DLSw supports SNA and NetBIOS in multiprotocol routers. SNA and NetBIOS provide connection-oriented services. SNA and NetBIOS use IEEE 802.2 LLC2 protocol over LANs. DLSw also provides SNA connectivity over WAN links for devices attached by SDLC peripheral links. For conceptual information on how data link switching works for LANs, see RFC 1795. The NETBuilder bridge/router family of hardware and software fully implements this standard.

Figure 249 shows a typical network configuration using data link switching with SNA and NetBIOS traffic to connect three bridge/routers across an IP internetwork. Each connection is a tunnel, which consists of two TCP ports: one to send data (port #2067) and one to receive data (port #2065).

Figure 249 Simple Data Link Switching Configuration

Multiple sessions between different ports are multiplexed onto a single tunnel. For instance, if there is a session connecting LAN server 1 and LAN requester 1, and a concurrent session connecting PC1 and host 1, traffic is multiplexed onto a single tunnel between the NETBuilder II bridge/router at Dallas and the SuperStack II NETBuilder bridge/router at New York.

Media Addressing and NetBIOS Name Caching

When the 3Com DLSw router receives an explorer or NetBIOS name type frame, the router first checks the statically defined table for the existence of a predefined route. The router also checks the DLSw caching tables for a match. If a match is found, the frame is forwarded on the static or cached DLSw tunnel. If no match exists, then the frame is forwarded to each DLSw tunnel. When the DLSw router receives a DLSw explorer or NetBIOS name type frame, the router adds the media address or the NetBIOS name to its caching tables.

A cached item is deleted when the DLSw router uses a cached route to forward an explorer frame but fails to get a response. The result is that the first explorer or query frame is sent using the cache tunnel. When that frame fails to get a response, the cached item is deleted and the query is resent on all tunnels.

DLSw Configuration and STP

DLSw is not aware of the Spanning Tree Protocol (STP). Because of this limitation, you must avoid configuring a second data path that can loop SNA and NetBIOS traffic back to an originating router. Do not configure either bridge or tunnel paths as second data paths. Avoid the topology shown in Figure 250 because it may duplicate packets and cause failure.

Figure 250 Illegal DLSw Tunneling Configuration

Your site may require redundancy in a DLSw environment. If you need DLSw bridge/router redundancy, contact your network supplier for planning.

In token ring topologies, DLSW or LLC2 tunneling can support parallel paths in a source-routed-only environment.