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Data Link Switching (DLSw) carries legacy IBM Systems Network Architecture (SNA) and NetBIOS traffic between DLSw switches by encapsulating it in TCP/IP. For common LLC Type 2 connections, the switches terminate data-link control locally rather than extending one continuous data-link connection across the WAN. That distinction helps explain why DLSw is not simply bridging or full network-layer routing.
What Data Link Switching means
The Internet Engineering Task Force (IETF) defines DLSw as a forwarding mechanism for IBM SNA and NetBIOS protocols. Its Switch-to-Switch Protocol (SSP) describes how DLSw switches communicate; the initial SSP implementation uses TCP as a reliable transport between switches. In practical terms, DLSw carries selected legacy LAN traffic over an IP network while handling it at the data-link level, rather than routing it as ordinary network-layer traffic.
The specification is RFC 1795, “Data Link Switching: Switch-to-Switch Protocol,” published in April 1995. The earlier RFC 1434, published in March 1993, described DLSw over TCP/IP.
How DLSw carries a connection across a WAN
LLC Type 2 connections
In a common LLC Type 2 setup, the DLSw switch ends the local data-link control connection. It then multiplexes LLC connections onto a TCP connection to another DLSw switch. The remote switch relays the relevant information to the destination on its own local connection. As a result, the two endpoint stations do not maintain one uninterrupted LLC Type 2 connection through the WAN; each side has a local connection handled by its switch.
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Discovery and data transfer
Before data transfer in an SNA LLC2 setup, a station can start discovery with a TEST frame. IBM describes this as part of the LAN connection process. Once the relevant stations are found and the local connections are established, DLSw can carry the traffic between switches inside TCP packets.
Why DLSw differs from bridging
A conventional bridge preserves data-link control end to end. DLSw instead terminates that control at the switch for connection-oriented traffic. That matters when the path between networks is a WAN: LLC Type 2 is designed around predictable local-network transit times, while a long or variable WAN delay can cause acknowledgement timers to expire and provoke retransmissions.
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By handling local acknowledgements and flow-control functions at the switches, DLSw avoids requiring those control exchanges to traverse the WAN as part of one continuous LLC connection. Its aim is to carry legacy SNA or NetBIOS traffic across an IP network while addressing the behavior of data-link protocols over longer, less predictable paths.
What problems DLSw was designed to address
RFC 1795 identifies several issues the mechanism is intended to address:
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- Data-link control timeouts caused by WAN delay.
- Acknowledgements that would otherwise have to cross the WAN.
- Flow-control and congestion-control concerns.
- Control of broadcast search packets.
- Source-route bridging hop-count limits.
For NetBIOS datagrams using connectionless LLC Type 1, RFC 1795 notes that broadcast-search control and hop-count limits are the relevant issues among those listed.
Where DLSw fits in legacy networks
IBM documents a legacy branch-to-mainframe example: branch routers running DLSw could connect LAN-attached SNA workstations and SDLC-attached devices to mainframe systems. This could avoid requiring a dedicated communications link to each SDLC device. IBM notes that router definitions for DLSw reside on the router and that virtual MAC addresses are used for SDLC devices.
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This example establishes a historical use case, not how common DLSw is in current networks. IBM also identifies Cisco’s DLSw+ as an implementation with enhancements to original DLSw; that does not establish that all vendors’ implementations behave alike.
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RFC 1434 and RFC 1795: check implementation compatibility
RFC 1795 revised and expanded the earlier protocol specification, but it explicitly warns that its changes do not define interoperability details with RFC 1434 or enhanced implementations. If you are maintaining a legacy network, do not assume compatibility from the DLSw label alone. Check the specific devices’ implementation documentation and the protocol version they support.
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