Data Link Switching Plus

Data-link switching (DLSw) is documented in RFC 1795. IBM originally submitted it to the IETF as RFC 1434, to overcome some of the limitations on SRB networks, especially in WANs. The Cisco implementation of DLSw is called DLSw Plus (DLSw+). Some of the benefits of implementing DLSw+ are that link-layer acknowledgements and keep-alive messages of SNA and NetBIOS traffic do not have to travel through the WAN. Also, DLSw+ traffic can be rerouted around link failures and can be prioritized on WAN links, while SRB does not provide this ability. Figure 3-6 shows an example of a DLSw+ network.

Figure 3-6 DLSw+ Network

End systems can attach to the DLSw network from Token Ring, Ethernet, FDDI, Qualified Logical Link Control (QLLC), and Synchronous Data Link Control (SDLC) networks. Although there are other methods, the preferred method for establishing peer connections is 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 from traversing the DLSw network; Media Access Control (MAC) addresses can be configured in the same manner, to reduce SRB explorer frames.

The basic configuration of DLSw is quite simple. Each router with attached networks is configured as a local peer. Remote peers are then configured to remote routers. It is preferable to use an IP address assigned to an internal loopback interface as the local peer address. Loopback interfaces provide a virtual interface and do not fail if a physical link fails, thus providing a stable peer address. For designs with multiple branches connecting to a hub site, the promiscuous keyword can be used to permit remote peers to connect without being individually specified. See Figure 3-7 for a sample configuration.

Figure 3-7 DLSw Sample Network

The configuration for Router A is displayed in Example 3-1.

Example 3-1 DLSw Configuration—Router A !

source-bridge ring-group 100 !

dlsw local-peer peer-id 10.100.1.1 promiscuous dlsw remote-peer 0 tcp 10.200.1.1 !

interface tokenring 0

source-bridge 1 1 100 !

interface loopback 0

ip address 10.100.1.1 255.255.255.255

The configuration for Router B is displayed in Example 3-2.

Example 3-2 DLSw Configuration—RouterB !

source-bridge ring-group 200 !

dlsw local-peer peer-id 10.200.1.1

dlsw remote-peer 0 tcp 10.100.1.1 !

interface tokenring 0

source-bridge 2 1 200 !

interface loopback 0

ip address 10.200.1.1 255.255.255.255

The configuration for Router C is displayed in Example 3-3.

Example 3-3 DLSw Configuration—Router C !

bridge-group 1 protocol ieee !

dlsw bridge-group 1 !

dlsw local-peer peer-id 10.250.1.1

dlsw remote-peer 0 tcp 10.100.1.1 !

interface loopback 0

ip address 10.250.1.1 255.255.255.255 !

interface Ethernet 0 bridge-group 1

Notice that Router A only defines a remote peer for Router B. It does not need to define Router C as a peer because the promiscuous keyword is used to accept connections from remote peers without having to define them. Router C must define Router A as a remote peer because the promiscuous keyword was not used.

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