Figure 9: Sample DLSw Network
Data link switching (DLSw) was developed in the 1990s as a method to transport IBM Systems Network Architecture (SNA) traffic over an IP WAN network. Switch-to-Switch Protocol (SSP) is used to forward network traffic between routers configured for DLSw (DLSw peers). Then, to route traffic over a WAN link or the Internet, DLSw encapsulates the SNA network traffic into IP packets.
DLSw was developed as a forwarding mechanism for IBM Systems Network Architecture (SNA) protocol. Although DLSw does not provide full routing capabilities, it provides switching at the data link layer and encapsulation in TCP/IP for transport over the Internet.
Because DLSw provides support for SNA, a connection-oriented protocol, the Services Router supports Logical Link Control (LLC) type 2 as part of the DLSw implementation. Figure 9 shows a possible DLSw network.
Figure 9: Sample DLSw Network
Switch-to-Switch Protocol (SSP) is used between DLSw peers to establish connections, locate resources, forward data, and handle error recovery as well as flow control. Generally, SSP does not provide full routing between peers, because routing is typically handled by common routing protocols such as OSPF or BGP. Instead, packets are switched at the SNA data link layer and encapsulated in TCP/IP for transport over IP-based networks. TCP is used as reliable transport method between DLSw peers.
There are several operational stages that take place in DLSw connections. First, two DLSw peers establish a TCP connection with each other. After the connection is established, each peer router exchanges supported capabilities with the other router. The TCP connection ensures reliable and guaranteed delivery of IP traffic, and also ensures the integrity and delivery of traffic encapsulated in the IP protocol. After capability information is exchanged, the DLSw peers establish circuits between SNA end systems and begin transmitting information frames (I-frames) over the network.
DLSw capabilities exchange is based on a switch-to-switch protocol message describing the capabilities of the sending data-link switch. Sent just after the DLSw peers establish a connection, a capabilities exchange control message communicates the following operational parameters between the two peers:
Establishing DLSw circuits is a process in which local and remote DLSw peers locate each other and set up data link control (DLC) connections between the remote router and a local router. The specific details of establishing circuits are determined by the traffic type, but the process is the same for all types of traffic.
The first step in the process enables the SNA devices on a LAN to find other SNA devices by sending out an explorer frame with the MAC address of the target SNA device. When a DLSw peer receives the explorer frame, it sends a canureach message frame to each of its DLSw peer connections. The canureach message frame queries the DLSw peers to determine if one of the peers can locate the target SNA device. If one of the DLSw peers can reach the target SNA device, it returns an icanreach message frame to the originating DLSw peer to indicate that it can provide a path to the SNA device in question.
After canureach and icanreach message frames are exchanged, the two DLSw peers establish a circuit consisting of a DLC connection between each router and the local SNA end system and a TCP connection between the two DLSw peers. The resulting circuit is uniquely identified by source and destination circuit IDs. Each SNA DLSw circuit ID includes the following information:
Circuit priority is negotiated when the circuit is set up on the network.