Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 148

Cisco's implementation of DLSw is called DLSw+. Some of the benefits of implementing
DLSw+ are that link-layer acknowledgments and keepalive messages of SNA and NetBIOS
traffic do not have to travel through the WAN. Also DLSw+ traffic can reroute around link
failures and can be prioritized on WAN links, whereas SRB does not provide this capability.
Cisco routers can be configured to communicate with devices running standards-based DLSw.

Figure 4-28 displays a sample DLSw network.

DLSw Network Figure 4-28

DLSW Peers

End systems can attach to the DLSw network from the Token Ring, Ethernet, Fiber Distributed
Data Interface (FDDI), Qualified Logical Link Control (QLLC) (X.25), and SDLC networks.
Although there are other methods, the preferred method for establishing peer connections is by
using TCP. After a connection is established, the peer routers can exchange capabilities.
Circuits are established between end-systems (SNA and NetBIOS). NetBIOS names can be
configured to prevent NetBIOS Name Queries to traverse the DLSw network; MAC addresses
can be configured in the same manner to reduce SRB explorer frames.

DLSw uses the Layer-2 Switch-to-Switch Protocol (SSP) between DLSw peer routers. SSP
switches frame at the SNA layer. DLSw also encapsulates frames into TCP/IP for transport over
IP networks.

DLSw supports termination of RIFs at the local virtual ring, which is the default mode. DLSw
local termination eliminates the requirement for link-layer acknowledgments and keepalive
messages to flow across a WAN. With RIF termination, the virtual rings do not have to be the
same between the peers, and all remote hosts appear to be connected to the local router's virtual
ring. DLSw also supports RIF Passthru, which does not terminate the RIF at the local end. With
RIF Passthru, the virtual ring numbers of the DLSw peer routers must match, and the rif-
passthru keyword must be in the remote-peer statement.

SNA devices on a LAN find other SNA devices by sending an explorer frame with the MAC
address of the target SNA device. When a DLSw router receives an explorer frame, the router
sends a canureach frame to each of the DLSw partners. If one of its DLSw partners can reach