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Network Design with RSRB

Remote Source Route Bridging (RSRB) establishes tunnels between routers in the internetwork for connections. This permits source route bridging across non-Token Ring links and greatly increases the functionality within networks. Additional features, including local acknowledgement, work to improve response time and reliability. Figure 10.2 illustrates a simple RSRB installation across a serial connection.


FIGURE 10.2  Remote Source Route Bridging

There are five encapsulation protocols for use in RSRB configurations. These are outlined in Table 10.1.

TABLE 10.1 RSRB Encapsulation Protocols
Protocol Characteristics
Local SRB Available on end-to-end Token Ring networks. Requires little overhead, as no encapsulation is needed. LLC2 frames cross routers.
Direct Also requires little overhead, but encapsulation takes place in the data-link header. Useful for point-to-point links. Encapsulations may use HDLC, for example.
Frame Relay Using the specifications in RFC 1490, this transport encapsulates SNA into LLC2 frames on Frame Relay networks.
IP FST Fast Sequenced Transport over IP encapsulates LLC2 frames in IP datagrams. It involves more overhead than the previous methods, but it demands less overhead than TCP encapsulations. Designers must ensure that packets will arrive in sequence and without fragmentation.
TCP The TCP encapsulation wraps the LLC2 frame with a TCP packet. The trade-off for the obvious overhead is greater reliability and local acknowledgement. Packets may be fragmented and can arrive in any sequence—this encapsulation also reconstructs the packets. Most network designers will find TCP encapsulation the most consistent solution for their networks.

RSRB is not without limitations, and many new network designs will opt to use the DLSw (Data Link Switching) option, given its superior handling. DLSw is discussed in the following section. However, the long history of RSRB certainly requires designers of modern mainframe networks to understand the protocol—many organizations have been slow to adopt newer methodologies because of the lack of perceived benefits that come with upgrading and the required training and support demands. In the context of most organizations, which appear to be moving away from SNA, the strategic benefits of changing are dubious at best.

The steps to configure RSRB differ slightly for each encapsulation type; however, the primary steps are similar. A sample configuration for TCP encapsulation is shown in the following output. Note that the virtual ring is given the number 406 and has two remote peers and that the Marketing Segment on Token Ring 4/0 is linked to the virtual ring via the source-bridge command.

source-bridge ring-group 406 source-bridge remote-peer 406 tcp 10.100.105.254 source-bridge remote-peer 406 tcp 10.1.1.1 interface TokenRing4/0 description Marketing Segment ip address 192.168.19.1 255.255.255.0 no ip directed-broadcast no keepalive early-token-release ring-speed 16 source-bridge 226 3 406 source-bridge spanning source-bridge proxy-explorer

Network Design with DLSw

DLSw was developed to address some of the shortcomings in RSRB, and it is gaining popularity, but many organizations are resisting a changeover. This was likely the result of Year 2000 preparations and other new deployments that demand resources from organizations. In the context of network design, an entire chapter could be written regarding the proper installation and configuration of DLSw. However, for the purposes of the exam objectives, readers should be concerned only with a high-level understanding of the protocol itself. Consult RFC 1795 for additional information.

DLSw provides many features to the network designer. These include:

•  Supports LLC2 termination, which eliminates the need for keepalives to cross WAN links. This feature provides functionality similar to local acknowledgement and also avoids timeouts, which are a significant concern to designers with the time-sensitive SNA protocol. The local router acknowledges frames.
•  Supports SNA traffic over TCP, which adds reliability to the transport across WAN links.
•  Supports NetBIOS over TCP; however, few implementations use this function.
•  Provides for termination of the RIF (Routing Information Field). In RSRB, the RIF is incorporated into the WAN cloud. This feature limits SRB to seven hops. In DLSw, the RIF field is terminated in a virtual ring, which is the connection between two DLSw peer routers. This permits 13-hop installations; however, administrators should be cautioned that the RIF will be incomplete for troubleshooting. The greatest benefit to this feature is that explorer packets are contained on each side of the cloud, reducing traffic and preserving bandwidth.
•  Permits load balancing and allows for backup peer routers.
•  Is an open standard, and as such, it allows designers to interconnect different router brands.

In addition, Cisco offers enhanced DLSw features (referred to as DLSw+), including:

•  Peer groups
•  Border and on-demand peers
•  Backward compatibility with STUN and RSRB

Of these enhanced features, designers may find backward compatibility useful in migrations from STUN or RSRB to DLSw, which is generally regarded as the superior methodology. Peer groups can also assist the design. Routers within a peer group work to permit “any-to-any” connectivity, but peer groups also can simplify configuration and optimize explorer packet processing.

Peer routers also can provide the designer with load balancing. When configured, the router will use a round-robin method to balance sessions on a connection basis. This requires equal-cost paths. If load balancing is not enabled, the router will use a single preferred path for all explorer packets.

The following output provides a sample DLSw configuration, where the ring group has been defined and the router has been configured as a local peer in the group. This configuration uses its loopback address in order to circumvent interface failures.

source-bridge ring-group 9 dlsw local-peer peer-id 10.12.24.1 (loopback) dlsw remote-peer 0 tcp 10.14.10.1 dlsw remote-peer 0 tcp 10.10.18.1 dlsw bridge-group 9
It is very unlikely that the loopback interface will fail—unlike the physical interfaces. (Cisco defines the loopback as never failing, but sometimes an administrator will inadvertently delete the interface or remove its address.) Use of the loopback can greatly enhance the reliability and supportability of the router. The loopback notation in the previous output reflects the IP address of the router’s loopback interface—LO0. This is administratively assigned, as opposed to the traditional IP loopback of 127.0.0.1.


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