Data Link Switching (DLSw) is a way to carry legacy IBM Systems Network Architecture (SNA) and NetBIOS traffic between networks by relaying it between DLSw switches. The switches use the Switch-to-Switch Protocol (SSP) over TCP/IP; for LLC Type 2 traffic, each switch ends the local data-link connection rather than extending one connection across the WAN.
What DLSw does
The IETF defines DLSw as a forwarding mechanism for IBM SNA and NetBIOS protocols. It switches traffic at the data-link layer and encapsulates it for transport between switches over an IP network. That makes DLSw different from a full network-layer routing protocol: it carries selected legacy protocol traffic through a switch-to-switch arrangement. The initial SSP implementation specified by the IETF uses TCP as its reliable transport. RFC 1795
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How DLSw carries traffic
LLC Type 2 sessions
In the common LLC Type 2 case, the DLSw switch terminates the local data-link control connection. It then multiplexes LLC connections onto a TCP connection to another DLSw switch, which relays the relevant information to the destination-side local connection. The two stations therefore do not maintain one continuous LLC Type 2 connection across the WAN. RFC 1795
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In an SNA LLC2 setup, a station can begin finding a destination with a TEST frame before data transfer. IBM describes DLSw as carrying LAN traffic over a WAN by encapsulating that traffic in TCP packets. IBM: Data link switching (DLSw) IBM: DLSw and Logical Link Control 2 (LLC2)
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Why use DLSw instead of bridging?
A conventional bridge preserves data-link control end to end. DLSw instead terminates connection-oriented data-link control at each switch and relays traffic across the WAN. That distinction matters because LLC Type 2 was designed for the more predictable delays of a local network. Long or variable WAN delays can cause retransmission timers to expire and destabilize a link. By handling local acknowledgements and flow-control functions at the switches, DLSw avoids requiring those control exchanges to make every WAN round trip. RFC 1795
RFC 1795 identifies several problems DLSw is intended to address:
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- Data-link control timeouts when network transit is delayed.
- Acknowledgements that would otherwise have to cross the WAN.
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For NetBIOS datagrams using connectionless LLC Type 1, RFC 1795 says broadcast control and hop-count issues are the relevant concerns; the connection-oriented LLC Type 2 timeout and acknowledgement issues do not apply in the same way. RFC 1795
Where DLSw fits in legacy networks
IBM documents a branch-to-mainframe use case in which branch routers running DLSw connected LAN-attached SNA workstations and SDLC-attached devices to mainframe systems. In that setup, a separate dedicated communications link to each SDLC device was not required. IBM notes that router definitions are configured on the router and that SDLC devices can use virtual MAC addresses. IBM: DLSw and Logical Link Control 2 (LLC2)
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This is a documented legacy deployment pattern, not evidence of how common DLSw is in networks today. The sources establish its role for carrying SNA and NetBIOS traffic; they do not establish present-day deployment prevalence.
RFC 1434, RFC 1795, and implementation compatibility
The IETF published RFC 1434 in March 1993 to describe DLSw over TCP/IP. RFC 1795, published in April 1995, revised and expanded the specification. RFC 1795 warns that its changes do not define all interoperability details with RFC 1434 or enhanced implementations. As a result, compatibility between older DLSw equipment or software should be checked against the documentation for the specific implementations involved. RFC 1434 RFC 1795
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IBM identifies Cisco’s implementation as DLSw+, describing it as an enhanced form of the original DLSw. That label should not be taken to mean that all implementations or versions interoperate identically. IBM: Data link switching (DLSw)
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In brief: DLSw versus bridging
| Aspect | Conventional bridging | DLSw |
|---|---|---|
| Data-link control | Preserves control end to end. | For LLC Type 2, terminates local control at each DLSw switch and relays traffic between switches. |
| WAN behavior | Control acknowledgements and timing are exposed to the WAN path. | Handles local acknowledgements and flow-control functions at the switches while carrying traffic between them. |
| Typical purpose in the cited material | Not specified as a DLSw-specific use case. | Transporting legacy SNA and NetBIOS traffic over an IP WAN. |
These distinctions describe DLSw’s role, not a claim that it replaces bridging for general network traffic.
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