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DLSw+ Design Topologies 517

SNA applications have always been known to provide mission-critical business functions but
only to SNA equipment over SNA communication devices. IBM noticed the rapid growth of
the Internet in the mid-'90s and realized it must make SNA applications more IP-friendly and
accessible to the IP community. This paradigm shift can be compared to the migration of the
voice world to voice over IP today. Organizations needed to make adjustments to the SNA/IP
paradigm shift similar to the paradigm shift of VoIP.

DLSw version 1 required full-mesh connectivity between peers. This requirement was
eliminated with the introduction of DLSw version 2, RFC 2166. DLSw 2 also added several
other benefits:

One TCP session instead of two between peers

The use of UDP unicast instead of TCP

Finally, Cisco developed its own proprietary version of DLSw called DLSw+. DLSw+ differs
from DLSw primarily in the types of encapsulations that can be used. DLSw+ peers may be
connected via TCP, Fast Sequence Transport (FST), or Direct. FST and Direct are Cisco's
own methods.

You might need to read Chapters 12 and 13 or have some understanding of DLSw+ before you
attempt to read this chapter. In the early days of DLSw+, there probably was a full-mesh
configuration connecting all the peers. Full-mesh configurations have many drawbacks. Like
most protocols, DLSw+ works best in a hierarchical topology. This section discusses a method
of dealing with a DLSw+ hierarchical structure called border peers. Border peers provide a
hierarchical structure similar to a default gateway function in IP and eliminate unnecessary
traffic that can be compared to the way a router blocks broadcasts. This section also discusses
some of the key components of DLSw+ design and eliminating unnecessary traffic. Some of
the key components include design rules and the border-peer design theory of operation. Border
peers help reduce the number of physical connections in a full-mesh environment.

DLSw+ is sometimes called an any-to-any (full-mesh) network.

Hierarchical

Let's take a look at some of the key components involved in supporting a DLSw+ network:

The number of physical units is a driving factor. Because an SNA physical unit has either
an SDLC or LLC2 connection to another device, it must send keepalive messages at
regular intervals. Keepalive messages can be processor-intensive. If there are many
connections, this could affect router selection.

NOTE

A Cisco 4700 router can handle about 4000 physical units.

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