Data link switching (DLSw)

Data link switching carries LAN traffic over the wide area network by encapsulating the LAN traffic in TCP packets. Data link switching was first developed by IBM to provide SNA support within multi-protocol routers. The DLSw formats and protocol were then made available to the wider community and published as RFC 1434. The RFC was then later enhanced and republished as RFC 1795.

Cisco's implementation of data link switching is known as DLSw+, and it contains additional enhancements to the original DLSw. The working infrastructures of most organizations include Cisco routers.

DLSw over the WAN

To illustrate how DLSw carries LAN (LLC2) traffic over the WAN, the distance between workstation A and workstation B is extended and two locations connect using a communication line and routers that implement DLSw. Figure 1 depicts the new configuration.

Figure 1. Connecting two LAN-attached workstations over a WAN using DLSw

As in the previous example, workstation A initiates a connection to workstation B by sending a TEST frame broadcast. Router A, which is attached to the same LAN, receives the TEST frame, encapsulates it in a TCP packet, and transmits it to all its peer DLSw routers. The TCP broadcast packet is called CANUREACH.

CANUREACH is received by all DLSw routers, including Router B. Router B has already learned the MAC addresses of the workstations, hosts, and servers that are attached to its LAN. When the CANUREACH packet with the MAC address of workstation B reaches router B, the router converts the CANUREACH packet to a TEST LLC2 frame.

Because the TEST is a broadcast frame, all stations on the LAN receive the TEST frame. Workstation B, which has been assigned the 4200.0000.0002 MAC address, responds with a +RSP. Because ICANREACH is a unicast message, Router B encapsulates the +RSP in a TCP packet and sends an ICANREACH packet to workstation A only. Router A decapsulates the ICANREACH packet to an LLC2 +RSP frame.

LLC2 requires timely responses and acknowledgments between the two end stations. Extending the distance between workstation A and workstation B, and using a slower media than the rated speed of a LAN, might cause LLC2 timers to expire and terminate the connection. Serial WAN link speeds are from 256 Kbps to 2 Mbps, while LAN speed is 100 Mbps.

With DLSw, connections are terminated at the DLSw routers, which acknowledge packets locally instead of transmitting these across the WAN. This technique is known as spoofing; it reduces WAN traffic, eliminating potential LLC2 time-outs.