Ospf Network

Types of OSPF Packets

OSPF uses five different packets in its protocols. Table 3-2 describes the different OSPF packet types. This section discusses each protocol and the role that the packets play. Table 3-2 Overview of OSPF Packet Types Table 3-2 Overview of OSPF Packet Types Requests LSAs that need to be downloaded to requesting router. Only sent during Exchange, Loading, or Full state. Contains a list of the LSAs that are to be updated. Often used in flooding, as discussed later in this chapter. Acknowledges the...

Redistribution Configuration

This section reviews the command that is used to configure redistribution on a Cisco router. Note that the redistribute command can be used in any routing protocol, not just OSPF. Redistributing routes into OSPF from other routing protocols or from static routes causes these redistributed routes to be labeled in the routing tables as OSPF external routes. This makes sense if you recall the external route discussion in Chapter 5, Routing Concepts and Configuration. Because these routes are from...

Understanding the Seven Layers of the OSI Reference Model

The seven layers of the OSI reference model can be divided into two categories upper layers and lower layers. The upper layers are typically concerned only with applications, and the lower layers primarily handle data transportation. The sections that follow examine the three upper layers, the four lower layers, and the functions of each. The upper layers of the OSI reference model 5, 6, and 7 are concerned with application issues. They are generally implemented only in software programs. The...

Link State Versus Distance Vector Routing Protocols

This section describes the two most common and relevant routing protocols that TCP IP has available for use, namely RIP and OSPF. Controversy surrounds the debate over link-state versus distance vector routing algorithms regarding which is better. NOTE Link-State and distance vector routing protocols are also known as interior gateway protocols (IGPs) this concept is discussed later in the discussion of OSPF and border gateway protocol (BGP) interoperability. Chapter 7, Summarization, discusses...

Routing Types Within an OSPF Network

The following sections provide general descriptions of these route types. As you move further into designing and implementing OSPF networks, you will be exposed to these different types of routes. Intra-area routing describes routes to destinations within a logical OSPF area. Intra-area routes in OSPF are described by router (Type 1) and network (Type 2) LSAs. When displayed in the OSPF routing table, these types of intra-area routes are designated with an O. Inter-area routing describes...

Area Configuration

Areas have additional roles and functions similar in scope and purpose to OSPF route types. This section discusses OSPF areas and how to configure them within OSPF. You should review OSPF Areas in Chapter 2 and Area Design in Chapter 4. You should know how to activate a basic OSPF area by using the network command, as described earlier in this chapter. To configure a normal OSPF area, the network command is used when in OSPF router configuration mode. Example 5-6 initializes OSPF routing...

Normal NSSA Operation and Configuration

Table 5-11 presents shows which LSAs are active in a normal NSSA. Table 5-11 LSA Operation in a Normal NSSA Router link connected intra-area routes ABR summary links inter-area via ABR AS external links external routes via ASBR Not-so-stubby areas NSSA routes via ABR In Figure 5-16, you can see how and where the LSAs are transmitted within a stub area. When configuring an area as an NSSA, all routers in the area must be configured to identify the area as an NSSA. In Example 5-13, you change...

Introduction to OSPF

It seems appropriate in this chapter to share with you a caption from a small picture that my wife gave me when we celebrated our 13 th wedding anniversary. I keep it on my desk to remind myself of the bigger picture. My daughter also likes it because it has a picture of a family of dolphins swimming, and it struck a chord that I felt was essential to have in my life The family is a harbor of safety in an ocean of change. It is an association established in nature and guided by enduring...

Types of OSPF Routers

Four different types of routers designate the hierarchical routing structure used by OSPF. Each router has a unique role and set of defining characteristics within the hierarchy. Figure 2-9 shows a typical OSPF network, with multiple areas containing the different types of OSPF routers. Link to Another ' Autonomous i System (AS) ASBR Autonomous System Border Router ASBR Autonomous System Border Router The following sections provide general descriptions for the four types of OSPF routers....

How OSPF Authentication Works

When OSPF authentication has been configured on a router, the router authenticates the source of each routing update packet that it receives. This is accomplished by the exchange of an authenticating key (sometimes referred to as a password) that is known to both the sending and the receiving router. The following types of OSPF neighbor authentication are used Message Digest Algorithm Version 5 (MD5) authentication Both forms work in essentially the same way, with the exception that MD5 sends a...

Configuration Example 3 Redistributing Connected and Loopback Interfaces

This example looks at redistributing connected networks into OSPF, the results of this, and alternative ways of handling connected networks. In cases where you must redistribute connected networks into OSPF instead of configuring OSPF to recognize them via the network command, all the connected networks show up as external routes in OSPF. In this example, Router Neo is configured with a loopback interface, which is assigned the network 192.168.253.0 24. Figure 6-11 shows the overall network for...

Benefits of Summarization

Summarization, in general, provides many benefits to your network and its operation some of these benefits are apparent and others have a smaller impact on the health of network. There are four primary reasons to implement proper summarization Shrink the routing table This benefit should be clear to you after the preceding discussion. The use of summarization allows a smaller number of route entries in your table, with the added benefits of decreasing the amount of memory used. Specifically, it...

OnDemand Circuits

The OSPF on-demand circuit operational capability is an enhancement to the OSPF protocol that allows efficient operation over on-demand circuits such as ISDN, X.25, switched virtual circuits (SVCs), and dial-up lines. This feature set is fully supported by Cisco (in certain Cisco IOS Software versions) and follows the standard as described in RFC 1793, Extending OSPF to Support Demand Circuits. This RFC is worth consulting if you plan to configure OSPF to operate within this type of networking...

Verifying OSPF Operation

After you enable OSPF routing on each of the three routers, verify its operation using show commands, as demonstrated in Example 2-10. Invalid after 0 seconds, hold down 0, flushed after 0 With this command output, you can gain some useful data regarding the operation of OSPF in your network. You can see the networks that OSPF is routing as well the RIDs of the other routers in the network that have sent routing information. The output from the show ip ospf command, as demonstrated in Example...

Implementing Your Design

This section discusses some of the design topics to consider within this case study and how to implement them in the network given the preceding discussion. These can be both OSPF-specific topics and other all-encompassing network issues, such as IP addressing. You are able to obtain a contiguous block of 32 Class C ( 24 or 255.255.255.0 mask) subnets for this network from the IP address manager. The address block is 172.17.64.0 19, which allows clean summarization into the backbone area after...

Case Study Developing the Link State Database

Earlier in this chapter, you learned how LSAs were used to send information about the links between OSPF routers. These LSAs are stored in the router in a database with each LSA as a record in that database. Figure 2-16 shows the OSPF network topology for this case study. Figure 2-16 OSPF Network Topology for Link-State Database Case Study Example 2-6 shows entries generated by the show ip ospf database command from the router HAL9000. Example 2-6 Viewing the LSA Database for the HAL9000 Router...

SPF in Operation

The ARPAnet used one of the first distance vector routing protocols. This protocol evolved into RIP, which is still in use today. Serious limitations were encountered with RIP as networks grew. This caused a demand for a new protocol that could run within an AS and had the capability to grow (scale) to a large-sized network comprised of many routers and network links. Into this gap stepped OSPF version 1, published as Request for Comments (RFC) 1131 in October 1989 by the OSPF Working Group of...

Redistributing into OSPF with Route Tagging

There are many reasons to tag routes, some of which have already been covered. Some others reflect the desire of network engineers to manipulate routes based on tags. This is a nice way to quickly identify which routes you need. Consider that you want to know which routes are from RIP, say on Router Apoc. You could create an access list and a route map, but what if you tagged all the routes during the redistribution on Router Trinity, thus allowing a more flexible and faster identification of...

Hello Process Protocol

Although this is an OSPF book, many different protocols use a concept of Hello packets just like OSPF, for example EIGRP. Therefore, understanding the rationale behind the use and implementation of Hello is important. Specifically in OSPF, the Hello protocol is used for the following purposes To ensure that communication between neighbors is bidirectional (two-way) To discover, establish, and maintain neighbor relationships To elect the DR and BDR on broadcast and NBMA networks To verify that...

Default Routes

The ability to generate and propagate a default route in a network is important. Chapter 4, Design Fundamentals, discussed the methods of default routes in OSPF areas. This chapter covers the redistribution of an existing default route into OSPF. You can create a default route using one of the following two methods Advertise 0.0.0.0 inside the domain, but only if the ASBR itself already has a default route. Advertise 0.0.0.0 regardless of whether the ASBR has a default route. The latter method...

TCPIP Protocol Suite

A protocol is a set of rules and conventions that govern how devices on a network exchange information. This section discusses one of the more commonly used protocol suites TCP IP. This discussion does not provide sufficient information for an in-depth study of TCP IP. Nevertheless, TCP IP needs to be covered to some degree so that you can better understand the overall operation of network protocols these discussions are expanded in later chapters concerning OSPF. The TCP IP protocol suite is...

IP Addressing

This section discusses IP addressing methodology, basic subnetting, variable-length subnet masking (VLSM), and classless interdomain routing (CIDR). In a properly designed and configured network, communication between hosts and servers is transparent. This is because each device that uses the TCP IP protocol suite has a unique 32-bit IP address. A device reads the destination IP address in the packet and makes the appropriate routing decision based on this information. In this case, a device...

Types of Network Topologies

The preceding sections discussed the evolution of today's advanced networks and the building blocks that have evolved to make them what they are today that is, the OSI reference model and the TCP IP protocol. The sections on the OSI reference model described the essential means of how data is transported between the various layers that are running on all intranet devices. The TCP IP section reviewed the protocols' characteristics. This section addresses the media that operates in your network....

OSPF Routing Concepts

OSPF is a dynamic link-state routing protocol that uses a link-state database (LSDB) to build and calculate the shortest path to all known destinations. It is through the use of Dijkstra's SPF algorithm that the information contained within the LSDB is calculated into routes. The shortest path algorithm by itself is quite complicated, and its inner workings were covered in depth in Chapter 3, OSPF Communication. It is important to have a good understanding of Dijkstra's SPF algorithm to achieve...

Loopback Interfaces

OSPF uses the highest IP address configured on an active interface as its RID. If the interface associated with this IP address is ever unavailable, or if the address is removed, the OSPF process must recalculate a new RID and flood all its routing information out its interfaces. The highest IP address on a router would be the largest numerical IP address assigned to an active interface. If a loopback interface is configured with an IP address, OSPF defaults to using this IP address as its RID,...

Simple Network Management Protocol

Department of Defense (DoD) Advanced Research Projects Agency (ARPA) in the late 1960s and early 1970s, it was accepted as a USDoD networking standard. The further development of ARPA, or rather the ARPA network (ARPAnet), allowed the continued development of a global set of networks based on the TCP IP protocol. This global set of networks evolved into the Internet. The Internet quickly outgrew the capability of network engineers to manually monitor and...

Adding OSPF to the Center of a RIP Network

A common first step in converting a RIP network to an OSPF network is to convert backbone routers into running both RIP and OSPF, while the remaining network edge devices run RIP. These backbone routers automatically become OSPF ASBRs (redistributing RIP into OSPF). Each ASBR controls the flow of routing information between OSPF and RIP. In Figure 6-8, Routers Morpheus, Neo, and Trinity are configured as ASBRs when redistributing RIP into OSPF. RIP does not need to run between the backbone...

Basic Routing Protocol Operation

Consider an example of a router that is initially configured with two networks to which it directly connects. The router has only these two networks in its routing tables. However, other networks beyond the initial two are not entered into the routing table because they do not directly connect to the router. So how does the router recognize these other networks This can be accomplished in the following ways Static routing A manually defined and installed type of route within the router as the...

Configuration Example 3 Subnetting with Summarization

Summarization is a wonderful concept in networking that can give networks a variety of benefits, as discussed earlier in this chapter. It is important to provide a template that demonstrates how you might go about designing or redesigning an OSPF network with summarization in place from the beginning. This latter case is the more likely scenario, and you might have already been involved in projects to renumber and readdress networks that needed a new and improved logical look. These situations...

Hello Protocol Packet Format

The OSPF Hello protocol packets are formatted in only one way. All OSPF packets start with a standardized 24-byte header, which contains information that determines whether processing is to take place on the rest of the packet. The packets contain the fields that are shown in Figure 3-23, always in the same order. All the fields in this format are 32-bit fields, except for the following fields The following list describes what each of the packet fields represents Version Identifies the OSPF...

Case Study Adding a New OSPF Router to a Network

This case study provides a scenario that covers most of the information presented in this chapter. Suppose that a new OSPF router is added to a network. With this scenario, follow the case study to understand the ramifications of how adding a new OSPF router would affect an operating network. Refer to Figures 2-12 through 2-15, which detail each step of the process as it occurs in the following sequence 1 A new OSPF router is added to the network. 2 This new router immediately transmits a...

Neighbor Stuck in 2Way State

In the topology in Figure 3-35, all routers are running OSPF neighbors over the Ethernet network. Example 3-18 provides sample output of the show ip ospf neighbor command on R7. Example 3-18 Output from the show ip ospf neighbor Command for Router 7 Example 3-18 provides sample output of the show ip ospf neighbor command on R7. Example 3-18 Output from the show ip ospf neighbor Command for Router 7 R7 establishes full adjacency only with the DR and BDR. All other routers have a 2-way adjacency...

Database Exchange State Changes

The following is a brief description of the possible OSPF neighbor state changes when the routers are exchanging DDs. These steps occur when two routers decide to form an adjacency. For example, on broadcast media, a router becomes full only with the DR and the BDR it stays in the 2-way state with all other neighbors ExStart This state indicates the first step in creating an adjacency, the goal of which is to decide which router is the master and which is the slave. The master router is the...

Configuring OSPF Authentication in an Area

Review for a moment why you would want to configure OSPF authentication. You want to protect and preserve the routing updates that flow between our routers, but why Someone might spoof routing updates, which leads you to the conclusion that if someone could gain access to spoof your routing updates, protecting him her with a plain text password is useless. A nice side benefit of this configuring section is that if you understand MD5 you should have no difficulty with plaintext which is less...

Router Configuration Examples

Example 4-11 shows the configurations contained within the routers shown in Figure 4-35. These configurations were built with Cisco IOS Software Release 11.3. They do not work with older releases of Cisco IOS Software. For older releases, add the frame-relay map ip Example 4-11 Case Study Router Configuration for the Network Topology Shown in Figure 4-35 Example 4-11 Case Study Router Configuration for the Network Topology Shown in Figure 4-35 (Continued) Example 4-11 Case Study Router...

Type 5 Autonomous System External LSAs

Type 5 LSAs are generated by the ASBRs. These LSAs describe routes to destinations that are external to the AS. Type 5 LSAs are flooded everywhere, with the exception of stub areas. External links are an indication of networks outside of the OSPF routing process in the AS. These outside networks can be injected into OSPF via different sources, such as static and redistribution. The ASBR has the task of injecting these routes into an AS. Figure 3-9 illustrates the operation of Type 5 LSAs....

Type 7 NotSoStubby Area LSAs

These LSAs describe routes within a not-so-stubby-area (NSSA). Type 7 LSAs can be summarized and converted into Type 5 LSAs by the ABRs for transmission into other OSPF areas. After Type 7 LSAs are converted to Type 5 LSAs, they are distributed to areas that can support Type 5 LSAs. Figure 3-11 illustrates the operation of Type 7 LSAs, and Figure 3-12 illustrates the format of Type 7 LSA packets. makes the stub area router ID) translates makes the stub area...

NotSoStubby Areas

As mentioned in Chapter 2, NSSAs have their own RFC and are an interesting concept to the normal operation of OSPF. The advent of this new type of hybrid stub area also introduced a new LSA, Type 7, which is responsible for carrying external route information. NSSAs are similar to regular OSPF stub areas except that an NSSA does not flood Type 5 external LSAs from the core into the NSSA, but as a hybrid stub area, an NSSA has the capability to import AS external routes in a limited fashion...

About the Technical Reviewers

Henry Benjamin , CCIE No. 4695, holds three CCIE certifications (Routing and Switching, ISP Dial, and Communication and Services). Formerly with the Cisco Systems CCIE global team, Henry is now an independent consultant for a large security firm in Australia. He has served as a proctor for the CCIE Lab exams and is the author of CCNP Practical Studies Routing from Cisco Press and CCIE Routing and Switching Exam Cram from Coriolis. Matthew H. Birkner, CCIE No. 3719, is a technical leader at...

Understanding LSA Group Pacing

The OSPF LSA group pacing feature allows the router to group OSPF LSAs and pace the refreshing, checksumming, and aging functions. The group pacing results in more efficient use of the router. NOTE Cisco has made LSA group pacing the default behavior for OSPF in Cisco IOS Software Each OSPF LSA is recorded and tracked with an age that indicates whether the LSA is still valid. When the LSA reaches the maximum age (60 minutes), the LSA is discarded from the link-state database. A router keeps...

Altering Neighbor Cost

In your network, you want to prioritize or alter traffic flow based on the cost of a link. Suppose that you want to alter (increase or decrease) the default cost that is associated with a link to a neighbor. You can change this cost by assigning a cost associated with that neighbor as follows On point-to-multipoint broadcast networks, there is no need to specify neighbors. However, you can specify neighbors with the neighbor command in which case you should specify a cost to that neighbor. On...

Multipoint Subinterfaces

Cisco serial interfaces are multipoint interfaces by default, unless specified as a point-to-point subinterface. However, it is possible to divide the interface into separate virtual multipoint subinterfaces. Multipoint interfaces or subinterfaces are still subject to the split horizon limitations, as previously discussed. All nodes attached to a multipoint subinterface belong to the same network number. Typically, multipoint subinterfaces are used in conjunction with point-to-point interfaces...

Network Data Encryption

To safeguard your network data, Cisco provides network data encryption and route authentication services in Cisco IOS Software. This section briefly discusses how route authentication in OSPF is done and how it can benefit your network. Network data encryption is provided at the IP packet level. IP packet encryption prevents eavesdroppers from reading the data that is being transmitted. When IP packet encryption is used, IP packets can be seen during transmission, but the IP packet contents...

Altering OSPF Convergence

One of the most attractive features of OSPF is its capability to quickly adapt to topology changes. The two essential components to routing convergence are as follows Detecting changes to the network topology Rapid recalculation of routes OSPF uses the following mechanisms to detect topology changes Interface status changes (such as carrier failure on a serial link). The failure of OSPF to receive a Hello packet from its neighbor within a specified timing window, called a dead timer. Both of...

Using Tunable OSPF Parameters

A group of tunable OSPF parameters can help you design an area to more readily meet your network's specific needs. All of these commands and their associated values generally default to good values. If you are considering changing these defaults, it is good practice to change them in all routers in an area, or your routers might have trouble communicating. Make sure you take precautionary measures before you change the default values. TIP Cisco routers do not show default values in their...

Administrative Distance and Metrics

Regardless of the reason that you have encountered redistribution, there are some characteristics of how it operates within OSPF and on Cisco routers. When redistributing from one routing protocol to another, keep in mind the following items Previous chapters have discussed the metrics used by OSPF and how to manipulate them. Administrative distances help with route selection among different routing protocols, but they can cause problems for redistribution. These problems can be in the form of...

New WAN Requirements

Terrapin wants to remove the leased lines and 3174 cluster controllers and to network its sales offices using Frame Relay. The new WAN must seamlessly integrate into the existing corporate internetwork. Be aware of the following items from a WAN design perspective The existing corporate campus network infrastructure is 10-Mbps Ethernet, with an established router base of 30 Cisco routers of various models. The current network protocols in use are IPX RIP and TCP IP with Interior Gateway Routing...

Virtual Link Configuration Example

Previous sections discussed the characteristics of OSPF virtual links and provided an example. You now learn how to configure a virtual link in a real network. You are also going to see some packet captures of the virtual link in operation as well as understand the operation of a virtual link. This can help you understand how the packets flow throughout the network. The network you are to configure is shown in Figure 4-31. Note that a new area exists that cannot be physically connected to area...

VLSM Pitfalls

Some of the disadvantages of VLSM are as follows It is easy to make mistakes in address assignment, so double-check your calculations before deploying them. It is more confusing and difficult to monitor your network with all the new subnets. There are more routing entries, and it is difficult to configure summarization. There is an increased possibility of assigning overlapping subnets. When using VLSM, be careful about assigning addresses, for example, with a Class B network number of...

LSA Operation Example

Now that all nine implemented LSAs have been discussed and you understand how they operate within the OSPF functional environment, refer to Figure 3-15 for a visual representation of the operation and interaction among the various types of LSAs within an OSPF network. Type 5 LSAs are flooded to all areas. Type 5 LSAs are flooded to all areas. Type 4 ASBR Summary LSAs are sent to describe the ASBR by the ABRs. Type 3 ABR Summary LSAs are sent describing the networks, but only to the area that...

Neighbor Stuck in Init STATE

The init state indicates that a router sees Hello packets from the neighbor, but two-way communication has not been established. A Cisco router includes the router IDs of all neighbors in the init (or higher) state in the neighbor field of its Hello packets. Example 3-15 shows sample output of the show ip ospf neighbor command. Example 3-15 Output from the show ip ospf neighbor Command For two-way communication to be established with a neighbor, a router must also see its own router ID in the...

Selecting the Designated Router

The DR and BDR on a LAN generally have the most OSPF work to do. Select routers to be the DR and BDR that are not already heavily loaded with CPU-intensive activities. This can be accomplished by using the ip ospf priority priority command, which enables you to organize the DRs as needed. In review, some of the characteristics and behavior of Cisco routers when using priority are as follows By default, all Cisco routers have a priority of 1, thus forcing the use of the router ID (RID) as the...

OSI Reference Model Layers and Information Exchange

The seven OSI layers use various forms of control information to communicate with their peer layers in other computer systems. This control information consists of specific requests and instructions that are exchanged between peer OSI layers. Control information typically takes one of two forms Headers Appended to the front of data passed down from upper layers Trailers Appended to the back of data passed down from upper layers OSI layers are not necessarily required to attach a header or...

Designated Routers

OSPF builds adjacencies between routers for purposes of exchanging routing information. However, when OSPF has to deal with NBMA or broadcast networks, a problem presents itself. In these types of networks, there are multiple routers, which would result in too many adjacencies. To combat superfluous adjacencies, the Designated Router (DR) was introduced. OSPF designates a single router per multiaccess network to build adjacencies among all other routers. You can calculate the number of...

Route Tagging

Route tagging is one of the neatest possible ways of manipulating routes. Having learned how to manipulate routes the long way with access control lists, you are now ready for the more efficient manner of manipulation using route tagging. While many network engineers use route tagging as a way to filter routes (which is certainly a good use), there are many other uses. Although this is not a book on route filtering, I have included tagging in this redistribution chapter because it is of value...

Other Passwords

Almost all passwords and other authentication strings in Cisco IOS Software configuration files are encrypted using the weak, reversible scheme used for user passwords. To determine which scheme has been used to encrypt a specific password, check the digit preceding the encrypted string in the configuration file. If that digit is a 7, the password has been encrypted using the weak algorithm. If the digit is a 5, the password has been hashed using 476 Chapter 8 Managing and Securing OSPF...

Configuration Example 4 Redistributing OSPF and EIGRP

EIGRP is a popular routing protocol for those wishing to deploy a single-vendor network (and obviously that vendor is Cisco). Is that a smart move That is a Layer 8 political discussion. Having options and flexibility does not mean that you only look at buying one vendor's product. I like Cisco equipment and Cisco's routing protocol EIGRP, but having both together puts too many constraints on the business of running a network. The use of EIGRP is solid, and it is useful to understand how to get...

Area Design Overview

When creating large-scale OSPF internetworks, the definition of areas and assignment of resources within areas must be done with a pragmatic view of your OSPF internetwork. This assignment of resources includes both physical and logical networking components so that optimal performance results. This section discusses some of the items that are applicable to designing any type of OSPF area. Specific considerations are discussed after each area type. Areas are essentially small networks contained...

Altering OSPF Administrative Distance

An administrative distance is a rating of the priority (that is, trustworthiness) of a routing information source, such as an individual router or a group of routers. Numerically, an administrative distance is an integer from 0 to 255. Specifically, the higher the numerical value of administrative distance, the lower the trust rating. An administrative distance of 255 means that the routing information source cannot be trusted and should be ignored. Table 5-14 shows administrative distance...

Multiple Community Strings

For SNMPvl operation, Cisco permits multiple community strings so that a router can belong to multiple communities. An SNMP community string is simply a password in the sense that if you know the community string, you can access SNMP data. Furthermore, community strings can be either read-only or read write. This feature provides further security by restricting the capability to alter the configuration of Cisco devices to those that have the community string assigned the read write capability....

Flooding Process Protocol

Flooding in OSPF is responsible for validating and distributing link-state updates to the link-state database whenever a change or update occurs to a link. Changes or updates are key concepts regarding when flooding occurs. Flooding is part of the LSDB synchronization mechanism within OSPF. The goal of this mechanism is to keep the LSDBs of the routers in an OSPF domain synchronized within time in the presence of topological changes. In the event of a link-state change (for example, from up to...

Interoperability Issues with VLSM

Routers in a single segment must agree on the network mask. For example, if every router does not agree on the same mask for an Ethernet segment or a Frame Relay link, a breakdown in communication will occur. Consider that IGRP does not support VLSM, so when information is redistributed from OSPF to IGRP or RIP version 1 (RIP-1), only a single mask is used. The best way to make redistribution work is to hide all VLSMs from IGRP. OSPF should summarize the networks to achieve one mask per network...

Distribute List Concerns

This is an important point to note regarding the operation of OSPF. Recall that redistribution occurs on an OSPF ASBR because in OSPF an ASBR connects to other routing domains. This concept will become important shortly. There are two ways to apply a distribute list on a router To affect routing updates coming inbound Applied outbound to affect the routing updates be sent by the router. Use the following commands to achieve applying a distribute list distribute-list out When applying a...

Configuration Example 5 Redistributing OSPF and RIP and Tagging Routes

In your network, you have connected Router Trinity to OSPF area 10 and a RIP network as well. The entire OSPF network needs to know about the following networks These networks are found on Router Trinity and are part of the RIP routing domain. Because the objective is for the entire OSPF network to learn about them, you are going to be monitoring the routing table of Router Apoc to see when it learns of these routes. As you would expect, in the routing table in Example 6-59, Router Apoc has no...

Using Route Tagging to Protect Against Routing Loops

This section shows you the configuration steps used to identify routes via a route tag during mutual redistribution to prevent loops. At first, you have the same basic redistribution configuration that was used in the previous example. From the base configuration in Example 6-55, you build the route tag configuration. Example 6-55 Basic Redistribution Configuration During the steps to configure and assign a route tag, you are first going to create a route map that tags each route. One route map...

OSPF Network Scalability

Your ability to scale an OSPF internetwork depends on your overall network structure and IP addressing scheme. Adopting a hierarchical network design with a structured address assignment (that is, using summarization whenever possible) is the most important factor in determining the overall scalability of your OSPF network. Network scalability is affected by both operational and technical considerations. Operationally, OSPF networks should be designed so that areas do not need to be split to...

OSPF Security

The two kinds of security mechanisms applicable to routing protocols are as follows The routers that participate in an OSPF network are controlled. OSPF contains an optional authentication field. You might think it is possible to control the routing information within an OSPF area. However, for OSPF to operate properly, all routers within an area must have the same database. As a result, it is not possible to use route filters in an OSPF network to provide security because OSPF exchanges route...

NSSA Type 7 LSA Filtering

Review the network that was just discussed, and consider how you can accomplish stopping the network 192.168.254.0 24 from being propagated to the rest of the OSPF domain. Because OSPF is a flexible protocol, it has a built-in way of filtering out routes. This is a way to control which Type 7 LSAs are translated and forwarded outside the NSSA. Use the following configuration on either the NSSA ASBR or the NSSA ABR to selectively block the translation of LSAs router ospf 100 summary-address...

Telnet Session Timeouts

Setting the login and enable passwords might not provide enough security in some cases. The timeout for an inactive Telnet session (10 minutes by default) provides an additional security measure. If the console is left unattended in privileged mode, any user can modify the router's configuration. You can change the login timeout via the exec-timeout mm ss command, where mm is minutes and ss is seconds. The commands in Example 8-14 show what happens before, during, and after changing the timeout...

Link State Database Size Considerations

Issues relating to the size of the LSDB are important and deal directly with the LSDB operation in relation to the topology of the network. The LSDB is everywhere within an OSPF network, and the LSDB has the following functional characteristics, which do not change A router has a separate LSDB for each area to which it belongs. All routers belonging to the same area have an identical LSDB. A router performs separate SPF calculations on associated LSDBs for each area. LSA flooding occurs only...

Using Private Addressing to Summarize

Private addressing is another option often cited as being simpler than developing an area scheme using bit-wise subnetting from a network range assigned to you by your ISP. However, in many cases, you do not get what is optimal for your needs. Therefore, your business might need to develop its network address plan using solid business reasons to ensure success. Recall that networks are designed to meet a business need. This is especially true when making the decision on whether to use private...

Even More ECMP

OSPF performs ECMP per destination over up to four paths to a destination, but what happens when you have five or even six equal-cost paths do they go unused Absolutely not The maximum-path command, when applied to OSPF, allows you increase from four to six the maximum number of paths supported. The example that follows shows this in action with OSPF. In Example 5-32, eight ECMPs route to network 192.168.1.0 from Router 1 to Router 2 have been configured. By default, OSPF places four ECMP route...

Backbone Design Golden Rules

Use the following guidelines when designing an OSPF backbone (area 0) Understand that area 0 is a transit area, not a destination for traffic. Ensure that the stability of the backbone area is maintained and monitored. Ensure that redundancy is built into the design whenever possible. Ensure that OSPF backbones are contiguous. Keep this area simple. Fewer routers are better. Keep the bandwidth symmetrical so that OSPF can maintain load balancing. Ensure that all other areas connect directly to...

NSSA defaultinformationoriginate

Used on the NSSA ABR (Router Neo) and in addition to the normal NSSA behavior that you would expect to see (inter- and intra-area routes), with this keyword option you also force the ABR to inject a default route (0.0.0.0 0) into the NSSA. Be aware of the following rules in this regard An NSSA ABSR can generate a default route but only when it has a default route in its routing table therefore, if it does not know about a default route, it cannot be expected to tell the NSSA about it. AN NSSA...

Using Route Maps to Protect Against Routing Loops

This section shows you the configuration steps used to deploy route maps to stop routing loops from occurring. At first, you have the same basic redistribution configuration as shown in Example 6-49. You build the route map from this base configuration. Example 6-49 Beginning Basic Redistribution Configuration To configure the route map to protect the networks from route feedback and thus the creation of a routing loop, take the following steps Step 1 List all the networks to be permitted...

Type 3 ABR Summary LSAs

Summary LSAs are generated only by area border routers (ABRs) and describe inter-area routes to various networks. Specifically, Type 3 LSAs describe networks that are within the OSPF autonomous system but outside of the particular OSPF area that is receiving the LSA. A Type 3 LSA has a flooding scope of being transmitted only into the area where the network or subnet is not found. For example, if an ABR is connected to area 1 and area 0 with a network that has subnet 172.16.1.0 24 in area 1, a...

Mending a Partitioned Area

OSPF does not actively attempt to repair area 0 partitions. When an area becomes partitioned, the new sections simply become separate areas. As long as the backbone can reach both of these areas, it continues to route information to them. A virtual link functions as if it were a point-to-point link. Physically, however, the link is composed of the two backbone routers, each of which is connected to area 0. The two backbone routers establish a virtual adjacency so that LSAs and other OSPF...

OSPF Authentication

OSPF incorporates a minimal amount of security within its design. That sounds contradictory. How can a protocol be designed with security, yet be minimal OSPF is not responsible for transmitting data, OSPF is responsible for transmitting routing updates and building a routing table. The authentication capacity provided is sufficient to protect its design goals. When OSPF was designed, the necessary fields required for security were included in the design of OSPF's packets. Nevertheless, the...

Agent Response to NMS Request

The flow chart presented in Figure 8-8 describes the second part of the SNMP operation in which the SNMP request is received by the managed device, which passes it on to the agent who processes and answers the request. This flow of events is presented in a generic format from a high-level perspective. As with any complex network operation, many events also occur that allow the operation to take place. Figure 8-8 Agent Response Flow Chart to NMS Request Step 1 Agent uses ASPN.1 Basic Encoding...

Creating Access Lists

Access list definitions provide a set of criteria that are applied to each packet that is processed by the router. The router decides whether to forward or block each packet based on whether the packet matches the access list criteria. Typical criteria defined in access lists are packet source addresses, packet destination addresses, or upper-layer protocol of the packet. However, each protocol has its own specific set of criteria that can be defined. For a given access list, you define each...

Altering LSA Retransmissions

Cisco routers have the capability to alter the timing in which they retransmit LSAs on a perinterface basis. When a router runs OSPF and when it transmits an LSA to a neighbor, the normal operation of OSPF is to hold that LSA until the router receives an acknowledgment that the LSA was received successfully. By default, a router waits 5 seconds for the acknowledgment and, if needed, the LSA is retransmitted. In certain instances, this waiting period is not long enough for the round trip when a...

OSPF and Eigrp Mutual Redistribution

Referring to the network in Figure 6-12, you can see in Example 6-46 that Router Cypher has no idea if any of the OSPF routes are possible via its connection to Router Tank. Example 6-46 Routing Table Before Mutual Redistribution Codes C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i -...

Stub Area Design

A stub area is a powerful network design technique. In OSPF, this area carries a default route and inter-area routes but does not carry external routes. Placing sections of the network in stub areas reduces network overhead because stub areas are essentially dead end areas. This reduces the routes being advertised across the network. For example, consider that you were redistributing 10,000 routes into area 0 of your OSPF network. Standard areas receive all sent routes however, stub area do not...

Configuration File Examples

Compare the partial configuration options in Examples 6-25, 6-26, and 6-27 with the full configuration files in Examples 6-28, 6-29, and 6-30 for the routers in question. Example 6-28 Full Configuration for Router Morpheus ip address 130.10.62.1 255.255.255.0 interface serial 1 ip address 130.10.63.1 255.255.255.0 interface ethernet 0 ip address 130.10.8.1 255.255.255.0 interface tokenring 0 ip address 130.10.9.1 255.255.255.0 router rip default-metric 10 network 130.10.0.0 Example 6-28 Full...