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definitions. The DSPU feature lets you define downstream PU type 2 devices in the router.
DSPU simplifies host configuration by letting you replace multiple PU definitions that
represent each downstream device with one PU definition that represents the router. In Figure
12-18, the Cisco router appears as the only PU to the host; the four PCs are viewed as LUs by
the host. In Figure 12-18, the router is configured for the Token Ring PCs as a PU, not the host.

Figure 12-18

DSPU

A big advantage of this is, because you define the downstream PUs at the router rather than at
the host, you isolate the host from changes in the downstream network topology. Therefore, you
can insert and remove downstream PUs from the network without making any changes on
the host.

Another major benefit of concentrating downstream PUs at the router is that this reduces
network traffic on the wide-area network (WAN) by limiting the number of sessions that must
be established and maintained with the host. Terminating downstream sessions at the router
ensures that idle session traffic does not appear on the WAN. See Figure 12-18.

LLC2

This section looks at Logical Link Control (LLC) type 2 frames (see Figure 12-19) and how
they perform session establishment. There are three types of LLC frames:

Type 1 provides unacknowledged connectionless service.

Type 2 provides connection-oriented service.

Type 3 provides acknowledged connectionless service.

IBM S/370

Token Ring

ÒI see one PU and four LUsÓ

Cisco router appears to

the PCs as the

mainframe

Cisco router appears as one

PU to the mainframe

PCs appear as LUs to

the mainframe

DSPU

Router A

87200333.book Page 452 Wednesday, August 22, 2001 2:53 PM