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428 Chapter 8 • Designing the Infrastructure such as variable length subnet mask (VLSM), classless inter-domain routing (CIDR), and a routing protocol that can support these methods. Possible Types of Topology Design Once you have established your internetwork scheme, you must design a way for handling interconnections among sites within the same region or area of admin- istrative control. In designing regional WANs, whether you are using packet- switching services or point-to-point interconnections, three basic design approaches are common throughout the industry: ■ Star topologies ■ Fully meshed topologies ■ Partially meshed topologies In the following pages, I will try to help you understand these topologies and how you can use them to your advantage. Remember, though, that the discus- sions presented in this chapter address the application of these topologies specifi- cally to packet-switching services. NOTE Illustrations in this chapter use lines to show the connection of specific routers on the PSDN network. These connections are considered virtual connections, as the circuits are mapped within the routers themselves. Normally, all physical connections are generally made to switches within the PSDN. Unless otherwise specified, the connecting lines represent vir- tual connections within the PSDN. Star Topologies The star topology (also known as a hub and spoke) is a grouping of network devices that has a single internetworking hub, and provides connections for the external cloud networks to the backbone and access to each other, although only through the core router. Figure 8.3 illustrates a packet-switched star topology for a regional internetwork. One of the main advantages of a star topology is that there is simplified man- agement and minimized tariff costs or tolls.Whereas tolls aren’t much of a factor www.syngress.com 130_ASP_08 6/19/01 2:52 PM Page 428 Designing the Infrastructure • Chapter 8 429 these days, they were in the past, and with some of the things that are happening politically, they could be again. However, there are significant disadvantages. First, the core router is a single point of failure for the entire internetwork. Second, the core router can limit overall performance for access to backbone resources; the core may not be robust enough, or have enough bandwidth to handle all of the traffic from the external networks.Third, this topology is not very scalable, as there are generally only a certain number of ports on the core router. Fully Meshed Topologies In a fully meshed topology, each routing node on the edge of a packet-switching network has a direct path to every other node on the cloud. Figure 8.4 illustrates this type of arrangement. One of the best reasons for creating a fully meshed environment is that it provides for a high level of redundancy.A fully meshed topology helps to facili- tate the support of all routing protocols, but it is not tenable in large packet- switched internetworks. Some of the main issues are due to the large number of virtual circuits that are required (one for every connection between routers).There are also problems associated with the large number of packet and broadcast replications necessary for routing protocols or application traffic, and the configuration complexity for routers in the absence of multicast support in nonbroadcast environments. www.syngress.com Figure 8.3 Star Topologies for a Regional Internetwork Router Router Core Router Router Router Router Regional Packet- Switching Network Connection to Backbone CloudCloud Cloud Cloud Cloud 130_ASP_08 6/19/01 2:52 PM Page 429 430 Chapter 8 • Designing the Infrastructure There is a middle ground, though; by combining fully meshed and star topologies into a partially meshed environment, you can improve fault tolerance without encountering the performance and management problems that are nor- mally associated with a fully meshed internetwork.The following section dis- cusses the partially meshed topology. Partially Meshed Topologies As discussed earlier, a partially meshed topology reduces several of the problems that are inherent in star and fully meshed topologies.There is a reduction in the number of routers within a region that need direct connections to all other nodes in the region. Not all nodes need to be connected to all other nodes. For a non- meshed node to communicate with another nonmeshed node, it will send traffic through one of the hub routers.This is a lot like the star topology, but there is a redundant path available in the event the hub router becomes inoperable. Figure 8.5 illustrates such a situation. There are many forms of partially meshed topologies. Generally, partially meshed implementations provide the optimum balance for regional topologies in terms of the number of virtual circuits, their ability to provide redundancy, and their overall performance. By providing a greater amount of connectivity and high availability, you will be able to use your bandwidth more effectively. www.syngress.com Figure 8.4 Fully Meshed Topology Router Router Router Router Router Router Packet-Switching Network 130_ASP_08 6/19/01 2:52 PM Page 430 Designing the Infrastructure • Chapter 8 431 Broadcast Issues Broadcast traffic presents problems when it is introduced into a packet-service environment. Broadcasts are necessary for a node to reach multiple other node stations with a single packet when the sending node does not know the address of the intended recipient, or when the routing protocols needs to send hello packets and other miscellaneous services. As an example, the level of broadcast traffic that is generated in an Enhanced IGRP environment depends on the setting of the enhanced IGRP hello-timer interval.The size of the internetwork determines other issues. In a small network, the amount of broadcast traffic generated by Enhanced IGRP nodes might be higher than with comparable internal gateway routing protocols that run on the Internet. However, for large-scale internetworks, Enhanced IGRP nodes generate sub- stantially less broadcast traffic than RIP-based nodes, for example. NOTE Usually, it is a good practice to manage packet replication when going over your design considerations. When integrating broadcast-type LANs (such as Ethernet) with nonbroadcast packet services (such as X.25), you should try to figure out where replication will cause bottlenecks within your network. With the multiple virtual circuits that are characteristic of connections to packet-switched environments, routers need to replicate broadcasts for each virtual circuit on a given physical link. www.syngress.com Figure 8.5 Partially Meshed Topology Router Router RouterRouter Hub Router (Redundant) Hub Router (Redundant) Frame Relay Network 130_ASP_08 6/19/01 2:52 PM Page 431 432 Chapter 8 • Designing the Infrastructure Within a highly meshed environment, the replicating broadcasts can be resource intensive in terms of increased required bandwidth and number of CPU cycles. Because of this, highly meshed networks are impractical for large packet- switching networks. However, circuit meshing is essential to enable fault tolerance. You really need to balance the trade-offs in performance with requirements for redundancy. Also remember that as you scale your network, there will be other issues that fall within the same vein; as you add routing nodes, you will want to add redundancy, which will add at least two paths to your core infrastructure. Performance Issues When designing your WAN around a specific application service type, you should consider the characteristics of the virtual circuit. Sometimes the perfor- mance of a virtual circuit will depend on its capability to handle mixed-protocol traffic. Depending on how the traffic is queued and streamed from one node to the next, certain applications may require special handling. One solution might be to assign specific circuits to specific application and protocol types. There are always going to be performance concerns for specific packet- switching services.That is why there is the ability to include Committed Information Rates (CIR) in Frame Relay internetworks and window size limita- tions in X.25 networks. (The CIR matches the maximum average rate per con- nection for a period of time.) What is highly common within the ISP market is to sell guaranteed CIRs to your customers and give them the ability to “burst” (for a fee, of course) outside of the limits that they were given.As it is, a CIR is the minimum amount of bandwidth that your client is guaranteed at any point in time.The CIR is usually defined within the service level agreement (SLA) to which you and the customer agreed. Frame Relay Internetwork Design Considerations A major concern when designing a Frame Relay implementation is scalability.As the number of remote clients and their links grows, your network must be able to grow to accommodate these growth spurts.The network must also provide a high level of performance, yet minimize support and management requirements. Meeting all these objectives can be quite a feat.The following sections focus on some of the critical factors for Frame Relay internetworks, such as: www.syngress.com 130_ASP_08 6/19/01 2:52 PM Page 432 Designing the Infrastructure • Chapter 8 433 ■ Hierarchical design ■ Regional topologies ■ Broadcast issues ■ Performance issues The following are suggestions to provide a solid foundation for constructing scalable networks that can balance performance, fault tolerance, and cost. Again, I am only using Frame Relay as a template; it is not the only technology that you can use. Hierarchical Design for Frame Relay Internetworks As discussed earlier in this chapter, the arguments supporting hierarchical design for packet-switching networks apply to hierarchical design for Frame Relay net- works. Remember the three factors that lead us to recommend the implementa- tion of a hierarchical design: ■ Scalability ■ Manageability ■ Optimization of broadcast and multicast control traffic One of the ways in which many Frame Relay vendors charge for services is by data link connection identifier (DLCI) numbers.These DLCI numbers iden- tify a Frame Relay permanent virtual connection (PVC, also known as a perma- nent virtual circuit, which is the X.25 terminology).The DLCI number is locally significant, and defines the connection between Frame Relay elements.The number of Frame Relay PVCs within the network is highly dependent on what protocols are in use and actual traffic patterns. To figure out how many DLCIs are going to be used within your environ- ment, and how many will be mapped to your interfaces, depends on several fac- tors that should be considered together: ■ What protocols are being routed? Any protocol that is broadcast intensive will constrain the number of assignable DLCIs. For example, AppleTalk is a routed protocol characterized by high levels of broadcast overhead.Another example is Novell Internetwork Packet eXchange (IPX), which sends both routing and service updates, which results in www.syngress.com 130_ASP_08 6/19/01 2:52 PM Page 433 434 Chapter 8 • Designing the Infrastructure higher broadcast bandwidth overhead. In contrast, IGRP is less broadcast intensive, because it will send routing updates less often (by default, every 90 seconds).You can modify the timer for IGRP, so it can become broad- cast intensive if timers are modified to send updates more frequently. ■ What are the levels of broadcast traffic? Broadcasts, such as routing updates, are one of the most important considerations when determining the number of DLCIs that can be defined.The amount and type of broadcast traffic will be a factor in your ability to assign DLCIs within this general recommended range. ■ What is the speed of the connection? Broadcast traffic levels are expected to be high, so you should consider faster links and DLCIs with higher CIR and higher burstable limits.You should also consider imple- menting fewer DLCIs. ■ Are there any static routes? If static routing is implemented, you can use a greater amount of DLCIs per line, because a larger number of DLCIs will help to reduce the level of broadcasting. To assist in your design considerations, here are two forms of hierarchical design that you can implement: ■ The hierarchical meshed Frame Relay internetwork ■ The hybrid meshed Frame Relay internetwork These designs have their advantages and disadvantages, and are compared in the following sections. Hierarchical Meshed Frame Relay Internetworks Implementing a hierarchical mesh for Frame Relay environments can assist you in avoiding implementing an excessively large number of DLCIs.This will allow for a more manageable, segmented environment.The hierarchical meshed envi- ronment features full meshing within the core PSDN and throughout the sur- rounding networks. Locating routers between network elements creates the hierarchy. Figure 8.6 illustrates a simple hierarchical mesh.The internetwork shown illustrates a fully meshed backbone, with meshed regional internetworks and broadcast networks at the outer edges. www.syngress.com 130_ASP_08 6/19/01 2:52 PM Page 434 Designing the Infrastructure • Chapter 8 435 The advantage of the hierarchical mesh is that it scales well and helps to localize traffic. By placing routers between fully meshed portions of your net- work, you limit the number of DLCIs that need to be configured per physical interface, segment your internetwork, and make the network more manageable. However, please remember these two issues when implementing a hierarchical mesh: ■ Broadcast traffic and packet replication In an environment that has a many routers with multiple DLCIs per interface, there will be exces- sive broadcast and packet replication, which can impair overall perfor- mance. Due to a high level of meshing throughout a network, there will be excessive broadcasts and packet replication that will be a significant resource threat. In the core, where throughput requirements are typically high, the prevention of bandwidth loss due to broadcast traffic and packet replication is particularly important. www.syngress.com Figure 8.6 Fully Meshed Hierarchical Frame Relay Environment X1 X2 Meshed Region X Meshed Region Y Frame Relay Backbone Y1 Y2 Z1 Z2 Meshed Region Z Router Router Router Router Router Router Router Router Router Router Router 130_ASP_08 6/19/01 2:52 PM Page 435 436 Chapter 8 • Designing the Infrastructure ■ Increased costs associated with additional router interfaces When compared with a fully meshed topology, additional routers will be necessary to split the meshed core from the meshed edge networks. However, by implementing these routers, you are creating much larger networks that scale ad infinitum when compared to a fully meshed internetwork. Hybrid-Meshed Frame Relay Internetworks The cost-effective and strategic significance of the core network often forces net- work designers to implement a hybrid-meshed network for their WAN internet- works. A hybrid-meshed network is composed of redundant, meshed lines in the WAN core, and partially (or fully) meshed Frame Relay PSDNs on the network edge. Routers separate the two networks. Figure 8.7 illustrates such a hybrid arrangement. The hybrid hierarchical mesh designs can provide higher performance on the core because they can localize traffic and simplify the scaling of the network. Hybrid-meshed networks for Frame Relay can provide better traffic control in www.syngress.com Figure 8.7 Hybrid Hierarchical Frame Relay Internetwork X1 X2 Access Networks X1 and X2 Also Use Partially Meshed Topology Router Router Router Router Router Partially Meshed Regional Internetwork Connected by Frame Relay PSDN Router Fully Meshed Backbone Interconnected with Point-to-Point Leased-Lines Router Router Router Router Frame Relay Regional Network Router Router Point-to-Point Backbone 130_ASP_08 6/19/01 2:52 PM Page 436 Designing the Infrastructure • Chapter 8 437 the core and allow the backbone to be composed of dedicated links, which results in greater stability. Some of the disadvantages of hybrid hierarchical meshes include the high costs associated with leased lines, and increased broadcast and packet replication traffic. Regional Topologies for Frame Relay Networks There are generally three accepted designs that are relevant for a Frame Relay- based packet service regional network: ■ Star topology ■ Fully meshed topology ■ Partially meshed topology Each of these topologies is discussed in the following sections. Generally, I have emphasized partially meshed topologies as those that are integrated into a hierarchical environment; star and fully meshed topologies are discussed more for their structural context. Star Topologies Star topology was addressed earlier in the section,“Possible Types of Topology Design.” Star topologies are attractive because they minimize the number of DLCIs that are required, which will result in a lower-cost solution. However, some inherent issues are associated with the star topology because bandwidth limita- tions. In an environment in which a backbone router is attached to a Frame Relay cloud at 768 Kbps, and the remote sites are attached at 256 Kbps, there will be some throttling of traffic coming off the core that is intended for remote sites. A star topology does not offer the fault tolerance that is necessary for many networking situations. For example, if the link from the hub router to a specific cloud router is lost, all connectivity to that router is lost. Fully Meshed Topologies A fully meshed topology requires that every routing node connected to a Frame Relay network is logically linked by an assigned DLCI to every other node on the cloud.This topology is not easy to manage, support, or even implement for larger Frame Relay networks for several reasons: www.syngress.com 130_ASP_08 6/19/01 2:52 PM Page 437 [...]...130 _ASP_ 08 4 38 6/19/01 2:52 PM Page 4 38 Chapter 8 • Designing the Infrastructure s Large, fully meshed Frame Relay networks require many DLCIs.There is a requirement for each logical link between nodes to have a DLCI As shown in Figure 8. 8, a fully connected topology requires the assignment of [x(x-1)]/2 DLCIs, where x is the number of routers that will be directly connected Figure 8. 8 Fully Meshed... eliminate single point-of-failure issues.Virtual interfaces allow you to create networks using partially meshed Frame Relay designs, as shown in Figure 8. 9 www.syngress.com 130 _ASP_ 08 6/19/01 2:52 PM Page 439 Designing the Infrastructure • Chapter 8 Figure 8. 9 A Twin-Star Router, Partially Meshed Network Router Frame Relay Network Router Router Router Star Router (Redundant) Star Router (Redundant) To... Interior Gateway Routing Protocol (EIGRP) (Cisco only) was designed by Cisco for multiprotocol Cisco networks It supports IP, IPX, and AppleTalk networks www.syngress.com 447 130 _ASP_ 08 4 48 6/19/01 2:52 PM Page 4 48 Chapter 8 • Designing the Infrastructure Strengths: s Incremental updates to reduce broadcast traffic s Supports VLSMs s Uses multiple metrics s Fast convergence s Retains backward compatibility... www.syngress.com 130 _ASP_ 08 6/19/01 2:52 PM Page 453 Designing the Infrastructure • Chapter 8 NOTE The 80 /20 traffic rule has been steadily changing due to the rise of intranets and distributed applications With new and existing applications moving toward a distributed applications and storage model, which are accessed though Web retrieval, the traffic pattern is going toward the 20 /80 model, where only... applications will also increase.The premium service may be configured to not drop those packets until the buffer has reached 90 percent of its full capacity www.syngress.com 457 130 _ASP_ 08 4 58 6/19/01 2:52 PM Page 4 58 Chapter 8 • Designing the Infrastructure Since it is harder to reach the higher thresholds (unless the network becomes overly congested due to a large amount of higher priority applications),... links Microsoft has built this technology into Windows 2000 so that any Win2K server can activate this remote control capability www.syngress.com 130 _ASP_ 08 6/19/01 2:52 PM Page 455 Designing the Infrastructure • Chapter 8 Application-Aware Networking ASPs who want to deploy their applications need to realize that their success of mission-critical applications over both the internal LAN and clientele... popular, private addressing is almost completely up to you.The three private address spaces are: s Class A 10.x.x.x s Class B 172.16.x.x s Class C 192.1 68. x.x www.syngress.com 130 _ASP_ 08 6/19/01 2:52 PM Page 451 Designing the Infrastructure • Chapter 8 These are private addresses, and cannot be routed on the Internet Using subnet masks will allow you to configure Network and Host IDs to suit your needs... Maintenance Protocol (RTMP) have an overhead that increases exponentially as you add connections These protocols should be the exception rather than the rule in an ASP environment www.syngress.com 459 130 _ASP_ 08 460 6/19/01 2:52 PM Page 460 Chapter 8 • Designing the Infrastructure Multimedia Services According to a study by the Telecommunications Industry Association, the multimedia application market (such... order, as this is probably how you will see them listed in other reference manuals Some of the strengths and limitations for each protocol are also listed www.syngress.com 445 130 _ASP_ 08 446 6/19/01 2:52 PM Page 446 Chapter 8 • Designing the Infrastructure Routing Information Protocol The Routing Information Protocol (RIP) was derived from Xerox Corporation’s XNS for IP networks It supports IP and IPX... IETF for IP networks It supports IP networks Strengths: s Usable on most vendors’ equipment s Only broadcasts routing table when changes are made www.syngress.com 130 _ASP_ 08 6/19/01 2:52 PM Page 447 Designing the Infrastructure • Chapter 8 Weaknesses: s Uses only bandwidth as a metric s Restricts some topologies Integrated Intermediate System-to-Intermediate System The Integrated Intermediate System-to-Intermediate . Figure 8. 9. www.syngress.com Figure 8. 8 Fully Meshed Frame Relay Network DLCIs Router Router Router Router Router Router 130 _ASP_ 08 6/19/01 2:52 PM Page 4 38 Designing the Infrastructure • Chapter 8. effectively. www.syngress.com Figure 8. 4 Fully Meshed Topology Router Router Router Router Router Router Packet-Switching Network 130 _ASP_ 08 6/19/01 2:52 PM Page 430 Designing the Infrastructure • Chapter 8 431 Broadcast. www.syngress.com Figure 8. 5 Partially Meshed Topology Router Router RouterRouter Hub Router (Redundant) Hub Router (Redundant) Frame Relay Network 130 _ASP_ 08 6/19/01 2:52 PM Page 431 432 Chapter 8 • Designing