Using OSPF Across Multiple Areas

11 Mar

This chapter covers the following topics, which you need to understand to pass the CCNP/CCDP/CCIP BSCI exam:
■ The purpose of using OSPF in a multiple area network
■ The features of multiple area OSPF
■ The operation of OSPF across multiple areas
■ Design considerations in multiple area OSPF

Using OSPF Across Multiple Areas
The topics in this chapter detail the routing protocol OSPF across multiple areas. This chapter assumes your knowledge of Chapter 6, “Using OSPF in a Single Area,” and Chapter 7, “Configuring OSPF in a Single Area,” which dealt with OSPF concepts and configuration in a single area. Chapter 8 builds on this understanding and explains how OSPF works within a large multiarea network. This chapter and the following chapter cover two major sections. Chapter 8 deals with how the protocol works theoretically. Chapter 9, “Configuring OSPF Across Multiple Areas,” covers how to implement and manage an OSPF network. This chapter introduces OSPF areas and explains the operation of the protocol across those areas.

Before you can configure OSPF in multiple areas, you need to understand the motivation for using OSPF in multiple areas. You then must understand how to determine the area boundaries in OSPF. In order to design a multiarea OSPF network in this way, you need a comprehensive grasp of the features of multiarea OSPF, the operation of OSPF across multiple areas and, of course, the design considerations of such a network. This chapter discusses each of these topics.

“Do I Know This Already?” Quiz
The purpose of the “Do I Know This Already?” quiz is to help you decide what parts of this chapter to use. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.

The 12-question quiz, derived from the major sections in “Foundation Topics” portion of the chapter, helps you to determine how to spend your limited study time.

Table 8-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

Table 8-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

Using OSPF Across

CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong for purposes of the self-assessment. Giving yourself credit for an answer you correctly guess skews your self-assessment results and might provide you with a false sense of security.

1. When using OSPF, which of the following is a problem with large routing tables?
a. The routing table is sent out every 30 seconds, which will cause congestion.
b. As the routing table increases in size, the longer each lookup becomes. The memory requirements on the router also increase.
c. The process will time out before a path to the destination is found.
d. Fast switching cannot be used if the routing table exceeds a certain size.

2. Why does the frequency of the SPF algorithm increase with the size of the area?
a. Each router will have more neighbors, and synchronizing the databases takes priority.
b. The topology databases will subdivide after a certain size, requiring multiple SPF calculations.
c. As the area size increases, each recalculation will also take longer, which might result in errors, requiring the algorithm to rerun.
d. The larger the network, the greater the probability of a network change and, thus, a recalculation of the entire area.

3. Multiple areas are one of the main features of OSPF. Which of the following statements explain why this feature is such an important enhancement on earlier routing protocols?
a. It is easier to implement security.
b. All computation is kept within the area, with minimum communication between the areas, allowing the network to scale to larger sizes.
c. The network domain, when divided into areas allows for the use of both IANA classful addressing and private addressing.
d. The use of multiple areas allows for the use of prioritization.

4. What is an internal router?
a. An OSPF process running on a multilayer switch.
b. When multiple OSPF processes are run on the same router, the internal router is responsible for managing the processes.
c. A router responsible for maintaining a current and accurate database of every subnet within the area. All interfaces on this router are within the same area.
d. A router running OSPF with no external links to another autonomous system.

5. What is the purpose of an ABR?
a. A router responsible for connecting to outside the autonomous system
b. A router responsible for connecting two or more areas
c. A logical group of nodes forming a backbone that connects other areas
d. A group of routers running OSPF with no external links

6. What do the initials ASBR represent?
a. Authority Subnet Boundary Router
b. Autonomous System Border Router
c. Automatic Summarization Boundary Router
d. Autonomous System Boundary Router

7. How are routes that are generated within an area propagated throughout the area?
a. Type 3 and 4 LSAs
b. In the summary LSA sent out every 30 minutes
c. In the Hello packet between neighbors
d. Type 1 and 2 LSAs

8. Which of the following conditions must be met before any LSAs can be flooded out of all the interfaces?
a. The interface is in a state of exchange or full adjacency.
b. The interface is not connected to a totally stubby area.
c. The LSA was not received through the interface.
d. The interface is connected to a totally stubby area.

9. Which of the following OSPF characteristics affect how the routing table is created?
a. Whether there are multiple areas in the domain
b. Whether MD-5 security has been configured
c. The type of area in which the router is located
d. Whether there are communications outside the autonomous system

10. Which is the best design for OSPF?
a. Hierarchical with summarization
b. Tiered
c. Flat with summarization
d. Elliptical

11. Why does the type of area determine the number of routers that can be placed in the area?
a. Each LSA packet type has a fixed header, limiting the number of paths that can be listed.
b. The area type determines the number of LSAs and how often and how much CPU and memory each SPF computation requires.
c. Stub areas are not summarized, requiring additional resources.
d. The backbone area requires fewer resources because it simply sends summarized path information into other areas, which does not require the SPF algorithm to be run.

12. Which of the following must be observed when creating a virtual link?
a. Both routers must share a common area.
b. Both routers must share the same subnet address.
c. One of the routers must be connected to area 0.
d. Both routers must share the same process ID.

The answers to this quiz are found in Appendix A, “Answers to Chapter ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows:

■ 6 or less overall score—Read the entire chapter. This includes the “Foundation Topics” and “Foundation Summary” sections, the “Q&A” section, and the “Scenarios” at the end of the chapter.
■ 7–9 overall score —Begin with the “Foundation Summary” section, and then go to the “Q&A” section and the “Scenarios” at the end of the chapter. If you have trouble with these exercises, read the appropriate sections in “Foundation Topics.”
■ 10 or more overall score —If you want more review on these topics, skip to the “Foundation Summary” section, and then go to the “Q&A” section and the “Scenarios” at the end of the chapter. Otherwise, move to the next chapter.

Foundation Topics
The Purpose of Using OSPF in a Multiple Area Network

This section explains what multiple area networks are and how they overcome some of the shortcomings of single area networks. Multiple areas in OSPF provide one of the main distinguishing features between the distance vector protocols and the link-state OSPF.

As you learned in Chapter 6, an OSPF area is a logical grouping of routers that are running OSPF with identical topological databases. An area is a subdivision of the greater OSPF domain, sometimes known as the autonomous system. Multiple areas prevent a large network from outgrowing its capacity to communicate the details of the network to the routing devices charged with maintaining control and connectivity throughout the network.

The division of the autonomous system into areas allows routers in each area to maintain their own topological databases. This limits the size of the topological databases, and summary and external links ensure connectivity between areas and networks outside the autonomous system.

Problems with OSPF in a Single Area
To understand the true benefits of multiple areas, consider why you might decide to create multiple areas from one area.

The following symptoms that you might observe on the network provide a clue that a single area is becoming overpowered:

■ The SPF algorithm is running more frequently. The larger the network, the greater the probability of a network change and, thus, a recalculation of the entire area. Each recalculation also takes longer.
■ The larger the area, the greater the size of the routing table. The routing table is not sent out wholesale, as in a distance vector routing protocol; however, the greater the size of the table, the longer each lookup becomes. The memory requirements on the router also increase.
■ The topological database increases in size and eventually becomes unmanageable for the same reasons as in the previous point. The topology table is exchanged between adjacent routers at least every 30 minutes.

As the various databases increase in size and the calculations become increasingly frequent, the CPU utilization increases while the available memory decreases. This will make the network response time sluggish (not because of congestion on the line, but because of congestion within the router itself). It can also cause congestion on the link. These can result in various additional problems, such as loss of connectivity, loss of packets, and system hangs.

NOTE To check the CPU utilization on the router, use the show processes cpu command. To check the memory utilization, issue the show memory free command.

How to Determine Area Boundaries
Although you might have an obvious need for multiple areas, the practical question is how you should implement multiple areas. There are two approaches, as follows:

■ To grow a single area until it becomes unmanageable
■ To design the network with multiple areas, which are very small, in the expectation that the networks will grow to fit comfortably into their areas

Both approaches are valid. The first approach requires less initial work and configuration. Great care should be put into the design of the network, however, because this can cause problems in the future, particularly in addressing.

In practice, many companies convert their networks to OSPF from a distance vector routing protocol when they realize that they have outgrown the existing routing protocol. This allows the planned implementation of the second approach.

The Features of Multiple Area OSPF
Now that you understand why you need to control the size of the areas, you should consider the design issues for the different areas, including the technology that underpins them and their communication (both within and between the areas).

OSPF Within an Area
One of the main strengths of OSPF is its capability to scale and to support large networks. It does so by creating areas from groups of subnets. The area is seen internally almost as if it were a small organization or entity of its own. It communicates with the other areas, exchanging routing information; this exchange is kept to a minimum, however, allowing only that which is required for connectivity. All computation is kept within the area.

In this way, a router is not overwhelmed by the entirety of the organization’s network. This is crucial, because the nature of a link-state routing protocol is more CPU- and memory-intensive.

Router Types
Given the hierarchical nature of the OSPF network, there are routers operating within an area, routers connecting areas, and routers connecting the organization or autonomous system to the outside world. Each of these routers has a different set of responsibilities, depending on its position and function within the OSPF hierarchical design.

The following list identifies the different OSPF routers:

■ Internal router —Within an area, the functionality of the router is straightforward. It is responsible for maintaining a current and accurate database of every subnet within the area. It is also responsible for forwarding data to other networks by the shortest path. Flooding of routing updates is confined to the area. All interfaces on this router are within the same area. This router is the only router that can operate in a single area OSPF network, other than an Autonomous System Boundary Router (ASBR).
■ Backbone router —The design rules for OSPF require that all the areas be connected through a single area, known as the backbone area, Area 0, or 0.0.0.0. A router within this area is referred to as a backbone router. It can also be an internal router, an ASBR, or an Area Border Router (ABR).
■ ABR—This router is responsible for connecting two or more areas. It holds a full topological database for each area to which it is connected and sends LSA updates between the areas. These LSA updates are summary updates of the subnets within an area. Summarization should be configured for OSPF at the area border because this is where the LSAs make use of the reduced routing updates to minimize the routing overhead on both the network and the routers.
■ ASBR—To connect to the outside world or to any other routing protocol, you need to leave the OSPF domain. OSPF is an interior routing protocol or Interior Gateway Protocol (IGP); gateway is an older term for a router. The router configured for this duty is the ASBR. If any routing protocols are being redistributed to OSPF on a router, the router will become an ASBR because the other routing protocols are outside the OSPF autonomous systems. Although you can place this router anywhere in the OSPF hierarchical design, it should reside in the backbone area. Because any traffic leaving the OSPF domain is also likely to leave the router’s area, it makes sense to place the ASBR in a central location that all traffic leaving its area must traverse. This router could be configured within a single OSPF area, pointing to the outside world.

Figure 8-1 shows how the different router types are interrelated. All the routers in the backbone area, area 0, are not only performing the function of ABR, or ASBR as labeled, but are also backbone routers.

Router Definitions for OSPF

Figure 8-2 shows the connectivity and functionality of the different areas. The routers will send out routing updates and other network information through LSAs. The function or type of router will determine the LSAs that are sent.

Figure 8-2 The Different Types of OSPF Areas and LSA Propagation

Different Types of OSPF

Link-State Advertisements
Five commonly used types of link-state advertisements (LSAs) exist. Cisco uses six LSAs, which are briefly described here:

■ The router link LSA —This LSA is generated for each area to which the router belongs. This LSA gives the link states to all other routers within an area. This LSA is flooded into an area. This is identified as a Type 1 LSA.
■ The network link LSA —This LSA is sent out by the designated router and lists all the routers on the segment for which it is the designated router and has a neighbor relationship. The LSA is flooded to the whole area. This is identified as a Type 2 LSA.
■ The network summary link LSA —This LSA is sent between areas and summarizes the IP networks from one area to another. It is generated by an ABR. This is identified as a Type 3 LSA.
■ The AS external ASBR summary link LSA —This LSA is sent to a router that connects to the outside world (ASBR). It is sent from the ABR to the ASBR. The LSA contains the metric cost from the ABR to the ASBR. This is identified as a Type 4 LSA.
■ The external link LSA —This LSA is originated by AS boundary routers and is flooded throughout the AS. Each external advertisement describes a route to a destination in another autonomous system. Default routes for the AS can also be described by AS external advertisements. This is identified as a Type 5 LSA.
■ The NSSA external LSA —Identified as Type 7, these LSAs are created by the ASBR residing in a not so stubby area (NSSA). This LSA is similar to an autonomous system external LSA,except that this LSA is contained within the NSSA area and is not propagated into other areas, but it is converted into a Type 5 LSA by the ABR.

In the section “The ABRs and ASBR Propagation of LSAs,” Figure 8-3 shows the relationships between the different LSAs. This section discusses the router and network LSAs. The LSAs concerned with communication outside an area are considered later.

The Different Types of Areas
It is possible to create an OSPF network with only one area. This area is known as the backbone area or Area 0. In addition to the backbone area, which connects the other areas, OSPF networks use several other types of areas. The following are the different types of areas:

■ An ordinary or standard area —This type of area connects to the backbone. The area is seen as an entity unto itself. Every router knows about every network in the area, and each router has the same topological database. However, the routing tables are unique from the perspective of the router and its position within the area.

■ A stub area —This is an area that will not accept external summary routes. The LSA that is blocked is Type 5. The consequence is that the only way that a router within the stub area can see outside the autonomous system is by the use of a default route. Every router within the area can see every network within the area and the networks (summarized or not) within other areas. It is typically used in a hub-and-spoke network design.
■ A totally stubby area —This area does not accept summary LSAs from the other areas or the external summary LSAs from outside the autonomous system. The LSAs blocked are Types 3, 4, and 5. The only way out of the totally stubby area is via a default route. A default route is indicated as the network 0.0.0.0. This type of area is particularly useful for remote sites that have few networks and limited connectivity with the rest of the network. This is a proprietary solution offered only by Cisco. Cisco recommends this solution if you have a totally Cisco shop because it keeps the topological databases and routing tables as small as possible.
■ An NSSA—This area is used primarily to connect to ISPs, or when redistribution is required. In most respects, it is the same as the stub area. External routes are not propagated into or out of the area. It does not allow Type 4 or Type 5 LSAs. This area was designed as a special stub area for applications such as an area with a few stub networks but with a connection to a router that runs only RIP, or an area with its own connection to an Internet resource needed only by a certain division.

An NSSA is an area that is seen as a stub area but can receive external routes, which it will not propagate into the backbone area and thus the rest of the OSPF domain.
Another LSA, Type 7, is created specifically for the NSSA. This LSA can be originated and communicated throughout the area, but it will not be propagated into other areas, including Area 0. If the information is to be propagated throughout the AS, it is translated into an LSA Type 5 at the NSSA ABR.

It is not always possible to design the network and determine where redistribution is to occur. RFC 1587, “The OSPF NSSA Option,” deals with this subject.

■ The backbone area —This area is often referred to as Area 0, and it connects all the other areas. It can propagate all the LSAs except for LSA Type 7, which is translated into LSA Type 5 by the ABR.

Some restrictions govern creating a stub area or a totally stubby area. Because no external routes are allowed in these areas, the following restrictions are in place:

■ No external routes are allowed.
■ No virtual links are allowed.
■ No redistribution is allowed.
■ No ASBR routers are allowed.
■ The area is not the backbone area.
■ All the routers are configured to be stub routers.

The Operation of OSPF Across Multiple Areas
As you have learned so far in this chapter, there are many pieces to the puzzle of OSPF across multiple areas. Having identified the various pieces, you need to fit them together. Then you will see how the routing protocol operates across the various areas to maintain a coherent and accurate understanding of the autonomous system.

The ABRs and ASBR Propagation of LSAs
When a router is configured as an ABR, it generates summary LSAs and floods them into the backbone area. Routes generated within an area are Type 1 or Type 2, and these are injected as Type 3 summaries into the backbone. These summaries are then injected by the other ABRs into their own areas, unless they are configured as totally stubby areas. Any Type 3 or Type 4 LSA received from the backbone are forwarded into the area by the ABR.

The backbone also forwards external routes both ways unless the ABR is a stub router, in which case they are blocked.

If a summary is received from within the area, it cannot be forwarded. Summaries received from the backbone cannot be further summarized.

The flow and propagation of LSAs within and between areas is illustrated in Figure 8-3.

Certain conditions need to be met before any LSAs can be flooded out of all interfaces. The conditions that each interface must meet before an LSA can be transmitted out of that interface are given in the following list:

■ The LSA was not received through the interface.
■ The interface is in a state of exchange or full adjacency.
■ The interface is not connected to a stub area (no LSA Type 5 will be flooded).
■ The interface is not connected to a totally stubby area (no Type 3, 4, or 5 will be propagated).

OSPF Path Selection Between Areas
The OSPF routing table that exists on a router depends on the following factors:

■ The position that the router has in the area and the status of the network
■ The type of area in which the router is located
■ Whether there are multiple areas in the domain
■ Whether there are communications outside the autonomous system

Propagation of LSAs

Remember the sequence of events: The router receives LSAs. It builds the topological database. Then it runs the Dijkstra algorithm, from which the shortest path is chosen and entered into the routing table. The routing table is therefore the conclusion of the decision-making process. It holds information on how that decision was made by including the metric for each link. This enables you to view the operation of the network.

Different LSAs are weighted differently in the decision-making process. It is preferable to take an internal route (within the area) to a remote network rather than to traverse multiple areas just to arrive at the same place. Not only does multiple-area traveling create unnecessary traffic, but it also can create a loop within the network.

The routing table reflects the network topology information and indicates where the remote network sits in relation to the local router.

The router will process the LSAs in this order:

1. The internal LSA (Type 1 and 2).
2. The LSAs of the AS (Type 3 and 4). If there is a route to the chosen network within the area (Type 1 or 2), this path will be kept.
3. The external LSAs (Type 5).

Calculating the Cost of a Path to Another Area
There are paths to networks in other areas, and then there are paths to networks in another autonomous system. The costs of these paths are calculated slightly differently.

The path to another area is calculated as the smallest cost to the ABR, added to the smallest cost to the backbone. Thus, if there were two paths from the ABR into the backbone, the shortest (lowestcost) path would be added to the cost of the path to the ABR.

External routes are routes passed between a router within the OSPF domain and a router in another autonomous system or routing domain. The routes discovered by OSPF in this way can have the cost of the path calculated in one of two ways:

■ E1—The cost of the path to the ASBR is added to the external cost to reach the next-hop router outside the AS.
■ E2—The external cost of the path from the ASBR is all that is considered in the calculation. This is the default configuration. This is used when there is only one router advertising the route and no path selection is required. If both an E1 and an E2 path are offered to the remote network, the E1 path will be used.

At the side of the routing table is a column indicating the source of the routing information. Typically, this is the routing protocol. In the instance of OSPF, however, it includes the LSA type that provided the path.

Table 8-2 shows the codes used in the routing table.
Now that you understand the components and operation of multiple area OSPF, you should focus on some of the design implications of creating multiple areas, as described in the next section.

OSPF Routing

Design Considerations in Multiple Area OSPF
The major design consideration in OSPF is how to divide the areas. This is of interest because it  impacts the addressing scheme for IP within the network.

An OSPF network works best with a hierarchical design, in which the movement of data from one area to another comprises only a subset of the traffic within the area itself.

It is important to remember that with all the interarea traffic disseminated by the backbone, any reduction of overhead through a solid hierarchical design and summarization is beneficial. The entire network benefits when fewer summary LSAs need to be forwarded into the backbone area. When network overhead is minimized, the network grows more easily.

With this in mind, summarization is the natural consequence. As shown in Chapter 2, “IP Addressing,” summarization is not something that can be imposed on a network. It must be part of the initial network design. The addressing scheme must be devised to support the use of summarization.

In designing any network, you need to consider the resources available and to make sure that none of these resources are overwhelmed, either initially or in the future. In the creation of areas, OSPF has tried to provide the means by which the network can grow without exceeding the available resources. However, this does not remove your responsibility as the network administrator to design a network that can run efficiently within the limits of the resources available. Cisco has laid down guidelines to help in the design of stable, responsive, and flexible OSPF networks.

It is also important in any design to allow for transitions or breaks in the network. OSPF has provided a cunning device called the virtual link that allows areas disconnected from the backbone area to appear directly connected to the backbone as required.

Finally, in any network design, you must consider the traditionally tricky topology of the WAN, in particular the nonbroadcast multiaccess (NBMA) connections that fall into neither one network topology nor another.

The following sections consider all of these subjects as they pertain to multiarea OSPF design.

Capacity Planning in OSPF
Although it is possible to have more than three areas (per router) in OSPF, the Cisco Technical Assistance Center (TAC) recommends that a greater number of areas be created only after careful consideration. The results of having more areas will vary depending on the router (memory and CPU), as well as network topology and how many LSAs are generated. The recommendation is not to exceed 50 routers in an OSPF area, but again, this is a guideline and not a strict rule. Remember that OSPF is very CPU-intensive in its maintenance of the databases and in the flooding of LSAs, as well as when it calculates the routing table, a process based on LSAs.

Therefore, it is not strictly the number of routers or areas that is important, but the number of routes and the stability of the network. You must consider these issues because the number of LSAs in your network is proportional to the amount of router resources required.

With this understanding, the general rules stated by Cisco for OSPF design are that the following
numbers should not be exceeded:
■ Routers per area: 50
■ Neighbors per router: 60
■ Areas per router: 3
■ A router may not be a DR or BDR for more than 1 LAN
These are not hard and fast rules. The number of routers within an area depends on many factors; for example, a stub area with a 2500 router running over Ethernet is very different from area 0,

running 7500 routers over ATM. Some of the factors that influence the number of routers per area include the following:
■ What type of area is it: stub, totally stub, or backbone? This determines the number of LSAs and how often and how much CPU and memory each SPF computation requires.
■ What level of computing power do you have in the routers within the area? The smaller routers are not designed to manage large databases and to run the SPF algorithm continually.
■ What kind of media do you have? The higher the bandwidth on the link, the less congestion within the router as it queues the packets for transmission.
■ How stable is the network? How often LSAs will be propagated because of topology changes determines the need for bandwidth, CPU, and memory resources.
■ If the area is running over NBMA, is the cloud fully meshed? To overcome the resources required to maintain a fully meshed network, Cisco suggests that a well-designed partial mesh over low-bandwidth links reduces the number of links and thus the amount of traffic and resources required.
■ If the area has external connections, is there a large number of external LSAs? If the external connections are serviced with a default link, far less memory and CPU are required than if 500 external Internet links are propagated into the network.
■ Do you have a hierarchical design with summarization? The greater the summarization, the smaller and fewer the LSA packets that need to be propagated.

Cisco states that, normally, a routing table with less than 500 KB could be accommodated with 2 to 4 MB RAM; large networks with greater than 500 KB might need 8 to 16 MB, or 32 to 64 MB if routes are injected from the Internet.

NOTE Further information is available on the Cisco web site at http://www.cisco.com/warp/ public/104/3.html#17.0 in the OSPF Design Guide.

The following sections describe how to determine the appropriate number of neighbors to which a router should be connected, or the number of areas to which an ABR should be connected. In designing a network, elements in the network that use resources, CPU, memory, and bandwidth must be evaluated and provided for, where appropriate. Luckily, Cisco has performed extensive tests to provide clear guidelines for the design and implementation of an OSPF network.

Number of Neighbors per Router
Increasing the number of neighbors increases the resources on the router that are allocated to managing those links. More importantly if there is a designated router (DR), the router that performs the DR function might become overloaded if there are many routers on the link. It might be advisable to select the DR through manual configuration to be the router with the most available CPU and memory on the segment and to ensure that the router is not selected to be the DR for more than one link.

Number of Areas per ABR
For every area to which an ABR is connected, it will have a full topology table for that area. This could result in overloading the router before it has attempted to compute the best path. How many areas a router can support obviously depends on the caliber of the router and the size of the area. A good hierarchical design—where the maintenance of the areas is spread over a few routers—not only shares the resources, but also builds in a level of redundancy.

Summarization
One of the strengths of OSPF is the ability to scale the network. You can scale the network not only through the creation of multiple areas that limit the computation and propagation of routing updates, but also through the use of summarization. In Chapter 2, summarization was dealt with in great depth. This section builds on that knowledge and applies it to the design and implementation of multiarea OSPF.

In OSPF, two types of summarization exist:
■ Interarea summarization —This is performed at the ABR and creates Type 3 and 4 LSAs.
■ External summarization —This is performed at the ASBR and creates Type 5 LSAs.
Both have the same fundamental requirement of contiguous addressing.

OSPF is stringent in its demand for a solid hierarchical design, so much so that it has devised some commands to deal with situations that break its rules of structure.

The concept of the virtual link is explained in this section, while the commands with which to implement it are given in Chapter 9 in the section, “The area virtual-link Command.”

The Virtual Link
The main dictate in OSPF is that the multiple areas must all connect directly to the backbone area. The connection to the backbone area is through an ABR, which is resident in both areas and holds a full topological database for each area.

OSPF has provided a solution for the unhappy occasion when this rule cannot be followed. The solution is called a virtual link. If the new area cannot connect directly to the backbone area, a router is configured to connect to an area that does have direct connectivity.

The configuration commands create a tunnel to the ABR in the intermediary area. From the viewpoint of OSPF, the ABR has a direct connection.

The reasons such a situation might occur are as follows:

■ There is no physical connection to Area 0. This might be because the organization has recently merged with another or because of a network failure.
■ There are two Area 0s because of a network merger. These Area 0s are connected by another area (for example, Area 5).
■ The area is critical to the company, and an extra link has been configured for redundancy.

Although the virtual link feature is extremely powerful, virtual links are not recommended as part of the design strategy for your network. Instead, they are a temporary solution to a connectivity problem. You must ensure that you observe the following when creating a virtual link:

■ Both routers must share a common area.
■ The areas involved cannot be stub areas.
■ One of the routers must be connected to Area 0.
Figure 8-4 illustrates the use of a virtual link to provide a router in Area 10 connectivity to the backbone in Area 0.

Multiple Area OSPF Over an NBMA Network
Another design consideration is the design of the NBMA network as part of the OSPF domain. There are two main ways to approach the inclusion of an NBMA network:

■ The NBMA network can be defined as Area 0. The reasoning is that if the NBMA is used to connect all remote sites, all traffic will have to traverse this network. If the remote sites are made satellite areas, all traffic will have to traverse the NBMA, so it makes sense to make it the backbone area. This works well in a full-mesh environment, although it results in a large number of LSAs being flooded into the WAN and puts extra demands on the routers connecting to the NBMA network.
■ In a hub-and-spoke NBMA network, it makes sense to assign the hub network as Area 0 with the other remote sites and the NBMA network as other areas. This is a good design if the satellite areas are stub areas because it means that the routing information—and, thus, network overhead—is kept to a minimum over the NBMA cloud. Depending on the design, the rest of the network might constitute one other area or multiple areas. This will depend on the size and growth expectations of the OSPF domain.

The configuration of a basic OSPF over an NBMA network is provided in Chapter 7.
282 Chapter 8: Using OSPF Across Multiple Areas
Figure 8-4 Virtual Links in a Multiple Area OSPF Network

OSPF Network

Foundation Summary
The “Foundation Summary” section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a wellprepared candidate should, at a minimum, know all the details in each “Foundation Summary” before going to take the exam.

The following list identifies the different OSPF routers:

■ Internal router —Within an area, the functionality of the router is straightforward. It is responsible for maintaining a current and accurate database of every subnet within the area. It is also responsible for forwarding data to other networks by the shortest path. Flooding of routing updates is confined to the area.
■ Backbone router —The design rules for OSPF require that all the areas be connected through a single area known as the backbone area or Area 0. A router within this area is referred to as a backbone router. It can also be an internal router, an ASBR or an ABR.
■ ABR—This router is responsible for connecting two or more areas. It holds a full topological database for each area to which it is connected and sends LSA updates between the areas. These LSA updates are summary updates of the subnets within an area. It is at the area border that summarization should be configured for OSPF, because this is where the LSAs make use of the reduced routing updates to minimize the routing overhead on both the network and the routers.
■ ASBR—To connect to the outside world, or to any other routing protocol, you need to leave the OSPF domain. OSPF is an interior routing protocol or Interior Gateway Protocol (IGP); gateway is an older term for a router. The router configured for this duty is the ASBR. Although you can place this router anywhere in the OSPF hierarchical design, it should reside in the backbone area. Because any traffic leaving the OSPF domain is also likely to leave the router’s area, it makes sense to place the ASBR in a central location that all traffic leaving its area must traverse.

The five LSAs are as follows:

■ The router link —This LSA states all the links to the router sending out the LSA. The list is of all the neighbors attached to the router. The LSA is flooded to the area.
■ The network link —This LSA is sent out by the DR and lists all the routers on the segment for which it is the DR and has a neighbor relationship. The LSA is flooded to the whole area.
■ The network summary link —This LSA is sent between areas and summarizes the IP networks from one area to another. It is generated by an ABR.

■ The AS external (ASBR) summary link —This LSA is sent to a router that connects to the outside world (ASBR). It is sent from the ABR to the ASBR. The LSA contains the metric cost from the ABR to the ASBR.
■ The external link —This LSA is originated by AS boundary routers and flooded throughout the AS. Each external advertisement describes a route to a destination in another autonomous system. Default routes for the AS can also be described by AS external advertisements.

Table 8-3 shows the codes used in the routing table.

Codes and Associated

Some restrictions govern creating a stub area or a totally stubby area. Because no external routes are allowed in these areas, the following restrictions are in place:

■ No external routes are allowed.
■ No virtual links are allowed.
■ No redistribution is allowed.
■ No ASBR routers are allowed.
■ The area is not the backbone area.
■ All the routers are configured to be stub routers.

In designing an OSPF network, it is important to consider the following:
■ Summarization:
— Interarea: Performed at the ABR, creating type 3 and 4 LSAs
— External: Performed at the ASBR creating type 5 LSAs
■ Capacity planning:
— Router per area: 50
— Neighbors per router: 60
— Areas per router: 3
— A router may not be a DR or BDR for more than one LAN
■ Virtual links: As a temporary solution during transition or after a break in the network
■ NBMA networks:
— Creating the NBMA network as area 0 if it is a fully meshed network used to connect all other sites
— In a hub and spoke network, defining the hub as area 0, with the spokes forming other areas

Q&A
As mentioned in the introduction, “All About the CCNP, CCDP, and CCIP Certifications,” you have two choices for review questions. The questions that follow next give you a bigger challenge than the exam itself by using an open-ended question format. By reviewing now with this more difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of this chapter. The answers to these questions are found in Appendix A.

For more practice with examlike question formats, including questions using a router simulator and multichoice questions, use the exam engine on the CD.

1. In a totally stubby area, which routes are not propagated into the area?
2. Can a virtual link contain a stub area?
3. An ABR must be resident in which area?
4. What LSAs will the ABR forward?
5. State two advantages in creating areas in OSPF.
6. What is an external route, and on which type of router will this route be introduced?
7. Why is the use of summarization important in the design of OSPF?
8. How many routers does Cisco suggest is the limit to have in a single area?
9. What are the restrictions to be considered in the creation of a stub area or a totally stubby area?
10. A virtual link in OSPF is used to solve what problem?
11. State one disadvantage for making an NBMA cloud Area 0.
12. State one advantage in making the centralized routers and network resources dwell in Area 0 while the Frame Relay cloud and the stub remote LANs reside in satellite stub areas.
13. How does creating a number of areas in OSPF reduce the number of SPF calculations?
14. How does a stub area differ from the backbone area?
15. How does a totally stubby area differ from a stub area?
16. State the different LSA types.
17. Where does the backbone router reside, and what is its function?
18. There are two types of summarization. What are they?
19. For how many LANS does Cisco suggest a router should serve as a DR or a BDR?
20. Which router type creates LSA Types 3 and 4?

Scenarios
The following scenarios and questions are designed to draw together the content of the chapter and to exercise your understanding of the concepts. There is not necessarily a right answer. The thought process and practice in manipulating the concepts is the goal of this section. The answers to the scenario questions are found at the end of this chapter.

Scenario 8-1
A new network administrator recently joined the company and has found little documentation for the network. On drawing the topology of the network, the administrator has found a surprising configuration of a virtual link. Luckily, the administrator has come across such configurations in his previous job and understands their purpose.

After studying the figure provided, answer the following question.
1. Explain the purpose of the virtual link in Figure 8-5.

Network Diagram

The following figure shows the network of another company for which the administrator worked previously. Examine the figure and answer the questions.

288 Chapter 8: Using OSPF Across Multiple Areas

OSPF Across

2. Does the topology map in Figure 8-6 show a valid design?
3. Why would a company implement this design?

Scenario Answers
The answers provided in this section are not necessarily the only possible answers to the questions. The questions are designed to test your knowledge and to give practical exercise in certain key areas. This section is intended to test and exercise skills and concepts detailed in the body of this chapter.

If your answer is different, ask yourself whether it follows the tenets explained in the answers provided. Your answer is correct not if it matches the solution provided in the book, but rather if it has included the principles of design laid out in the chapter.

In this way, the testing provided in these scenarios is deeper: It examines not only your knowledge, but also your understanding and ability to apply that knowledge to problems.

If you do not get the correct answer, refer back to the text and review the subject tested. Be certain to also review your notes on the question to ensure that you understand the principles of the subject.

Scenario 8-1 Answers
1. Explain the purpose of the virtual link in Figure 8-5.
In this example, Area 1 does not have a direct physical connection into Area 0. A virtual link must be configured between Router A and Router B. Area 2 is to be used as a transit area, and Router B is the entry point into Area 0. This way, Router A and Area 1 will have a logical connection to the backbone.

2. Does the topology map in Figure 8-6 show a valid design?
Yes, the topology map in Figure 8-6 shows a valid design.
3. Why would a company implement this design?

OSPF allows for linking discontinuous parts of the backbone using a virtual link. In some cases, different Area 0s need to be linked together. This can occur, for example, if a company is trying to merge two separate OSPF networks into one network with a common Area 0. In other instances, virtual links are added for redundancy in case some router failure causes the backbone to be split in two. Whatever the reason might be, a virtual link can be configured between separate ABRs that touch Area 0 from each side and that have a common area between them.

No comments yet

Leave a Reply

You must be logged in to post a comment.