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Designing & Implementing an OSPF Network Table of Contents Designing & Implementing an OSPF Network OSPF Network Design Cisco's Implementation of OSPF Network Design Goals Functionality Scalability Adaptability Manageability Cost Effectiveness Network Design Issues Network Design Methodology Step 1: Analyze the Requirements Step 2: Develop the Network Topology Step 3: Determine Addressing & Naming Conventions Step 4: Provision the Hardware Step 5: Deploy Protocol and IOS Features Step 6: Implement, Monitor, and Manage the Network Configuring OSPF on Cisco Routers Enabling OSPF on an Inter-Area Router Configuring an Area Border Router (ABR) Configuring an Autonomous System Boundary Router (ASBR) Configuring a Backbone Router Configuring a Simplex Ethernet or Serial Interface Configuring OSPF Tunable Parameters Configuring Route Calculation Timers Creating a Loopback Interface Configuring OSPF for Different Network Types Configuring OSPF for Broadcast or Nonbroadcast Multiaccess Networks Configuring OSPF for Nonbroadcast Networks Configuring OSPF for Point-to-Multipoint Networks Configuring OSPF Area Parameters Configuring OSPF Not-So-Stubby Areas (NSSAs) NSSA Implementation Considerations Configuring Route Summarization Between OSPF Areas Configuring Route Summarization when Redistributing Routes into OSPF Generating a Default OSPF Route during Redistribution Configuring Lookup of DNS Names Forcing the Router ID Choice with a Loopback Interface http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm (1 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network Disable Default OSPF Metric Calculation Based on Bandwidth Configuring OSPF Over On-Demand Circuits Implementation Considerations for OSPF over On-Demand Circuits Multicast OSPF OSPF and the Multi-Protocol Router (MPR) OSPF & Novell's MPR Chapter Summary Case Study: Designing an OSPF Frame Relay Network Wide Area Network Design Requirements Determining the Frame Relay PVC Architecture Determining if There Will Be Multi-Protocol Support Determining the Application Data Flow Determining the Number of Routers Determining TCP/IP Addressing Determining Internet Connectivity Determining Enterprise Routing Policies Establishing Security Concerns OSPF Network Design TCP/IP Addressing OSPF Area Organization Specifying the OSPF Network Type Implementing Authentication Configuring Link Cost Tuning OSPF Timers Strategizing Route Redistribution Case Study Conclusions Frequently Asked Questions Designing & Implementing an OSPF Network "Imagination: A mind once stretched by a new idea never regains its original dimensions." Successories This chapter covers the actual process of sitting down and designing your OSPF network. The real process of putting the pen to paper and the true process behind it is covered. It is this chapter's intention to take the mystery out of designing any type of network. The concepts and steps discussed have universal application whether your network is BGP or OSPF; of course, the latter is emphasized. Chapter 6, "Advanced OSPF Design Concepts," covered many of the commands necessary for configuring OSPF. In this chapter, you will become familiar with the necessary steps to actually begin the OSPF process on a Cisco router. You already know there are many potential network architectures where you would have to configure OSPF, and the most common are covered in this chapter. This chapter has two specific sections as follows: ● OSPF Network Design. This section reviews the specific network design goals that should be the general basis of every network. There are certain issues that you must be aware of as network designers, and they are discussed in this section. The six fundamental steps that make up the Network Design Methodology are covered with special enhancements given to issues regarding OSPF. ● Configuring OSPF on Cisco Routers. At this point in the book, you have everything you need to know about how http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm (2 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network OSPF works and how to go about designing and implementing an OSPF network. But how do you turn on OSPF? This section addresses basic and advanced configuration issues as relate to Cisco routers. A bonus area is covered as well that deals with multi-protocol routers. OSPF Network Design This book has discussed the various design techniques for OSPF, from the various golden rules to the number of routers per area. It is now time to actually take this information and begin the process of designing an OSPF network. Let's begin the process by determining what is actually supported by Cisco Systems. Cisco's Implementation of OSPF As discussed in Chapter 4, "Introduction to OSPF," there is a variety of RFCs that deal with OSPF. By now, you should be familiar with the many different features available within the OSPF protocol. But which RFCs does Cisco support within its products? ● RFC 1253: Open Shortest Path First (OSPF) MIB. This RFC contains the information, which provides management information relating to OSPF. ● RFC 1583: OSPF Version 2. Cisco's implementation conforms to the specifications as detailed in this RFC. They support the following key features: stub areas, route redistribution, authentication (covered later), tunable interface parameters, and virtual links. ● RFC 1587: Not-So-Stubby-Areas (NSSA). Cisco equipment supports the use of all types of stub areas. ● RFC 1793: OSPF over Demand Circuits. Cisco supports this RFC as well. Network Design Goals It is not necessary to get into the reasons behind your decision to build an OSPF network or any of the previously covered definitions of what a network is. However, the five basic goals that you should keep in mind while designing your OSPF network (or any network for that matter) should be adhered to: ● Functionality ● Scalability ● Adaptability ● Manageability ● Cost effectiveness Functionality "The network must work" is the absolute bottom line. Because networks are an integral part of enabling individual users to do their jobs, this is essential. It is here that the use of Service Level Agreements (SLAs) is essential. You must know what is expected of the network in order to design it properly. http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm (3 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network Scalability As your organization grows, the network must be able to keep pace. Your network and its initial design must enable it to expand accordingly. A network that cannot keep pace with the organization's needs is not much use. Routing summarization is a major factor in the success of designing your network. If you want to ensure your network can scale properly, the summarization is the biggest factor on your success. Without summarization, you will have a flat address design with specific route information for every host being transmitted across the network, a very bad thing in large networks. To briefly review summarization, remember that routers summarize at several levels, as shown in Figure 7-1. For example, hosts are grouped into subnetworks, subnetworks are then grouped into major networks, and these are then consolidated in areas. The network can then be grouped into an autonomous system. Note There are many smaller networks that desire to use a "standard" routing protocol such as OSPF. These networks can, for example, have 100 or less routers with a relatively small IP space. In these situations, summarization may not be possible and might not gain much if it were implemented. Adaptability Adaptability refers to your network's capability to respond to changes. In most cases, adaptability refers to your network's capability to embrace new technologies in a timely and efficient manner. This becomes extremely important as the network ages because change within networking is racing forward at breakneck speeds. Though it is not necessary to always be on the leading or bleeding edge there is a lot to be said for letting others find the bugs! Figure 7-1: Route summarization affects network scalability. :on0407.fm Manageability To provide "true" proactive network management is the goal here. The network must have the proper tools and design to ensure you are always aware of its operation and current status. Cost Effectiveness In this case, I have saved the true bottom line of network design for last. The reality of life is that budgets and resources are limited, and building or expanding the network while staying within the predetermined budget is always a benefit to your career and proper network design. Although there are five basic goals of network design that can be followed in any situation, I think there also should be a certain mindset during the process. This mindset is regarding the actual technology you will be using. It is very important to use state-of-the-art technologies whenever possible, though this does not mean to use unproven or inadequately tested technology. The reasoning behind this is that by spending a little extra money up front, you are investing with an eye to the future knowing that the network you are building will be able to grow, from a technological standpoint, longer than otherwise possible. Network Design Issues http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm (4 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network Up until this point, the various network design goals and the methodology needed to make the goals become a reality have been discussed. There are also certain design issues that you must consider when working through the network design process: ● Reliability. When designing networks, reliability is usually the most important goal, as the WAN is often the backbone of any network. ● Latency. Another big concern with users occurs when network access requests take a long time to be granted. Users should be notified about a latency problem in the network. ● Cost of WAN resources. WAN resources are expensive, and as such, frequently involve a tradeoff between cost efficiency and full network redundancy. Usually cost efficiency wins. ● Amount of traffic. This is a very straightforward consideration. You must be able to accurately determine the amount of traffic that will be on the network in order to properly size the various components that will make it up. As you implement the network, you should also develop a baseline that can be used to project future growth. ● Allowing multiple protocols on the WAN. The simplicity of IP is of great benefit to any network. For example, by only allowing IP-based protocols on the network you will avoid the unique addressing and configuration issues relating to other protocols. ● Compatibility with standards or legacy systems. Compatibility is always going to be an issue within your network throughout its life. As a network designer, you need to always keep this in mind as you proceed. ● Simplicity and easy configuration. Having been a network engineer for many years and involved in network management, this feature is doubly important to me. You might only be involved in the design and implementation of the network and not the management. In that case, the knowledge you will develop will need to be passed on to those who will manage the network. Ensure that you keep the ideas of simplicity and ease of configuration in mind while you develop your design documents for the network. ● Support for remote offices and telecommuters. In today's telecommunications environment, remote satellite offices are becoming commonplace and require network connectivity, so you must plan accordingly. The estimates say that every day you will see companies increase the number of telecommuters. You must keep this in mind as you determine the placement of network components to ensure that they can handle this requirement when it becomes a priority for your organization. Network Design Methodology There are six common steps that can be used to design your OSPF network, or any network for that matter. This are not set in stone and will not guarantee the "perfect" network, but they will provide you with realistic steps and considerations that if taken into account will make for well designed network. These steps will also help you avoid getting caught up in all the "bells and whistles" available in the new-enhanced-ultra-secret- turbo-series-network-equipment which is the answer to all your networking needs. These steps to designing a network have been proven not only over time, but also through countless networks that have been designed and implemented based upon this standard. 1. Analyze the requirements. http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm (5 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network 2. Develop the network topology. 3. Determine addressing and naming conventions. 4. Provision the hardware. 5. Deploy protocol and IOS features. 6. Implement, monitor, and maintain the network. Although your network might not have the technology du jour, it might not really need it if you objectively determine the needs of a network by following this design methodology (as shown in Figure 7-2). Figure 7-2: Network design methodology. Step 1: Analyze the Requirements This step will detail the process of determining expectations and then converting those into a real network or explaining why everyone can't have video conferencing on the desktop. Note What do you know? Going into Step 1, you know that an OSPF network is required but not what it will need to accomplish for your users or how you will need to physically design the network. http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm (6 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network Granted, the needs of users are always changing, and sometimes they do not even know what they need. There I said it! However, it is true; they know what they want and when they want it, which is always now or yesterday. Nevertheless, from a network design prospective, they do not always know what they need or why they need it. Nevertheless, you, as the network engineer involved in the design of the network, must still objectively listen and determine user needs. In the end, they are going to be the customers of network, and the customer is always right. You must also take into consideration what the future might hold for them. Therefore, you should ask the users what needs they see themselves having in the future. This question should be directed toward their jobs because it is your responsibility to take their response and turn that into the requirements of the network. A corporate vision is always important. For example, do the long-range corporate plans include having a Web site? If so, what will it be doing? How about running voice over the network? What about video conferencing; is that going to be a corporate need? Additional data you might want to consider gathering is the current organization structure, locations, and flow of information within the organization and any internal or external resources available to you. Armed with this information, your networks need analysis, you should then begin determining the cost and benefit analysis. Of course in many cases you will not be able to get all the equipment or bandwidth you think is necessary. Therefore, it is also advisable to create a risk assessment detailing the potential problems or areas of concern regarding the network design. OSPF Deployment As you go through the process of determining the network requirements, keep in mind some important questions regarding the requirements of OSPF. The answers to these questions will help you further define the requirements of your OSPF network. ● How should the OSPF Autonomous System be delineated? How many areas should it have and what should the boundaries be? ● Does your network and its data need to have built-in security? ● What information from other Autonomous Systems should be imported into your network? ● Which sites will have links that should be preferred (lower cost)? ● Which sites will have links that should be avoided (higher cost)? Load Balancing with OSPF As you go through the process of determining the network requirements, keep in mind the load balancing feature of OSPF. In the Cisco implementation of OSPF, any router can support up to four equal-cost routes to a destination. When a failure to the destination is recognized, OSPF immediately switches to the remaining paths. OSPF will automatically perform load balancing allow equal-cost paths. The cost associated is determined (default) by the interface bandwidth statement unless otherwise configured to maximize multiple path routing. Before Cisco's IOS release 10.3, the default cost was calculated by dividing 1,000,000,000 by the default bandwidth of the interface. However, with IOS releases after 10.3, the cost is calculated by dividing 1,000,000,000 by the configured bandwidth of the interface as illustrated in Figure 7-3. http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm (7 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network Note In IOS 11.3, this issue has been addressed with the command ospf auto-cost reference bandwidth. Figure 7-3: OSPF costs. OSPF Convergence OSPF convergence is extremely fast when compared to other protocols; this was one of the main features included within its initial design. To keep this desirable feature fully functional in your network, you need to consider the three components that determine how long it takes for OSPF to converge: ● The length of time it takes OSPF to detect a link or interface failure ● The length of time it takes the routers to exchange routing information via LSAs, rerun the Shortest Path First algorithm, and build a new routing table ● A built-in SPF delay time of five seconds (default value) Thus, the average time for OSPF to propagate LSAs and rerun the SPF algorithm is approximately 1 second. Then the SPF delay timer of five seconds must elapse. Therefor OSPF convergence can be a anything from 6 to 46 seconds, depending upon the type of failure, SPF timer settings, size of the network, and size of the LSA database. The worst case scenario is when a link fails but the destination is still reachable via an alternate route, because the 40 second default dead timer will need to expire before the SPF is rerun. Step 2: Develop the Network Topology This step will cover the process of determining the networks physical layout. There are generally only two common design topologies: meshed or hierarchical. The following sections take a look at each to see which is the most efficient design for today's networks. Note What do you know? Going into Step 2, you've developed a list of the requirements associated with this OSPF network. You have also begun to lay out the financial costs associated with the network based upon this information. These costs could include equipment, memory, and associated media. Meshed Topology In a meshed structure, the topology is flat and all routers perform essentially the same function, so there is no clear definition of where specific functions are performed. Network expansion tends to proceed in a haphazard, arbitrary manner. This type http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm (8 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network of topology is not acceptable to the operation of OSPF. It will not correctly support the use of areas or designated routers. Hierarchical Topology In a hierarchical topology, the network is organized in layers that will have clearly defined functions. In this type of network there are three layers: ● Core Layer. This would make an excellent place for OSPF Backbone Routers that are all connected through area 0. All of these routers would be interconnected, and there should not be any host connections. This is because its primary purpose is to provide connectivity between other areas. ● Distribution Layer. It is here that you would locate other OSPF areas all connected through Area Border Routers (ABRs) back to the Core Layer (area 0). This is also a good location to begin implementing various network policies such as security, DNS, etc ) ● Access Layer. This is where the inter-area routers that provide connections to the users would be located. This layer ID is where the majority of the hosts and servers should be connecting to the network. By using this type of logical layered network design, you will gain some benefits that will help you design the network as shown in Figure 7-4. Figure 7-4: OSPF hierarchical topology. The benefits of the OSPF hierarchical topology as implemented in Figure 7-4 are as follows: ● Scalable. Networks can grow easily because functionality is localized so additional sites can be added easily and quickly. ● Ease of Implementation. This physical topology fits easily into OSPF's logical hierarchy, making network implementation easier. ● Ease of Troubleshooting. Because functionality is localized, it is easier to recognize problem locations and isolate http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm (9 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network them. ● Predictability. Because of the layered approach, the functionality of each layer is much more predictable. This makes capacity planning and modeling that much easier. ● Protocol Support. Because an underlying physical architecture is already in place if you want to incorporate additional protocols, such as BGP, or if your organization acquires a network running a different protocol, you will be able to easily add it. ● Manageability. The physical layout of the network lends itself towards logical areas that make network management much easier. There are other variations of the three-layered hierarchical design that are available are one layer distributed, hub and spoke and two layers, but they are beyond the scope of this book. At this point, though, you can see that the three layered hierarchical model fits perfectly into OSPF's logical design, and it is this model on which you will be basing your network design. Before discussing how to implement and design this type of model, you need some basic OSPF backbone design suggestions. OSPF Backbone Design in the Hierarchical Model The process of designing the backbone area has been previously discussed, so it will be only briefly reviewed here. Always keep the backbone area as simple as possible by avoiding a complex mesh. Consider using a LAN solution for the backbone. The transit across the backbone is always one hop, latency is minimized, and it is a simple design that converges very quickly. Figure 7-5 illustrates a simple OSPF backbone design. Figure 7-5: Simple OSPF backbone design. You know that you should keep users off the backbone because it is only a transit area, but that is not enough. You also need to consider securing your backbone physically. As a network critical shared resource, the routers need to be physically secure. If you use the previously mentioned LAN backbone solution, then securing your network can be relatively easy; just put it in a secure closet or rack as shown in Figure 7-6. Figure 7-6: Isolate the backbone and secure it both physically and logically. http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm (10 of 58) [5/6/2001 3:47:31 PM] [...]... and engage in proactive network management Network Management and Monitoring Applications Network management applications that use Simple Network Management Protocol (SNMP) provide a useful array of tools to control internetwork support costs: q Cisco debug and show commands q Syslogd q Protocol analyzers q DNS q TFTP and FTP q DHCP and BOOTP q Telnet q TACACS q Cisco Works (Router configuration management,... on an OSPF interface ip ospf retransmit-interval seconds Specify the number of seconds between link-state advertisement retransmissions for adjacencies belonging to an OSPF interface http://www.cisco.com/cpress/cc/td/cpress/design /ospf/ on0407.htm (26 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network ip ospf transmit-delay seconds Set the estimated number of seconds it takes to transmit... appropriate network statements to the OSPF routing process with the correct Area ID, for example: router ospf 109 network 130.10.8.0 0.0.0.255 area 0 network 172.25.64.0 0.0.0.255 area 1 http://www.cisco.com/cpress/cc/td/cpress/design /ospf/ on0407.htm (24 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network 4 Then you will want to add the area range command so that the networks within... summarization improves OSPF network stability IP addresses in an OSPF network should be grouped by area, and you can expect to see areas with some or all of the following characteristics: q Major network number(s) q Fixed subnet mask(s) q Random combination of networks, subnets, and host addresses It is important that hosts, subnets, and networks be allocated in a controlled manner during the design and implementation... http://www.cisco.com/cpress/cc/td/cpress/design /ospf/ on0407.htm (27 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network OSPF_ Router# conf t Enter configuration commands, one per line End with CNTL/Z OSPF_ Router(config)# interface loopback 0 OSPF_ Router(config-if)# ip address 10.251.11.1 255.255.255.255 OSPF_ Router(config-if)# description Configured to be OSPF Router ID Configuring OSPF for Different Network Types... message-digest ip ospf authentication-key ip ospf hello-interval ip ospf dead-interval timers spf spf-delay spf-holdtime You can use the show ip ospf border-routers command to see the area border routers within your network This command is explained in more detail in Chapter 8, "Monitoring & Troubleshooting an OSPF Network. " Configuring an Autonomous System Boundary Router (ASBR) The process of configuring an autonomous... the OSPF backbone and beyond, causing unnecessary network traffic and router overhead Whenever an LSA is sent, all affected OSPF routers will have to recompute their LSA database and routes using the SPF algorithm Figure 7-12: No route summarization will cause network problems http://www.cisco.com/cpress/cc/td/cpress/design /ospf/ on0407.htm (16 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF. .. process-id command 3 Assign the appropriate network statements to the OSPF routing process with the correct Area ID, for example: router ospf 109 network 130.10.8.0 0.0.0.255 area 0 network 172.25.64.0 0.0.0.255 area 1 4 Then you will want to add the area range command so that the networks within each area can be properly summarized, for example: router ospf 109 network 130.10.8.0 0.0.0.255 area 0 network. .. configuration management, network analysis) http://www.cisco.com/cpress/cc/td/cpress/design /ospf/ on0407.htm (22 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network Configuring OSPF on Cisco Routers OSPF typically requires coordination among many internal routers, area border routers (routers connected to multiple areas), and autonomous system boundary routers At a minimum, OSPF- based routers,... the previous section on "Enabling OSPF on an Inter-Area Router." http://www.cisco.com/cpress/cc/td/cpress/design /ospf/ on0407.htm (23 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network 3 Assign the appropriate network statements to the OSPF routing process with the correct area ID, for example: router ospf 109 network 130.10.8.0 0.0.0.255 area 0 network 172.25.64.0 0.0.0.255 area . Designing & Implementing an OSPF Network Table of Contents Designing & Implementing an OSPF Network OSPF Network Design Cisco's. http://www.cisco.com/cpress/cc/td/cpress/design /ospf/ on0407.htm (2 of 58) [5/6/2001 3:47:31 PM] Designing & Implementing an OSPF Network OSPF works and how to go about designing and implementing