C H A P T E R29
• Describe the need for DLSw.
• Know the advantages of DLSw over source-route bridging.
• Specify the transport protocol between DLSw switches.
• Understand the basic structure of DLSw.
• Recognize DLSw processes by name and function.
• Understand the circuit establishment process.
Data-link switching (DLSw) provides a means of transporting IBM Systems Network Architecture
(SNA) and network basic input/output system (NetBIOS) traffic over an IP network. It serves as an
alternative to source-route bridging (SRB), a protocol for transporting SNA and NetBIOS traffic in
Token Ring environments that was widely deployed before the introduction of DLSw. In general, DLSw
addresses some of the shortcomings of SRB for certain communication requirements—particularly in
WAN implementations. This chapter contrasts DLSw with SRB, summarizes underlying protocols, and
provides a synopsis of normal protocol operations.
DLSw initially emerged as a proprietary IBM solution in 1992. It was first submitted to the IETF as RFC
1434 in 1993. DLSw is now documented in detail by IETF RFC 1795, which was submitted in April
1995. DLSw was jointly developed by the Advanced Peer-to-Peer Networking (APPN) Implementors
Workshop (AIW) and the Data-Link Switching Related Interest Group (DLSw RIG).
RFC 1795 describes three primary functions of DLSw:
• The Switch-to-Switch Protocol (SSP) is the protocol maintained between two DLSw nodes or
• The termination of SNA data-link control (DLC) connections helps to reduce the likelihood of link
layer timeouts across WANs.
• The local mapping of DLC connections to a DLSw circuit.
Each of these functions is discussed in detail in this chapter.
In 1997, the IETF released DLSw version 2 (RFC 2166) which provides enhancements to RFC 1795
document. The additional features include these:
rnetworking Technologies Handbook
DLSw Contrasted with Source-Route Bridging
• UDP unic