Specifications

12-7
Catalyst 4224 Access Gateway Switch Software Configuration Guide
OL-2031-02
Chapter 12 Configuring Other Routing Protocols
IBM SNA
Phase Two: IP Transport
Beginning in the 1980s, large organizations began building TCP/IP-based
networks to support client/server applications and systems. UNIX, a dominant
operating system for client/server applications, natively supports TCP/IP. As the
growth of TCP/IP-based systems continued, organizations often found that they
had built parallel networks, one running SNA and one running TCP/IP. This setup
is expensive because of the duplication of line costs, equipment, and personnel.
To eliminate the duplication, organizations had a choice—run the TCP/IP traffic
over the SNA backbone, or run the SNA traffic over the TCP/IP backbone.
Running TCP/IP over an SNA backbone was not a feasible choice because of the
lack of redundancy and openness of SNA. Routers, which formed the core of the
TCP/IP network, began to support the encapsulation of SNA in TCP/IP for
transport across the TCP/IP network.
This encapsulation brings many benefits. First and foremost, while it is
encapsulated in TCP/IP, the SNA traffic is dynamically routed around network
failures, a benefit that only recently has been added to SNA networks with
APPN/HPR. The encapsulation schemes also provide more flexible
configurations for SNA devices and reduced polling traffic across the backbone.
Cisco offered the first such encapsulation scheme with RSRB. Since then, the
industry has adopted a standard, data-link switching (DLSw), that has been very
widely accepted and implemented. Routers also provide features such as serial
tunnel (STUN) and Block Serial Tunneling (BSTUN) to encapsulate other types
of traffic (asynchronous, bisynchronous, and some proprietary protocols) in
addition to SNA.
In this second phase of integration, many organizations find that the same end
users who are running advanced SNA client emulators to access mainframe and
midrange systems are also accessing TCP/IP systems. This means that each PC
must run two different protocol stacks—SNA and TCP/IP—for access to host
systems.