pay for the
robustness with a little slowness.
TIP
When an interface is encapsulated with the encapsulate x25 command, it becomes DTE by
default. To make the interface a logical DCE (X.25 is required to make one end of a circuit a
DCE), the command encapsulate x25 dce needs to be applied to the interface.
Also, the maximum clock rate of an X.25 port is 2000000.
The Cisco proprietary STUN protocol can encapsulate and transport X.25, Frame Relay, and
other proprietary protocols. This is a very important tool for the CCDP to keep in mind when
companies migrate to a total-IP backbone and still need to support their legacy protocols. Let's
take a look at where STUN and XOT might work together on a large, complex network.
TIP
Each STUN tunnel occupies 32 K of memory.
Figure 10-7 shows a company using XOT over its ASN (Access Subnetwork) and STUN over
its BSN (Backbone Subnetwork). STUN is a form of serial tunneling, in which data is
encapsulated over a TCP tunnel. The endpoints of the STUN tunnel need to be defined. Using
loopback interfaces is a good design practice for terminating tunnels because loopback
interfaces will always be available.
Router left
Router right
X.25 device
X.25 device
<XOT>
IP cloud
87200333.book Page 336 Wednesday, August 22, 2001 2:53 PM