338 Chapter 10: X.25/Frame Relay Topologies
3
Router Dallas accepts the XOT connection, de-encapsulates the IP packets from Router
Houston, and examines the X.25 source and destination addresses for the call setup
request.
4
If the X.25 destination address is not local to Router Dallas, the router forwards it out S0
via X.25 to the X.25 packet switch.
5
The X.25 packet switch does an X.25 lookup and determines that the X.25 destination
address is via Core Site B packet switch. The packet switches communicate via a
proprietary protocol not supported by the Cisco routers.
6
The Cisco routers have created a STUN tunnel from their Serial 0 interfaces,
encapsulating the proprietary protocol to let the packet switch devices communicate.
7
The Core Site B X.25 switch receives the call setup request over the STUN tunnel, does
an X.25 table lookup, and determines that the address is out its X.25 connection to Router
Nashville.
8
Router Nashville receives the X.25 call setup request, determines that the destination X.25
address is its XOT tunnel to Router Memphis, and encapsulates the X25 message into IP.
9
Router Memphis de-encapsulates the IP packet and sends the X.25 call setup request to
the PAD, establishing the X.25 circuit from PAD to PAD.
TIP
Another difference between STUN and XOT is that when STUN tunnels are created, STUN
constantly uses up CPU cycles to keep the tunnels up. With XOT, however, if the circuit to
establish is an SVC, the tunnel is brought up only when needed. Hence, it requires fewer CPU
cycles. If the XOT circuit is a PVC, CPU processor cycles are used again.
WARNING
STUN tunnels can be very processor-intensive. STUN tunnels are process-switched, even if the
router is using VIP cards that have their own processor.
NOTE
When creating tunnels, use TCP as the tunnel method if the end stations are sensitive to
receiving frames in order. If the end stations are not sensitive to receiving frames in order, IP
can be used as the transport method.
NOTE
It is possible to use the AUX port as an X.25 PAD on a Cisco router for testing purposes.
87200333.book Page 338 Wednesday, August 22, 2001 2:53 PM