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Designing Large-Scale LAN/WAN, Part 2

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Designing Large-Scale LAN/WAN, Part 2 50-20-52 DATA COMMUNICATIONS MANAGEMENT DESIGNING LARGE-SCALE LAN/WANS, PART II John R. Vacca INSIDE Designing Large-Scale IP LAN/WAN Systems; Designing SRB LAN/WAN Systems; Designing SDLC, SDLLC, and QLLC LAN/WAN Systems; Designing APPN Internetworks; Designing DLSw+ Internetworks; Designing ATM; Designing Packet Service LAN/WAN Systems; Designing DDR Internetworks; Designing ISDN; Designing Switched LAN Systems; Designing LAN/WAN Systems for Multimedia DESIGNING LARGE-SCALE IP LAN/WAN SYSTEMS The following discussion provides a very quick overview of the key de- cisions one must make when selecting and deploying routing protocols for large-scale IP LAN/WAN systems. This discussion lays the foundation for subsequent discussions regarding specific routing protocols. Describing LAN/WAN Topology The complete set of routers, and the networks that connect them, de- scribe the physical topology of a LAN/WAN system. A logical topology is also included in a LAN/WAN. Different routing protocols establish the logical topology in different ways. A logical hierarchy is not used by some routing protocols. Within a given LAN/WAN system environment (and to establish a logical topolo- gy), such protocols use addressing to segregate specific areas or do- mains. For such nonhierarchical, or flat, protocols, no manual topology creation is required. Through the establishment of a backbone and logical areas, other pro- tocols require the creation of an explicit hierarchical topology. Examples of routing protocols that use a hierarchical structure are the OSPF and In- termediate System-to-Intermediate System (IS-IS) protocols. According to Cisco, the explicit topology in a hier- archical scheme takes precedence PAYOFF IDEA over the topology created through This article provides details on how to utilize LAN addressing. and WAN technologies available today to design large-scale LAN/WANs. 08/00 Auerbach Publications © 2000 CRC Press LLC The addressing topology should be assigned to reflect the hierarchy if a hierarchical routing protocol is used. Also, the addressing implicitly cre- ates the topology if a flat routing protocol is used. There are two recom- mended ways to assign addresses in a hierarchical LAN/WAN. The simplest way is to give each area (including the backbone) a unique LAN/WAN address. An alternative is to assign address ranges to each area. Logical collections of contiguous LAN/WANs and hosts are areas. All the routers having interfaces on any one of the included LAN/WANs are also areas. Each area runs a separate copy of the basic routing algorithm. Therefore, each area has its own topological database. DESIGNING SRB LAN/WAN SYSTEMS The following discussion addresses implementation issues that can affect large-scale, router-based SRB LAN/WANs, SRB-related technology, and features that provide support for SRB requirements. Specific topics in- clude typical SRB environments and multiport bridging. Using Typical SRB Environments Three types of user environments use SRB: 1. Any-to-any (flat): End users at one site need to access end stations at another site. 2. Many end stations to few end stations (hierarchical): In a hierarchical SNA network, end users from multiple access sites need connectivity to a host site through a limited number of front-end processors (FEPs). 3. Many end stations to several end stations (distributed): Many users need to access a limited number of servers or a limited number of de- vices, such as an AS/400. The following discussion evaluates SRB environment design issues in relation to these user environments. Multiport Bridging A two-port, ring-to-bridge-to-ring combination was the fundamental de- sign of an SRB, as initially created by IBM. A half-bridge configuration that consisted of a ring-to-wide-area-network (WAN) combination fol- lowed by a second WAN-to-ring half-bridge combination was also creat- ed by IBM. Multiport routers adopt an implementation that allows SRBs to include multiple rings on a single internetworking node to support more than two rings. The virtual ring capability accomplishes this. According to Cisco, a virtual ring is a conceptual entity that connects two or more Auerbach Publications © 2000 CRC Press LLC EXHIBIT 1 — A Multiport Bridge Using a Virtual Ring Concept to Permit Multiple Ring Interconnection physical rings together, locally or remotely. Exhibit 1 illustrates the con- cept of multiport bridges and a virtual ring. Virtual rings can be expanded across router boundaries as a concept. Cisco recommends that several access points can be connected to a cen- tral router with an FEP by a large virtual ring. Simple bridging, multiport bridging, and connections to both local and remote virtual rings are supported by routers. Required to communicate with remote rings is a virtual ring configuration. The half-bridge configu- ration is not supported. The IBM half bridge does not use the concept of virtual rings. Two IBM half bridges use two rings. The virtual ring advan- tage is in a topology that features many SRBs. In such an arrangement, only a single unit is required at a central site. A property not found in physical ring topologies is what makes up re- mote virtual rings. The LAN/WAN administrator determines the logical connectivity. Two options are available: partially meshed topologies (sometimes called redundant star topologies) or fully meshed topologies. In a partially meshed topology, a single central location (such as an FEP Token Ring) is connected to all access locations. Each access location is Auerbach Publications © 2000 CRC Press LLC logically connected to the central FEP rings and is not connected to any other ring. Partially meshed topologies using virtual rings do not permit direct communication between remote rings. However, communication is allowed from the central ring to the remote rings, which also allows communication among remote rings through the central ring. DESIGNING SDLC, SDLLC, AND QLLC LAN/WAN SYSTEMS This part of the article very briefly describes three techniques designed to enable LAN/WAN system capabilities for SNA-based network architectures: 1. SDLC via STUN 2. SDLLC implementation 3. QLLC conversion Tunneling SDLC via STUN SDLC via serial tunneling (STUN) encapsulates SDLC frames into Internet Protocol (IP) packets and routes the encapsulated packets over IP-support- ed LAN/WAN media. According to Cisco, the SDLC frame is transmitted without modification. Also, the information within the frame is transparent to the LAN/WAN. All SNA physical unit (PU) types are supported. Implementing SDLLC Serial-attached devices using the SDLC protocol are allowed to commu- nicate with LAN-attached devices using the Logical Link Control, type 2 (LLC2)1 protocol via the SDLLC function. To consolidate the traditionally disparate SNA/SDLC networks onto a LAN-based, multiprotocol, multi- media backbone network is the basic purpose of the SDLLC function. Routers use the SDLLC feature to forward the LLC2 traffic through re- mote source-route bridging (RSRB) over a point-to-point or IP LAN/WAN, terminate SDLC sessions, and to translate SDLC to the LLC2 protocol. According to Cisco, routers support SDLLC over all such media through IP encapsulation because a router-based IP LAN/WAN can use any arbitrary media, such as FDDI, Frame Relay, X.25, or leased lines. Exhibit 2 illustrates a general SDLLC media translation LAN/WAN inter- network arrangement.2 Converting QLLC A data-link protocol defined by IBM that allows SNA data to be transport- ed across X.25 networks is known as QLLC. According to Cisco, each SDLC physical link is replaced by a single virtual circuit with QLLC. Also, according to Cisco, QLLC conversion is a feature of its IOS Software3 that causes the router to perform all of the translation required Auerbach Publications © 2000 CRC Press LLC EXHIBIT 2 — SDLLC Media Translation to send SNA data over an X.25 network so that IBM devices that are con- nected to a router do not have to be configured for QLLC. DESIGNING APPN INTERNETWORKS Because APPN has many of the characteristics of the LAN networks and still offers the advantages of an SNA network, with APPN, one can con- solidate the two networks (an SNA subarea network and an interconnect- ed LAN network). The following are the major benefits of using APPN: • APPN provides an effective routing protocol to allow SNA traffic to flow natively and concurrently with other protocols in a single net- work. • APPN supports subarea4 applications as well as newer peer-to-peer applications over a single network. • Connections are peer-to-peer, allowing any end user to initiate a con- nection with any other end user without the mainframe (VTAM) in- volvement. • Traditional SNA class of service (COS)/transmission priority can be maintained. One feature has remained critical to many users: COS,5 as SNA has evolved. On an SNA session basis (on the backbone), this feature pro- vides traffic prioritization. This, in turn, allows a single user to have ses- sions with multiple applications, each with a different COS. In APPN, this feature offers more granularity and extends this capability all the way to the end node rather than just between communication controllers. Identifying Types of APPN Nodes An APPN LAN/WAN has three types of nodes: local entry networking (LEN) nodes, end nodes (EN), and network nodes (NN). According to Cisco, the control point (CP), which is responsible for managing a node’s Auerbach Publications © 2000 CRC Press LLC resources and adjacent node communication in APPN, is key to an APPN node. The APPN control point is the APPN equivalent of the signaling connection control part (SSCP).6 Using Local Entry Networking (LEN) Nodes LEN nodes are pre-APPN, peer-to-peer nodes. They can participate in an APPN LAN/WAN using the services provided by an adjacent network node. The CP of the LEN node manages the local resources but does not establish a CP-CP session with the adjacent network node.
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