| In this section, you discover how to plan a routing strategy. It is imperative that you first cover some basic material to be able to understand what you are planning and why you are planning it that way. As you are planning a routing strategy, you must also identify the routing protocols you will need to use in normal environments, as well as what protocols to implement while using multicast-based traffic. What Is Routing? Routing is the process of taking data from one network and moving it to another network. If you recall, in Chapter 2, "Planning, Implementing, and Maintaining a Network Infrastructure," you learned the differences in subnets and how networks are identified by devices, such as by using the subnet mask. We also covered how a routing decision is madefor example, when a packet has a destination address that does not match the network it is currently on and needs to get to the network it is destined for. This is why you need a default gateway , which can be a router or a device acting as a router. It is at the default gateway where the routing process truly takes place. Chapter 2 also covered the Open System Interface (OSI) and Department of Defense (DoD) models and TCP/IP. You learned how important these items are in today's network environment. You also learned about routing in general. Let's quickly review a simple network diagram showing one user accessing a single Web server. In Figure 4.1, it is clear that a user is accessing a Web page on a remote Web server. Figure 4.1. To get the Web page the user requests , the traffic is routed between networks across the Internet. Let's look at the details of the request in Figure 4.1. From this information, you can gain a fundamental understanding of routing and how data is routed from one network to another. -
First, the user on the PC LAN accesses a Web page by opening his Web browser (Internet Explorer) and entering the domain name or IP address of the server on which the page resides. In this case, that would be http://12.1.2.2/index.htm . -
Next , the user's computer, which is on the 10.0.0.0/24 subnet, makes a request to a server on the 12.1.2.0/24 subnet. Because they are two different subnets, the packets need to be sent to the default gateway configured on the client LAN. The default gateway is set to 10.0.0.1, which is where routing takes place. -
The packets enter the router's Ethernet port, which is connected to the local area network (LAN). This interface (the one connected to the LAN) is nothing more than a network interface card (NIC) that gives access to and from the Ethernet-based network. After the packets enter the router, the router strips them down to look inside. This is done to view the destination address, which is 12.1.2.2 on the destination network of 12.1.2.0/24. -
Next, the router (10.0.0.1) looks in its routing table. A routing table is nothing more than a list of networks that the router will service. In this case, the router has been configured with a routing entry to do the following: If a packet comes from the LAN and is destined to go to the 12.1.2.0 network, it sends the traffic out interface 1. This interface is connected to the far-side router, which is labeled 12.1.2.1. As such, after the packets come in, they are forwarded to the 12.1.2.1 router, where the Web server is located. -
After the 12.1.2.1 router receives the packets, it also looks inside the packets to see where the packets need to go. When the router opens and reads the packets, it sees that the packets are destined to go to the 12.1.2.0 network, and it is a host with an IP address of 12.1.2.2. What makes this process even easier is that a router also holds an Address Resolution Protocol (ARP) cache; as long as that router is up and directly connected to the same subnet as the Web server, the packet will more than likely be sent directly to the Web server. -
Finally, the Web server receives the packets (which are a request to see a Web page named index.htm ) and answers that request by producing the Web page to the remote client. The same process happens again, except in reverse order, to return the page to the requesting machine. In Figure 4.2, the scenario has been expanded to show a routing environment over a larger network. Figure 4.2. Complex routing environments offer multiple paths to the destination. Based on the previous example, you should be somewhat familiar with the basic process of getting the packets from the source to the destination. Now, however, you have a problem because more than one router separates the two networks that need to interact. So, how are the required routing decisions made in this situation? As you can see, there is a one-hop connection via Frame Relay to the Web server with 32KB of bandwidth. Also, you can see a two-hop connection from the client to the Web server that is separated by two T1 lines. So, in general, where would the router send the traffic? Over the one-hop connection, which is technically shorter, or over the two-hop connection, which has much more bandwidth? A router also makes decisions that are very complex, including deciding what could be fastera slower link over fewer hops or a multihop link that utilizes high bandwidth links. Static Versus Dynamic Routing Now we will look at what actually does the routing for you and what your involvement in the process is. For one, routing is not as simple as it was made to sound. This process can become very complex and, without a proper design, could actually ruin your network. We will describe the problems that can occur throughout the chapter, but you need to understand that you must plan your network for routing before you implement it. Major problems could arise if you don't plan. Following are some planning questions you should consider: -
Will you use a static or dynamic routing environment, and why would you use one over the other? -
Which routing environment is more secure? A static routing environment is one in which all routing entries in the routing table are entered manually. When static routing is used, the administrator responsible for the router must manually enter information into the router to allow it to perform its function. If you want the router to route packets intended for remote destinations, you need to configure its routing table so the router will forward the packets to the next hop along the path to their destination. This may be simply something such as "all traffic that is not addressed for the local subnet is to be forwarded out of the router on Interface 1" or perhaps something more complex if the router has multiple interfaces connected to different networks. A dynamic routing table is one that you create with a routing protocol such as Routing Information Protocol ( RIP ) or Open Shortest Path First ( OSPF ) , which we will explore more shortly. When dynamic routing is used, the chosen routing protocol actually builds the routing table on the fly by "learning" about the routes that are available to it and maintaining an accurate status on these routes. Should a route fail when dynamic routing is in use, all nearby routers will adjust their routing tables to prevent sending packets along the failed route. A simple example of a routing table is shown in Table 4.1. Table 4.1. Sample Routing Table | Destination Network | Interface to Use | Number of Hops | | 172.16.0.0/16 | Via 10.0.2.1 | 1 hop | | 10.0.0.0/8 | Via 10.0.2.1 | 1 hop | | 12.1.2.0/24 | Via 10.0.2.1 | 1 hop | | 0.0.0.0/0 | Via 10.0.2.1 | 1 hop | The router keeps this table in memory so that, when a packet comes in, it can look at the prefix (the first few bits), hop count, and destination network. It's that easy. A routing table tells packets where to go, and it can be created manually or dynamically. Routing tables compute the next hop for a packet. You must remember that a routing table needs to have two fields to function: the IP prefix and the next hop address (which must be a valid address). The router from which the packet is leaving and going to must be able to reach this valid address; you must be connected, or the route will not work. If you do not have a match (a packet comes in and does not have a routing table entry match), the packet is discarded, and you will most likely get an Internet Control Message Protocol ( ICMP ) notification that the destination host was unreachable. Following are some key points to consider about static versus dynamic routing: -
Static routing is tedious . You have to know exactly what you want to do because any mistake causes the router to not work properly, resulting in routing errors that can prevent network connectivity. -
You may want to implement static routing if you have only a few routes to maintain. Remember, each time a packet enters the router, the router needs to process it, so tables that are very long can cause the router to take longer to make a routing decision, which in turn could slow down your network. When you use dynamic routing protocols, it is possible to have a router learn as many as hundreds of routes, depending on network size. -
Static routes are more secure because only a few required routes are in the table, so no one can see too much information. In this case, if the router is compromised by an attacker, he or she cannot glean too much information from your compromised router. The attacker may know only a default route back to the core network, for example; whereas if the attacker compromises a core router with 300 networks in the table, he or she can map your whole network from that one router. -
Dynamic routing is easy to configure and, once configured, is easy to maintain. If changes are made to the network, more than likely the routers will learn the changes, and they can quickly establish convergence on the network. Convergence occurs when all routers know all other routers on the network, and the topology is accurate. -
Dynamic routing is becoming more secure as more and more routing protocols provide ways to encrypt and authenticate updates between peers. However, configuring dynamic routes requires that the administrator have a greater level of knowledge and experience than when configuring static routes. One of the key elements required to design and implement efficient routing solutions is to understand the differences between your available options and then to implement the correct one. You might be wondering whether static and dynamic routing protocols can be used at the same time. The answer to that question is yes. Distance Vector Versus Link State Now that you understand dynamic and static routing, let's dig deeper into dynamic routing and the complexities it offers. You need to know this information for the 70-293 exam because Microsoft Windows Server 2003 allows you to configure either static or dynamic routing. If you choose dynamic, you have the option of using either a Link State dynamic routing protocol or a Distance Vector dynamic routing protocol. You need to know the specifics of each and which one to select in a production environment. Link State Link State protocols are highly functional routing protocols that allow routers to pass information efficiently . A Link State routing protocol ensures that each router on the network maintains a map of the network. You should also know that any network is prone to have a problem or failure occur at any time. Frame Relay links have problems, Telcos have problems, and all of them affect your networks. For example, assume that one of your T1 lines experiences a failure that separates two routers on a subnet across the Internet. Both would be able to (through the routing protocol) adjust for this loss of the link between them because that is what they are programmed to do. When your router link fails, it is programmed to sense the loss of carrier on the line and then, through the routing protocol, send updates to the other routers adjacent to them so that they all know that the link is down and the IP subnet where they are maintained is no longer available for destination packets on the wire. When a network link changes state (up to down, or vice versa), a notification, called a link state advertisement (LSA) , is flooded throughout the network. Routers all over the network address this change and make sure that their routing tables are adjusted accordingly . Another configurable option of Link State protocols is that they can use something other than hop count to determine their path through the network. Link State protocols can also determine that going over more routers may be quicker if the available bandwidth is higher instead of choosing the shortest path deemed solely on how many routers away the destination is. So, what is so great about a Link State routing protocol? It is highly reliable and much less bandwidth intensive than a Distance Vector routing protocol. Also, it is highly configurable. Now, let's look at a Distance Vector protocol. Distance Vector Distance Vector protocols are easy to configure and maintain, but not as reliable or efficient as Link State protocols. Distance Vector protocols (such as RIP) allow for simple design and simple maintenance, but your bandwidth may suffer as a result. A Distance Vector protocol lets every router that is configured to use it inform every other adjacent router of its entire routing table. This means that each router on the network gets a full routing table from each neighboring router. These tables are used to create a metric based on hop count. Each router knows how far it is to another subnet in the network so that when incoming packets (when the router strips the header and reads the destination address) are read, the router will know the quickest way to get the packet there. This simplicity comes at the cost of more bandwidth utilized to keep the routing tables updated as compared to Link State protocols. Planning the Correct Routing Protocol to Use Plan a routing strategy. With that lengthy but necessary routing introduction behind you, let's move forward now and examine the routing protocols available for use in Windows Server 2003. Windows Server 2003 provides three routing protocols to choose from: RIP , RIPv2 , and OSPF . RIP and RIPv2 are Distance Vector based, and OSPF is Link State based. RIP Although not commonly used as a protocol of choice for new network designs, the Routing Information Protocol ( RIP ) is still widely used throughout the world. The reason is that RIP was deployed widely in the past. RIP is not a proprietary protocol either; it is universal between just about every router (or routing device) ever made, so your knowledge and understanding of RIP are critical. RIP is also part of just about every Windows-based server ever created. Consequently, you need to know about RIPhow to design it and how to troubleshoot problems that may occur when using it. As mentioned previously, RIP is a Distance Vector routing protocol. Remember that RIP is easy to configure and maintain, and should be used for smaller networks; it should not be used for very large enterprise deployments that span several routers. RIP is defined in Request for Comments (RFC) 1058 and updated by RFC 1388. To keep the amount of information you need down to a minimum (you will be asked about RIP on the 70-293 exam), we describe what you need to know for the exam here: -
RIP is limited to 15 router hops, with 16 hops being infinity. What does this mean? Picture a LAN with a PC that needs to communicate with a server on another LAN that is 15 routers away. The routers keep a list of which subnets are no more than 15 routers away, and if you try to expand the network past the 16 th router, communications do not happen. Remember, you have a 15-hop maximum between networks, and the 16 th is deemed infinity. -
RIP has problems with subnetted networks. In Chapter 2, we briefly covered what subnetted networks would resemble in your design. RIP was created and deployed before networks were subnetted to the degree that they are today, and because they have no direct support for RIP, your networks cannot support it. It really comes down to the information that each router sends to each other and what that packet contains. If the routing update does not contain a field in the packet to allow for subnetted networks, it does not carry over the information you may need it to. In other words, RIP is configured to look at the prefix of the IP address in the routing update and know whether it's either A, B, or C, and nothing more. If you subnet, your subnet will not be supported because RIP understands only that if a packet with an IP address of 10.0.0.1 comes in, it is automatically assigned a subnet of 255.0.0.0, even if you have it subnetted down to 255.255.255.0. -
RIP is bandwidth intensive, and although that isn't a problem on most networks, a smaller network with WAN links that are set small (such as Frame Relay links set with a 32KB committed information rate) could feel the effect of constant broadcasts every 30 seconds, especially if the routing table is large. If the routing table is too large, more than one update could be sent every 30 seconds, making it even more bandwidth intensive. By default, RIP broadcasts to its neighbors every 30 seconds lists of networks and subnets it can reach. -
RIP is not very secure. RIP contains no security features or configurable parameters to make it secure. Other protocols used today have configurable parameters to make them more secure; however, RIP does not contain any solution to make it secure by default. In sum, RIP is a Distance Vectorbased protocol that is available on Windows Server 2003 for configuration. It is easy to set up and configure for use, but is limited in what it can do for you. For larger networks, therefore, using RIP would not be wise. In real-world production environments, you most likely will not use RIP in a new deployment, but because so much of it still exists today, you would be wise to know it and understand it well, especially for the 70-293 exam. Some of the most important features of troubleshooting RIP and the reasons it is so important to think about RIP's limitations when considering design are covered in the section "Other Problems with Routing" later in this chapter. RIPv2 RIP was not the greatest protocol ever designed, but due to its ease of use, it was widely deployed and used. RIPv2, or RIP version 2, was created for RIP users to overcome problems with security. This version adds an option for authentication to the RIP packet as well as support for subnetted networks. RIPv2 allows for variable-length subnetted networks to be passed through routing updates. EXAM TIP Know the differences For the 70-293 exam, make sure you are at least familiar with the major differences between RIP and RIP version 2.
RIPv2 offers an opportunity for growth if a company feels that it is either underskilled to deploy a routing protocol such as OSPF or that it will never grow to a larger- sized company that could benefit more from a routing protocol like OSPF. RIPv2 is supported by Windows Server 2003 as well. OSPF Open Shortest Path First (OSPF) is a Link Statebased nonproprietary routing protocol. For today's networks growing in size and complexity, OSPF is a wise choice. It is configurable, scalable, and easy to troubleshoot. However, it is not easy to plan, design, and deploy. Understanding OSPF can take some time and effort because half of what you need to know is in the underlying terminology used to explain and use OSPF. For the 70-293 exam, you need to know how to configure and use OSPF on a Windows Server 2003 system; more importantly, however, you need to know how to design it, know its nomenclature , and know when to use it. You can get a detailed explanation on OSPF by reading RFC 1247. OSPF Version 2 is documented in RFC 1583. As mentioned previously, OSPF is a Link State routing protocol with a complex set of options and features. So, what is so great about OSPF that makes up for its high difficulty level in planning and design? Let's look at it further: -
OSPF is highly scalable. You will be hard pressed to build (or support) a network large enough to outdo OSPF. OSPF was designed to work in very large networks, and it works very well at that. It does not have a hop count restriction as RIP does. With OSPF's use of areas and a subdivided domain, the design possibilities are almost seemingly infinite. -
With RIP, you cannot use subnetted networks; with OSPF, you can. -
With OSPF, your bandwidth is spared. With RIP, an entire routing table is broadcast from every router on the network every 30 seconds. With OSPF (in stable environments that do not suffer from many changes), packets (called hello packets) are sent out intermittently to verify links between adjacent routers. This way, less bandwidth is used, and a major update check is performed only every 30 minutes. So, now you can see where the trade-off comes in. If you want to use a more configurable and scalable protocol, you should use OSPF, but you need to remember that all its benefits equal added work on your part. You have to put more effort into the design, rollout, and maintenance of an OSPF network. For the 70-293 exam, you should also remember the disadvantages to using OSPF. It adds complexity, and its demands on memory and computation for the routers that use it can be devastating if you do not have the proper router hardware to maintain it. What else do you need to know about OSPF? You should know how it is laid out in a design. OSPF divides the network (what it considers a routing domain) into areas. An area is a subdivision of the entire network and is given a label. Area 0 (zero) is considered the backbone of an OSPF network. If your network is small enough, you can set up the entire network to use Area 0, which is very important. For example, if the network becomes more subdivided (say you have three areas, including Area 0), you need to know that if all traffic must travel between areas, the packets are first routed to the backbone, or Area 0. When you plan a network this way, the design keeps subnets consolidated to areas, thus reducing the size of the link state database that is updated on every router in an OSPF network. This keeps your OSPF network running optimally. With OSPF, you also need to know which class of network you are using. There are three: -
Point-to-point A WAN serial link connecting two routers on a single subnet can be considered a point-to-point link. -
Multiaccess An Ethernet or token-ring segment is a multiaccess link. Because the class is multiaccess (like Ethernet), you need to consider that each router on the connected network wants to know about it; it is not good for this to happen because not every router needs to form an adjacency with every other router. This would diminish the benefits of using a protocol like OSPF. To avoid this problem, OSPF assigns a Designated Router (DR) to manage all the link state advertisements (LSAs) that are sent from router to router. Also, you need to know that there is a Backup DR (called the BDR), which is also selected from the available routers on the network to take over if the DR fails. -
Nonbroadcast multiaccess (NBMA) A Frame Relay or X.25 cloud is classified as NBMA. Nonbroadcast multiaccess networks use DRs just like multiaccess, but you must remember that because broadcasts lack support on a Frame Relay network, for instance, you must manually configure each router with its neighbor. If you do not manually configure the routers, you suffer from loss of connectivity between the routers running OSPF. EXAM TIP Take it easy When designing, you can bypass the DR and NBMA network design by utilizing a series of point-to-point links. They may be more intensive to configure, but they take the intricacies of DR election out of your equation.
Why is this information so important? Again, the focus of the 70-293 exam is planning and design. You must understand how a protocol works entirely before you plan to deploy it. You have to know its nuances , what makes it tick, what makes it run under the hood. Understanding what kinds of networks can be connected to utilize OSPF is just as important as knowing how to configure and maintain it. If you do not design this protocol correctly, no matter how helpful it was programmed to be, you won't reap any benefits from it. So, now that you understand the Link State routing protocol OSPF, what do you need to remember for the exam? For the 70-293 exam, you need to know the basic design features of OSPF. Most importantly, you need to remember the following points: -
OSPF sends out hello packets to each adjacent router connected to the network. OSPF uses hello packets to verify that the network is always ready to work as advertised. These hellos are sent out of every router interface every 10 seconds but are so small in size that they do not adversely affect your available bandwidth. -
Link state advertisements provide other functionality, such as providing a solution for a scenario in which a router does not hear from its neighboring router for more than 40 seconds. The router then sends out LSAs marking the other router as down so that all the other routers can adjust their tables with the change. -
Hellos can be adjusted. A hello has a timer value that, when configured properly throughout the OSPF network, allows proper and accurate communications to take place. It is important to know that if a hello timer is misconfigured (not all timers are identical), problems can occur. Make sure that when you plan an OSPF network, you ensure that all the hello timers match. They must all be consistent across all routers on a network segment. -
Because LSAs age, it is important to get a refreshed routing table (or database) from a neighboring router just in case anything has changed. If nothing has changed on a particular router for 30 minutes, the router flushes its information and seeks an updated database from its adjacent routers. This means that the network reconverges every 30 minutesa far cry from the forced reconvergence in a RIP network every 30 seconds! In sum, OSPF is a Link Statebased routing protocol that is difficult to plan, design, manage, and maintain, but is highly configurable and scalable, making it the best choice for large networks. Now that you are familiar with RIP and OSPF, as well as what routing is and all the other details that surround it, let's discuss how to plan a routed environment, one of the objectives of exam 70-293. OSPF VERSUS RIP As a Microsoft Certified Professional, you must know how to design networks. No longer can you be a simple server jockey; now you need to know the connecting highway , which is the underlying network that allows your domain controllers to communicate with each other. You can't have WINS push/pull partners if you don't have a WAN. And when you have a WAN, you have routers, and when you have routers configured to use a dynamic routing protocol, you have either RIP or OSPF. So, which do you choose? Because we discussed all the details of RIP and OSPF in the preceding sections, we provide a distilled list of OSPF versus RIP here. This information will help you to decide which protocol you should use over the other. You can consider this information a summary of what you have already read in this chapter. RIP can no longer handle the growth of business as we know it today. No business today has boundaries. A business that currently (from a network view) has only one core network and 10 remote sites could easily be acquired by another company through a merger and become part of that company's network overnight. Because most large companies that acquire other smaller companies generally have large networks, it would be safe to say that they use advanced routing protocols on their networks to handle their current size as well as their rapid growth. Your RIP-based network would be a thorn in their side, with route redistribution having to take place to communicate, or a complete network overhaul to take place to bring you into compliance with their network. RIP simply can't keep pace with businesses today; therefore, newly designed networks should not be configured to use RIP because RIP's lack of scalability does not match the nature of business. If you decide to use RIP, you must consider that your network could never span any size larger than 15 hop counts. Remember that 16 is infinity. Another vital flaw of RIP is that it cannot use subnetted networks other than the default subnetted classes, which are 255.0.0.0, 255.255.0.0, and 255.255.255.0. Any other variable-length subnet will not work in a RIP environment, and in today's networks this is not something you can afford. Subnetting the private IP address ranges within your network allows a large number of hosts to be used. You should not waste a single host, and being able to efficiently subnet allows you to spare every single IP needed. RIP cannot route between links that are variably subnetted, so it is not wise to deploy RIP in any network you feel may grow to that size. RIP is bandwidth intensive on networks that are built with very slow WAN links. This means that if you use very small circuit sizes (such as 32KB) per link, you are more than likely going to feel the effects of RIP on your wide area network. Remember that RIP tries to update its neighbors with its entire routing table every 30 seconds. RIP is slower to converge than OSPF. This problem is felt only in very large networks due to the size it could span in router hops. If the network design is poorly laid out, you, as an end user, could experience timeouts on your PC if network convergence takes too long. Having this happen every time a Frame Relay link drops could prove annoying to everyone in your company. Very large networks using RIP and dealing with very slow convergence do not work well and may need a protocol such as OSPF to handle the distributed nature of the network it is made to manage. Remember that RIP should not be used on very large networks because everyone on the network will be affected if convergence becomes a problem. RIP understands hop count only as a routable metric. If you want to get from one LAN to another over a few router hops, the shortest hop count from LAN 1 to LAN 2 will win. What happens if there is also another path to LAN 2? What if you have another path that consists of two router hops both separated by a T1 link? This means that you would have 1.544MB of transfer on each link. Say that nothing else is utilizing this path. Now, say the other one-hop path is an analog dial-up connection between two routers over 56Kbps. Which would be faster? Of course, the two T1 lines not being used would be the fastest option, but RIP believes the single hop path is faster because it has a shorter path via hop counts, which in this situation is actually the slowest path RIP could choose. Through OSPF's configurable metrics, it would know the shortest path because it is smart enough (or configurable enough) to be able to know that the T1 link is, of course, the shortest path. RIP also has a spin-off called RIP version 2, or RIPv2 for short. If you are thinking of using RIPv2, you might want to consider using OSPF instead if your network will likely experience future growth. RIPv2 fixes the problems that RIP hadwhich are subnetted networks and authentication for securitybut it still has slow convergence problems and a hop count limitation. OSPF is harder to plan, design, and implement, but once you do, you gain a world of flexibility and control not offered by protocols such as RIP. OSPF addresses all the problems that RIP experiences. OSPF can handle any kind of routed environment you can design because it was made to do so. If you are going to stay with a vendor-neutral routing protocol, all new networks designed and built today should use OSPF. It offers scalability and control over a corporation of just about any size. OSPF provides the following great enhancements over RIP: no hop count limitations; handling of variable-length subnetted networks; and the use of multicast packets (instead of broadcast packets) to give and send updates, which means less processing power used on routers not meant for the updates to process broadcast packets, which in turn eats CPU cycles for nothing. By using multicast packets, OSPF saves router CPU time. Bandwidth is also saved because routing updates are sent only as needed, not every 30 seconds. With OSPF, the network reconverges every 30 minutes, but that's to be expected and nowhere near as bandwidth consuming as every 30 seconds with RIP. With OSPF, you can set up areas. Area 0 is always the backbone, but as far as expansion, you can expand your network into other areas such as Area 1, Area 2, and Area 3. You could configure all four areas as separate subnets and set up a logical definition of subnetted networks to limit the number of link state updates needed per area. If you separate your network into areas, not every router needs to know what every other router does; if it did, a router 29 hops away would need to exchange a database with every other router on the network, and this would make convergence really slow. OSPF allows you to create areas so that such a problem never happens. |
Planning a Routing Environment Now that you are familiar with what routing is, how it is performed, what a router is, and what different protocols you can choose from, you need to know how to plan a routing environment. This section briefly covers what you can do to plan for your new routing environment. For one, you can plan your network logically. This section briefly describes some of the more common routing topologies, such as ring, mesh and star so that you understand what they mean as they are mentioned in the book as well as on the exam. The simplest routing topology to consider is the ring, which is also the least efficient and reliable of the common routing topologies. In the ring topology, shown in Figure 4.3, each site is connected to its two closest neighbors with a WAN link; thus, two WAN connections and two routers are needed at each site. The ring topology is fairly easy to install and configure, but is less efficient than the next topology we will explore, the mesh, because no one site can communicate directly with any other site except its neighbor on either side. Figure 4.3. The ring topology is easy to implement and configure, but not efficient when compared to the mesh. In a mesh topology, a shown in Figure 4.4, every site in the network is connected to every other site in the network by a WAN link. This arrangement is costly because it uses a high number of links and may become difficult to manage as the number of sites, and thus the number of links, grows. The benefit to this arrangement is complete redundancy against failure. If you lose a link anywhere in the network, the entire network is always up, no matter what. The drawbacks are cost and maintainability. Figure 4.4. The mesh topology is expensive to implement and maintain, but provides complete redundancy. The last topology we examine here, the star , as shown in Figure 4.5, is essentially a core location with remote sites linked to it. Remote sites are normally other business units, smaller locations, or business partners and need to access data from the core over the WAN. All communication from one remote site to another occurs via the core location. Having all resources centralized logically in a core location (such as the headquarters) makes the design easy to maintain from a routing perspective because every site, after it is connected, is only one hop away. Figure 4.5. The star topology is very efficient and inexpensive to implement and maintain. When planning a routing environment, other than planning for the topology you will use, you also have to plan your routing protocols, which we discussed previously in great detail. Remember all this information not only for the 70-293 exam, but also for all real-world production deployment scenarios. Windows Server 2003 Routing Solutions As far back as Windows NT and its variations, you could always (and still can today) build a router out of a server. Remember that all routers do is connect networks together. So, if you have a switch with 10 PCs connected using the subnet 10.0.0.0/24 and another switch with another 10 PCs connected using the subnet 10.0.1.0/24, all you would need to make them communicate is a router in between the two switches. You can use either a router with two Ethernet interfaces or a Windows server with two NICs installed to connect the two subnets. You could conceivably connect the two subnets, turn on RIP, and be routing in no time. Windows Server 2003 can act as a fully functional router using static routes or dynamic routing protocols, such as RIP (both versions) and OSPF. Would you use Windows Server 2003 for a router? Of course, you can; the only setback is that most routers today come with specialized hardware that allows them to contain telecom linkbased interfaces such as ISDN, Serial, T1 lines, T3 lines, and so on. If you want to connect two or more Ethernet-based networks and route between them, you could set a Windows Server 2003 system to do that task, but can you connect to the Internet via a T1? Not so easily said and done. Remember, Windows Server 2003 is fully functional as a router, but you also need to remember its weaknesses for the exam. Before you can configure RIP or OSPF routing to occur, you must first enable the Routing and Remote Access Service on the server, as outlined in Step by Step 4.1. STEP BY STEP 4.1 Enabling Routing and Remote Access Services -
Open the Routing and Remote Access console by choosing Start, Control Panel, Administrative Tools, and then Routing and Remote Access. By default, the local computer is listed as a server (see Figure 4.6). Notice the red arrow that indicates that the Routing and Remote Access Service is not presently enabled. Figure 4.6. The Routing and Remote Access console is used to configure the Routing and Remote Access Service for a variety of functions, including acting as a router.  -
Right-click the server you want to configure and select Configure and Enable Routing and Remote Access. The Routing and Remote Access Server Setup Wizard starts. -
Click Next to start configuring the Routing and Remote Access Service. The Configuration dialog box opens, as shown in Figure 4.7. Figure 4.7. The Routing and Remote Access Server Setup Wizard includes the most common configuration options to make the configuration process as easy as possible.  -
Select the Secure Connection Between Two Private Networks option and click Next to continue. The Demand-Dial Connections dialog box opens, as shown in Figure 4.8. Figure 4.8. Demand-dial routing allows you to open connections between routers when traffic needs to pass through the router and disconnect connections when not needed.  -
On the Demand-Dial Connections dialog box, select No and click Next to continue. The Completing the Routing and Remote Access Server Setup Wizard summary dialog box opens, as shown in Figure 4.9. Figure 4.9. The last screen of the setup wizard summarizes the changes made, lists the next steps, and allows you one final opportunity to cancel the configuration changes.  -
Click Finish to complete the enabling of routing services. |
With your Windows Server 2003 computer now enabled to provide routing, you must decide whether you will use static routing or dynamic routing. Using the route Command to Configure Static Routes The route command is primarily used to configure static routes within a network. It can also be used for troubleshooting by listing all the routes that this computer knows about. The syntax of the route command is as follows : route [-f] [-p] [command [destination] [mask subnetmask] [gateway] [metric costmetric]] The -f parameter clears the routing tables of all entries. The -p parameter makes the route persistent. When a route is added to the router, by default the entry is not kept across reboots of the system. The -p parameter ensures that the entry is maintained in the routing table across reboots of the system. The following commands are used to specify what can be done with the route command: -
print Prints the existing entries in the routing table. -
add Adds a new route to the routing table. -
delete Deletes an existing route from the routing table. -
change Modifies an existing route in the routing table. The destination parameter specifies the destination that you want to reach. The destination parameter can contain the following: -
Host address -
Subnet address -
Network address -
Default gateway The mask parameter defines what portion of the destination address must match for that route to be used. When the mask is written in binary, a 1 is significant (must match) and a 0 need not match. For example, a 255.255.255.255 mask is used for a host entry. The mask of all 255s (all 1s) means that the destination address of the packet to be routed must exactly match the network address for this route to be used. For another example, the network address 157.57.8.0 has a netmask of 255.255.248.0. This netmask means the first two octets must match exactly, the first 5 bits of the third octet must match (248=11111000), and the last octet does not matter. Thus, any address of 157.57 and the third octet of 8 through 15 (15=00001111) will use this route. The gateway parameter specifies where the packet needs to be sent. This can be the local network card or a router on the local subnet. The metric parameter specifies a cost that is to be associated with that route. This cost is used in the decision on how packets should be routed. Packets are routed through the route that has the lowest cost. Step by Step 4.2 shows how to configure a static route for the network 10.100.10.0. STEP BY STEP 4.2 Adding a Static Route to Windows Server 2003 -
Open the command prompt by choosing Start, Programs, Accessories, and selecting Command Prompt. The Command Prompt window opens. -
Display the system's existing routing table by typing route print and pressing Enter. Make note of the routes listed now to check against the routes listed after you add the static route. You should see the following information (the output will vary from computer to computer, based on your computer's interfaces, network addresses, and existing routes): [View full width] [View full width] IPv4 Route Table =========================================================================  Interface List 0x1 ........................... MS TCP Loopback interface 0x10003 ...00 b0 d0 da 90 1a ...... 3Com 3C920 Integrated Fast Ethernet 0x10004 ...00 20 35 e7 93 fd ...... IBM 10/100 EtherJet PCI Adapter ========================================================================= ========================================================================= Active Routes: Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 10.115.10.1 10.115.10 .2 30 0.0.0.0 0.0.0.0 10.226.136.185 10.226.136 .189 20 10.115.10.0 255.255.255.0 10.115.10.2 10.115.10 .2 30 10.115.10.2 255.255.255.255 127.0.0.1 127.0.0 .1 30 10.226.136.184 255.255.255.248 10.226.136.189 10.226.136 .189 20 10.226.136.189 255.255.255.255 127.0.0.1 127.0.0 .1 20 10.255.255.255 255.255.255.255 10.115.10.2 10.115.10 .2 30 10.255.255.255 255.255.255.255 10.226.136.189 10.226.136 .189 20 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0 .1 1 224.0.0.0 240.0.0.0 10.115.10.2 10.115.10 .2 30 224.0.0.0 240.0.0.0 10.226.136.189 10.226.136 .189 20 255.255.255.255 255.255.255.255 10.115.10.2 10.115.10 .2 1 255.255.255.255 255.255.255.255 10.226.136.189 10.226.136 .189 1 Default Gateway: 10.226.136.185 ========================================================================= Persistent Routes: None -
Within the Command Prompt window, type the following command and press Enter: Route Add 10.100.10.0 mask 255.255.255.0 10.100.5.1 metric 2 A successful entry returns you to the command prompt, with no message. If the entry addition is unsuccessful , you will receive an error message. -
To display the system's routing table with the newly added route, type route print and press Enter. This displays the following information (note the newly added route in boldface type): [View full width] [View full width] IPv4 Route Table ========================================================================= Interface List 0x1 ........................... MS TCP Loopback interface 0x10003 ...00 b0 d0 da 90 1a ...... 3Com 3C920 Integrated Fast Ethernet 0x10004 ...00 20 35 e7 93 fd ...... IBM 10/100 EtherJet PCI Adapter ========================================================================= ========================================================================= Active Routes: Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 10.115.10.1 10.115.10 .2 30 0.0.0.0 0.0.0.0 10.226.136.185 10.226.136 .189 20 10.100.10.0 255.255.255.0 10.115.10.2 10.115.10 .2 2 10.115.10.0 255.255.255.0 10.115.10.2 10.115.10 .2 30 10.115.10.2 255.255.255.255 127.0.0.1 127.0.0 .1 30 10.226.136.184 255.255.255.248 10.226.136.189 10.226.136 .189 20 10.226.136.189 255.255.255.255 127.0.0.1 127.0.0 .1 20 10.255.255.255 255.255.255.255 10.115.10.2 10.115.10 .2 30 10.255.255.255 255.255.255.255 10.226.136.189 10.226.136 .189 20 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0 .1 1 224.0.0.0 240.0.0.0 10.115.10.2 10.115.10 .2 30 224.0.0.0 240.0.0.0 10.226.136.189 10.226.136 .189 20 255.255.255.255 255.255.255.255 10.115.10.2 10.115.10 .2 1 255.255.255.255 255.255.255.255 10.226.136.189 10.226.136 .189 1 Default Gateway: 10.226.136.185 ========================================================================= Persistent Routes: None |
Implementing RIP Routing As discussed earlier, static routing is not necessarily a viable solution in today's business network; thus, you should consider implementing one of the available dynamic routing protocols that Windows Server 2003 supports. Step by Step 4.3 shows how to enable RIPv2 for routing. STEP BY STEP 4.3 Adding RIP to the Routing and Remote Access Service -
Open the Routing and Remote Access console, as shown in Figure 4.10, by choosing Start, Control Panel, Administrative Tools, and then Routing and Remote Access. Figure 4.10. A server with an active Routing and Remote Access Service appears in the Routing and Remote Access console with a green upward arrow.  -
In the left pane, expand the list under IP Routing and right-click General. From the context menu, select New Routing Protocol. The New Routing Protocol dialog box opens, as shown in Figure 4.11. Figure 4.11. The New Routing Protocol Wizard can be used to add protocols to your Windows Server 2003 server.  -
Select RIP Version 2 for Internet Protocol and click OK. RIP then appears under the IP Routing entry, as shown in Figure 4.12. RIP is now installed on your Windows Server 2003 server. Figure 4.12. RIP appears under the IP Routing heading in the Routing and Remote Access console.  -
Right-click the RIP entry and select New Interface from the context menu. The New Interface for RIP Version 2 for Internet Protocol dialog box opens, as shown in Figure 4.13. Figure 4.13. The first step in configuring RIP is to create a RIP interface.  -
Select the appropriate Local Area Connection and click OK. The RIP Properties dialog box opens, as shown in Figure 4.14. Figure 4.14. The RIP Properties dialog box allows you to configure the RIP protocol to work in your network environment.  -
On the General tab, under Outgoing Packet Protocol, select RIP Version 1 Broadcast. Under Incoming Packet Protocol, select RIP Version 1 and 2. Click OK to return to the Routing and Remote Access console and activate the changes. You have just configured the server to work with any legacy RIP version 1 routers that might still be on the network. |
Implementing Silent RIP RIP also has a feature known as silent RIP that allows a server running RRAS to build a routing table by listening to the RIP broadcasts on the network, but the host doesn't broadcast any updates to the network. A silent RIP router processes RIP announcements but does not announce its own routes. To enable silent RIP on your Windows Server 2003 router, perform the steps outlined in Step by Step 4.4. STEP BY STEP 4.4 Enabling Silent RIP on Windows Server 2003 -
Open the Routing and Remote Access console. -
Expand the console tree and select RIP. The list of available interfaces running RIP appears in the right pane of the console, as shown in Figure 4.15. Figure 4.15. You can find the complete list of interfaces supporting a particular protocol by looking in the right pane of the console.  -
Right-click the interface that you want to configure for silent RIP mode, and from the context menu, select Properties. The Local Area Connection Properties opens, as shown in Figure 4.16. Figure 4.16. The Silent RIP setting can be found in the Outgoing Packet Protocol pull-down, due to the fact that the configuration is related to which routes are broadcast, not received.  -
On the General tab, under Outgoing Packet Protocol, select Silent RIP from the pull-down menu. Click OK to return to the Routing and Remote Access console and apply the changes. |
Additional RIP Configuration Options Before we move on to implementing and configuring the OSPF protocol, let's look at some of the other RIP settings you might need to set. Step by Step 4.5 shows how to set these options. STEP BY STEP 4.5 Setting Additional RIP Configuration Options with Windows Server 2003 -
Open the Routing and Remote Access console. -
Expand the console tree and select RIP. The list of available interfaces running RIP appears in the right pane of the console. -
Right-click the interface that you want to configure, and from the context menu, select Properties. The Local Area Connection Properties opens. -
As you saw in Step by Step 4.4, the General tab deals with the way routes are broadcast and received. Click the Security tab, shown in Figure 4.17. Look at how the RIP updates can be managed. Figure 4.17. The Security tab allows you to restrict the networks that can send/receive RIP updates.  -
Click the Neighbors tab, shown in Figure 4.18. Review the settings that can be configured pertaining to how the router interacts with its RIP neighbors. Figure 4.18. The Neighbors tab allows you to configure how RIP broadcasts are sent by your Windows Server 2003 router.  -
Click the Advanced tab, shown in Figure 4.19. The Advanced tab allows you to configure the more complex settings associated with RIP and the way Windows Server 2003 supports it. Click OK to return to the Routing and Remote Access console. Figure 4.19. The Advanced tab allows you to configure advanced settings such as split horizon, poison reverse, and triggered updates.  |
Implementing OSPF Routing If your network is growing or expected to grow, or you want to provide a more robust routing solution, you should consider implementing OSPF routing. Step by Step 4.6 outlines the process to configure OSPF routing on your Windows Server 2003 computer. STEP BY STEP 4.6 Installing and Configuring OSPF on Windows Server 2003 -
Open the Routing and Remote Access console. -
Expand the console tree, and under IP Routing, right-click General. From the context menu, select New Routing Protocol. The New Routing Protocol dialog box opens. -
Select Open Shortest Path First and click OK to install it. It now appears under IP Routing in the Routing and Remote Access console, as shown in Figure 4.20. Figure 4.20. The OSPF protocol appears under the IP Routing node of the Routing and Remote Access console.  -
Select the newly installed OSPF protocol and right-click. From the context menu, select New Interface. The New Interface for Open Shortest Path First (OSPF) dialog box opens, as shown in Figure 4.21. Figure 4.21. The OSPF protocol can be tied to a single interface or to multiple interfaces.  -
Select Local Area Connection and click OK. The OSPF Properties dialog box opens, as shown in Figure 4.22. Figure 4.22. From the OSPF Properties dialog box, you can complete the configuration of the OSPF protocol.  -
On the General tab, select the Enable OSPF for This Address option. In Area ID, click the ID of the area to which the interface belongs (for this exercise, it should be 0.0.0.0). In Router Priority, click the arrows to set the priority of the router over the interface to 1. In Cost, click the scroll arrows to set the cost of sending a packet over the interface to 2. In the Password text box, type a password. Under Network Type, set the type of OSPF interface as Broadcast. Click OK to complete the installation of the interface and return to the Routing and Remote Access console. |
Routing Hierarchies Unlike RIP, OSPF operates within a hierarchy. The basic structure to this hierarchy includes areas, autonomous systems, and the OSPF backbone. The largest entity within the hierarchy is the autonomous system. An autonomous system (AS) is a collection of networks and routers under common administration, sharing a common routing protocol. NOTE When do I need more than one area ? A good rule of thumb is to start creating a multiple area OSPF network when you reach 50 routers. If you have 50 routers on your network, your network is probably complex enough to warrant creating additional areas.
Areas An autonomous system can be further divided into areas . Each area is identified by an area ID. This identifier has no relation to an IP address or IP network ID, although it uses the same dotted decimal formula. Area IDs are not used to reflect routing data, and are actually convenient labels to simplify the management of the area. Although the area ID does not signify any IP routing information, if all the networks within an area correspond to a single subnetted network ID, the area ID can be set to the network ID. This can be convenient for administration of the areas. For example, if an area contains all the subnets of the IP network 10.1.0.0, the area ID can be set to 10.1.0.0. The reason for defining areas within an autonomous system is to reduce the size of a Link State Database. EXAM TIP OSPF is a complex routing protocol If it seems as though OSPF is a really complex routing protocol, it is. Engineers who architect OSPF networks for large companies usually have extensive training and experience. For the exam, you should have a general understanding of the concepts associated with OSPF. You will not be asked to design a complex OSPF network as part of the exam.
To keep the size of LSDBs to a minimum, LSAs for an area's networks and routers are flooded within the area, but not to routers outside the area. Each area becomes its own link state domain with its own topological database. Routers with multiple interfaces can participate in multiple areas. These routers, which are called area border routers , maintain separate LSDBs for each area. The routing tables within these routers are a combination of the routing table entries of all the SPF trees for each topological database, as well as static routes, Simple Network Management Protocol (SNMP) configured routes, and routes learned from other routing protocols. To reduce the number of entries in the routing table, the networks inside the area can be advertised outside the area using summary route advertisements. When you keep area topologies separate, OSPF passes less routing traffic than it would if the autonomous system were not partitioned. NOTE The Link State Database (LSDB) The LSDB provides the overall picture of networks in relationship to routers within an area. The LSDB contains the collection of LSAs received from all routers in the same area. In an autonomous system with a large number of networks, each OSPF router must keep the LSA of every other router in its topological database.
The Default Route Each area within an OSPF network can be configured with a default route. A default route has a destination address of 0.0.0.0 with a subnet mask of 0.0.0.0. The default route is used to route any packets that are destined for an address not explicitly listed in the routing table. Default routes are typically used to reduce the size of routing tables because they can be used to summarize all routes external to the area into a single route. Default routes typically point to the OSPF backbone. OSPF Backbone Every OSPF network must have at least one area. An OSPF network with more than one area must have a backbone area to interconnect all other areas within the network. The backbone area always has an area ID of 0.0.0.0 and acts as the hub for all the other areas on the network. All other areas must communicate their routing information to the backbone area so that it can distribute the information to all other areas. Figure 4.23 shows an example of an internetwork with several areas and a backbone. Figure 4.23. An OSPF network with multiple areas. Backbone routers not only route host traffic between areas, but can also provide summary routes within areas to other routers on the backbone. They, in turn, provide those summaries to the other routers within their areas. This configuration ensures that any host within an area can reach any host in another area. This capability ensures that no area's routing tables contain detail information about another area's topology. This minimizes the size of routing tables and ensures that the backbone handles all inter-area traffic. Virtual Links In rare cases an area may not have direct physical access to the backbone. In this instance, you need to configure a connection from the new area to the backbone through a connection called a virtual link. A virtual link provides the disconnected area a logical path to the backbone. The virtual link has to be established between two area border routers that have a common area, with one area border router connected to the backbone. Virtual links are configured over a non-backbone area known as a transit area. Step by Step 4.7 shows how to configure a virtual link within RRAS. You need at least two OSPF areas to configure this interface. STEP BY STEP 4.7 Configuring a Virtual Routing Link for Windows Server 2003 -
Open the Routing and Remote Access console. -
Expand the console tree and select IP Routing. Right-click OSPF and select Properties from the context menu. The OSPF Properties dialog box opens, as shown in Figure 4.24. Figure 4.24. To setup a virtual routing link, you need to edit the OSPF properties.  -
On the Virtual Interfaces tab, click Add. In Transit Area ID, click the transit area over which you are connecting the virtual link. The OSPF Virtual Interface Configuration dialog box opens, as shown in Figure 4.25. Figure 4.25. Be sure you know the network parameters and characteristics of your virtual link before configuring the virtual interface.  -
In Virtual Neighbor Router ID, type the OSPF router ID of the router at the other endpoint of the virtual link. In Transit Delay (Seconds), click the arrows to set the transit delay in seconds. In Retransmit Interval (Seconds), click the arrows to set the retransmit interval in seconds. In Hello Interval (Seconds), click the arrows to set the hello interval in seconds. In Dead Interval (Seconds), click the arrows to set the dead interval in seconds. Click OK to add the virtual interface, and click OK again to return to the Routing and Remote Access console. |
Area Routing Area partitioning creates the following two types of OSPF routing, depending on whether the source and destination are in the same area or in different areas: -
Intra-area Intra-area routing occurs when the source and destination are in the same area. With intra-area routing, the source routes the packet to its default gateway (an internal area router). The internal area router then makes use of the explicit routes (as calculated by the SPF algorithm) maintained in the area routers and routes the packet through the appropriate interface to the destination internal area router. The destination internal area router then forwards the packet to the destination host. -
Inter-area Inter-area routing occurs when the source and destination are in different areas. When routing between areas, the source routes the packet to its default gateway (an internal area router); the area router then forwards the packet to an area border router using the shortest path. The area border router then forwards the packets through backbone routers using the shortest path to the area border router for the destination host. The area border router for the destination host then forwards the packets through internal area routers using the shortest path, until the packets reach their destination. Step by Step 4.8 allows you to create additional OSPF areas. STEP BY STEP 4.8 Creating an OSPF Area with Windows Server 2003's Routing and Remote Access Service -
Open the Routing and Remote Access console. -
Expand the console tree and select IP Routing. In the details pane, right-click OSPF and select Properties from the context menu. The OSPF Properties dialog box opens. -
Select the Areas tab. -
Click Add to open the OSPF Area Configuration dialog box, as shown in Figure 4.26. Figure 4.26. Be sure you know the network parameters and characteristics of your area before adding it to your OSPF network.  -
On the General tab, type a dotted decimal number that identifies the area. To use a plain-text password, verify that the Enable Plaintext Password option is selected. To mark the area as a stub, select the Stub Area option. In Stub Metric, click the arrows to set the stub metric. To import routes of other areas into the stub area, select the Import Summary Advertisements option. Click OK twice to apply the changes and return to the Routing and Remote Access console. NOTE What is a stub area ? A stub area is an OSPF area that doesn't advertise individual network routes from external areas. Instead, it relies on a default route (0.0.0.0 with a subnet mask of 0.0.0.0) to route all traffic out of the area. This is done to reduce the amount of memory required on the routers located in the stub area; they don't need to maintain a topology database.
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Autonomous System Routing Routing does not occur only within areas using OSPF. When internetworks are connected to other internetworks that are under different administrative control, routing must be established. This type of routing between autonomous systems is established using external routes. Autonomous system border routers running OSPF learn about exterior routes through other routing protocols, such as Interior Gateway Routing Protocol (IGRP), RIP, or Border Gateway Protocol (BGP). Autonomous system border routers are similar to area border routers, except that they route between different autonomous systems. EXAM TIP Supported protocols Some of the more common border protocols such as Interior Gateway Routing Protocol or Border Gateway Protocol are not supported by Windows Server 2003. They are important to know as general information, because they are commonly used in production networks, but will not be on the exam.
By default, autonomous system border routers advertise all external routes within their autonomous system. This allows all areas and networks within areas to reach destination networks that may lie outside the autonomous system. As a network manager, you may find it necessary to restrict the external routes that are advertised within the autonomous system. Using Windows Server 2003 Router and Remote Access Services, you can configure the autonomous system border router to accept or ignore the routes of certain external sources, such as routing protocols (RIPv2) or other sources (static routes or Simple Network Management Protocol). You can also configure the autonomous system border router to accept or discard specific routes by configuring one or multiple (Destination, Network Mask) pairs. Step by Step 4.9 shows how to enable autonomous system border routing. STEP BY STEP 4.9 Configuring Autonomous System Border Routing in Windows Server 2003's Routing and Remote Access Service -
Open the Routing and Remote Access console. -
Expand the console tree and select IP Routing. In the details pane, right-click OSPF and select Properties from the context menu. The OSPF Properties dialog box opens. -
On the General tab, shown in Figure 4.27, click Enable Autonomous System Boundary Router. Figure 4.27. You need to enable the Autonomous System Boundary Router parameter to accept routing information from RIP.  -
Select the External Routing tab, shown in Figure 4.28, and click Accept Routes from All Route Sources Except Those Selected. If you want to exclude a specific route source, select it in the window below. You can also select Ignore Routes from All Route Sources Except Those Selected if you want to accept routes from only one or two sources. Figure 4.28. You have some granularity when configuring the sources your router will accept updates from.  -
Click OK to return to the Routing and Remote Access console and enable the OSPF boundary routing. |
Other Problems with Routing Our last routing topic addresses problems that may occur and what the protocols themselves can do to fix those problems after they are configured. Here, we cover hop count issues, Split Horizons and Poison Reverse, and convergence problems. -
Hop count limit Hop count limitations restrict the network's scalability. This is a problem in design. If you have to keep building your network outward, and you suddenly can't route to a location, or the network starts experiencing major routing problems, you may have surpassed the maximum hop count limit. RIP permits a maximum hop count of 15, whereas OSPF does not have this hop count limitation. Remember that any packet traveling on a RIP network with a hop count greater than 15 hops is tagged as unreachable. -
Split Horizons Split Horizons is a service that RIP performs to stop two node routing loops; it should prevent routing loops between adjacent routers. Split Horizons does not advertise routing information on the interface in which it was learned. -
Poison Reverse The Poison Reverse service tries to stop routing loops in the larger internetwork, instead of just two nodes, as Split Horizons does. If a routing loop is occurring, Poison Reverse sends updates to adjacent routers to put that route (the one in question) in a hold-down state, which can be set by a timer. If these metrics are not fine- tuned , routing loops can occur. In this case, Poison Reverse is activated, and the network is put in a hold-down state. Depending on the timer, this problem could go on long enough to cause network timeouts that are experienced by the clients your network services. -
Convergence issues If you have a problem with convergence, you need to analyze a topology map of your network as it currently operates and figure out where (and why) the network is having this problem. As stated previously, convergence occurs when all routers on the network know all other routers, and the topology is accurate. If major activity occurs in your network so that it needs to constantly reconverge (perform updates and verify routing information), you may also experience problems in which routes are unknown and data cannot be accessed. In this case, you need to adjust your network settings or redesign the network so that it converges quickly or stays in a state of convergence longer. By using OSPF instead of RIP, you are already ahead of the game because RIP reconverges the entire network every 30 seconds, by default. |
