Which ospf router or routers would require the following command to define a stub area?

OSPF Router Types and Areas

OSPFv2 introduced areas as a way to cut down on the size of the link-state database, the amount of information flooded, and the time it takes to run the SPF algorithm, at least on areas other than the special backbone area.

An OSPF area is a logical grouping of routers sharing the same 32-bit Area ID. The Area ID can be expressed in dotted decimal notation similar to an IP address, such as 192.168.17.33. The Area ID can also be expressed as a decimal equivalent, so Area 261 is the same as Area 0.0.1.5. When the Area ID is less than 256, usually only a single number is used, but Area 249 is still really Area 0.0.0.249.

There are five OSPF area types. The position of a router with respect to OSPF areas is important as well. The area types are shown in Figure 14.5.

The OSPF Area 0 (0.0.0.0) is very special. This is the backbone area of an OSPF routing domain. An OSPF routing domain (AS) can consist of a single area, but in that case the single area must be Area 0. Only the backbone area can generate the summary routing topology information that is used by the other areas. This is why all interarea traffic must pass through the backbone area. (There are backdoor links that can be configured on some routers to bypass the backbone area, but these violate the OSPF specification.) In a sense, the backbone area knows everything. Not so long ago, only powerful high-end routers could be used on an OSPF backbone. On the Illustrated Network, each AS consists of only an Area 0.

If an area is not the backbone area, it can be one of four other types of areas. All of these areas connect to the backbone area through an Area Border Router (ABR). An ABR by definition has links in two or more areas. In OSPF, routers always form the boundaries between areas. A router with links outside the OSPF routing domain is called an autonomous system boundary router (ASBR). Routing information about destination IP addresses not learned from OSPF are always advertised by an ASBR. Even when static routes, or RIP routes, are redistributed by OSPF, that router technically becomes an ASBR. ASBRs are the source of external routes that are outside of the OSPF routing domain, and external routes are often very numerous in an OSPF routing domain attached to the global Internet. If a router is not an ABR or ASBR, it is either an internal router and has all of its interfaces within the same area, or a backbone router with at least one link to the backbone. However, these terms are not as critical to OSPF configurations as to ABRs or ASBRs. That is, not all backbone routers are ABRs or ASBRs; backbone routers can also be internal routers, and so on.

Configuring Virtual Links

As mentioned in step 5 of the “Stepping through the Initial OSPF Configuration” section, you must configure a virtual link for any area that does not connect directly to the backbone area. You configure the virtual link on both the ABR for the discontiguous area and another ABR that connects to the backbone. The virtual link acts like a point-to-point link.

The routing protocol traffic that flows along the virtual link uses intra-area routing only.

1

Create an area that does not connect directly to the backbone area, and configure an interface to be in that area.

In the Add A New Virtual Link field, enter the router ID of the remote endpoint of the virtual link.

Select the transit area from the drop-down box. This is the area that connects both to the backbone and to the discontiguous area. Additional fields will appear.

Configure the following parameters for the virtual link:

Hello interval Length of time, in seconds, between hello packets that the router sends on the interface. For a given link, this field must be the same on all routers, otherwise adjacencies do not form. The default hello interval value is 30 seconds.

Dead interval Number of seconds after the router stops receiving hello packets that it declares the neighbor is down. For a given link, this value must be the same on all routers, or adjacencies do not form. The value must not be zero and must fall within the range of 1–65535. The default dead interval value is 120 seconds.

Typically, the value of this field should be four times that of the hello interval.

Retransmit interval Specifies the number of seconds between LSA retransmissions for adjacencies belonging to this interface. This value is also used when retransmitting database description and link state request packets. Set this value well above the expected round-trip delay between any two routers on the attached network. Be conservative when setting this value to prevent unnecessary retransmissions. Range: 1–65535 in number of seconds. The default retransmit interval value is five seconds.

Auth Type Type of authentication scheme to use for a given link. In general, routers on a given link must agree on the authentication configuration to form neighbor adjacencies. This feature guarantees that routing information is accepted only from trusted routers. The available options are None, Simple, and MD5. The default auth type value is None.

If you selected MD5 for the auth type, you must also configure the following parameters:

Add MD5 Key If the Auth type selected is MD5, the Key ID and MD5 Secret fields appear. Specify the Key ID and its corresponding MD5 secret to configure a new MD5 key.

If you configure multiple Key IDs, the Key ID with the highest value is used to authenticate outgoing packets. All keys can be used to authenticate incoming packets.

Key ID The Key ID is included in the outgoing OSPF packets to enable the receivers to use the appropriate MD5 secret to authenticate the packet. The available Key ID range is 0–255.

MD5 Secret The MD5 secret is included in encrypted form in outgoing packets to authenticate the packet.

Click Apply.

To make your changes permanent, click Save.

Repeat this procedure on both the ABR for the discontiguous area and an ABR that connects to the backbone area.

Areas

IS–IS provides two-level network hierarchy using areas that are similar to OSPF. The routers in the backbone area are called L2 routers; the internal routers in the low-level areas are called L1 routers. A network that has any low-level (L1) areas, must also have at least one L1/L2 router that sits in the L1 area but is connected to the L2 (backbone) area by a link. Note that in IS–IS, a router is entirely within an area, unlike OSPF, where a router can sit on the border between two areas; connectivity between areas is only through a link.

Configuring OSPF Interfaces

The following commands can be used to configure a backbone and other areas, such as stub areas, for specified interfaces.

set ospf

area <backbone | ospf_area> range ip_prefix <on | off>

area <backbone | ospf_area> range ip_prefix restrict <on | off>

stub-network ip_prefix <on | off>

stub-network ip_prefix stub-network-cost <1-677722>

interface if_name area <backbone | ospf_area> <on | off>

interface if_name hello-interval <1-65535>

interface if_name hello-interval default

interface if_name dead-interval <1-65535>

interface if_name dead-interval default

interface if_name retransmit-interval <1-65535>

interface if_name retransmit-interval default

interface if_name cost <1-65535>

interface if_name priority <0-255>

interface if_name passive <on | off>

interface if_name virtual-address <on | off>

interface if_name authtype none

interface if_name simple password

interface if_name md5 key authorization key id secret md5 secret

interface if_name md5 key authorization key id

Table 11.17 describes the arguments for configuring the OSPF interfaces.

Table 11.17. OSPF Interface Configuration Arguments

Types of OSPF Areas

Building on our explanation of OSPF areas, there are actually a few different types of areas that serve different purposes. We will cover each of them in the following:

Backbone Area This is the central area in your OSPF routing domain which all areas must connect to. This area is labeled area 0 (or the longhand version 0.0.0.0). When traffic needs to pass from one area to another, it must traverse the backbone. Routes are advertised to all other areas from the backbone.

Stub Area This is an area which connects to the backbone. Traffic does not pass through the area, but rather originates from, or is destined to, this area. It must have a unique label value within the routing domain and cannot be labeled area 0. Routes learned here are advertised into the backbone, as well as other areas (often in a summarized form).

Totally Stubby Area This is similar to a Stub Area, except it does not accept any summarized routes to be injected into it.

Not So Stubby Area This is an area which connects to the backbone, but it might also connect to a remote network. In other words, traffic may traverse through this area. This area may advertise routes learned to other areas. Sometimes a Not So Stubby Area (NSSA) will contain VPN connections to other trading partners, or similar scenarios.

OSPF Router Types and Areas

OSPFv2 introduced areas as a way to cut down on the size of the link-state database, the amount of information flooded, and the time it takes to run the SPF algorithm, at least on areas other than the special backbone area.

An OSPF area is a logical grouping of routers sharing the same 32-bit Area ID. The Area ID can be expressed in dotted decimal notation similar to an IP address, such as 192.168.17.33. The Area ID can also be expressed as a decimal equivalent, so Area 261 is the same as Area 0.0.1.5. When the Area ID is less than 256, usually only a single number is used, but Area 249 is still really Area 0.0.0.249.

There are five OSPF area types. The position of a router with respect to OSPF areas is important as well. The area types are shown in Figure 15.5.

The OSPF Area 0 (0.0.0.0) is very special. This is the backbone area of an OSPF routing domain. An OSPF routing domain (AS) can consist of a single area, but in that case the single area must be Area 0. Only the backbone area can generate the summary routing topology information that is used by the other areas. This is why all interarea traffic must pass through the backbone area. (There are backdoor links that can be configured on some routers to bypass the backbone area, but these violate the OSPF specification.) In a sense, the backbone area knows everything. Not so long ago, only powerful high-end routers could be used on an OSPF backbone. On the Illustrated Network, each AS consists of only an Area 0.

If an area is not the backbone area, it can be one of four other types of areas. All of these areas connect to the backbone area through an Area Border Router (ABR). An ABR by definition has links in two or more areas. In OSPF, routers always form the boundaries between areas. A router with links outside the OSPF routing domain is called an autonomous system boundary router (ASBR). Routing information about destination IP addresses not learned from OSPF are always advertised by an ASBR. Even when static routes, or RIP routes, are redistributed by OSPF, that router technically becomes an ASBR. ASBRs are the source of external routes that are outside of the OSPF routing domain, and external routes are often very numerous in an OSPF routing domain attached to the global Internet. If a router is not an ABR or ASBR, it is either an internal router and has all of its interfaces within the same area, or a backbone router with at least one link to the backbone. However, these terms are not as critical to OSPF configurations as to ABRs or ASBRs. That is, not all backbone routers are ABRs or ASBRs; backbone routers can also be internal routers, and so on.

Open Shortest Path First (OSPF)

OSPF was the natural successor to the RIP. OSPF protocol is a hierarchical IGP that uses a link state in the individual areas that make up the hierarchy. A link state database (LSDB) creates a tree-image of the network topology. It then sends copies of the LSDB periodically to update all routers in the area of the OSPF network.

OSPF is the most widely used IGP in regards to large enterprise networks. It has a much larger network size range than RIP. The OSPF protocol can determine the best path by communicating with other routers and then saving the routes in their LSDBs securely.

An OSPF network is divided into areas, which contain area identifiers. These identifiers are 32-bit and are usually written in the format of an IP address. Be aware that area identifiers are not IP addresses, and may often times duplicate any IP address without conflict occurring. These areas are logical groupings of routers whose information may be communicated to the rest of the network. There are several types of areas in an OPSPF network:

Backbone Area The backbone area forms the central hub of an OSPF network. All other areas are connected to it, and inter-area routing happens via routers connected to the backbone area and to their own non-backbone areas. The backbone area distributes all routing information between the non-backbone areas. The backbone must be adjacent to all other areas, but does not need to be physically contiguous. Connectivity can be established and maintained through virtual links. All OSPF areas must connect to the backbone area. This connection, however, can be through a virtual link.

Stub Area The stub area is an area that does not receive external routes except the default route, but does receive inter-area routes. All routers in the area need to agree they are stub, so that they do not generate types of LSA not appropriate to a stub area. Stub areas do not have the transit attribute and thus cannot be traversed by a virtual link.

Not-so-stubby area (NSSA) The Not-so-stubby area (NSSA) is a type of stub area that can import autonomous system (AS) external routes and send them to the backbone, but cannot receive AS external routes from the backbone or other areas. The NSSA is a non-proprietary extension of the existing stub area feature, which allows the injection of external routes in a limited fashion into the stub area.

Large Network, Multi-Area IGP Design with IP/MPLS Access

This section details the system architecture for a transport model where the network organization between the core and aggregation domains is based on a single autonomous system, multi-area IGP design. This model, illustrated in Figure 4.9, follows the approach of enabling a seamless MPLSLSP using hierarchical-labeled BGP LSPs across the core and aggregation network, and presents two approaches for extending the seamless MPLSLSP into the mobile RAN access domain.

From a multi-area IGP organization perspective, the core network is either an intermediate system to intermediate system (IS-IS) Level 2 or an open shortest path first (OSPF) backbone area. The aggregation domains, in turn, are IS-IS Level 1 or OSPF non-backbone areas. No redistribution occurs between the core and aggregation IGP levels/areas, so the route scale in contained within each domain. The MPLS/IP mobile access networks subtending from aggregation or pre-aggregation nodes are based on a different IGP process, restricting their scale to the level of the local RAN. To accomplish this, the pre-aggregation nodes run two distinct IGP processes. The first process corresponds to core-aggregation network (IS-IS Level 1 or OSPF non-backbone area), and the second process corresponds to the mobile RAN access network. The second IGP process could be an OSPF backbone area or an IS-IS Level 2 domain. All CSGs that are part of the mobile RAN access network subtending from a pair of pre-aggregation nodes are part of this second IGP process.

Partitioning these network layers into such independent and isolated IGP domains helps reduce the size of routing and forwarding tables on individual routers in these domains, which, in turn, leads to better stability and faster convergence within each of these domains. LDP is used for label distribution to build intra-domain LSPs within each independent access, aggregation, and core IGP domain. Inter-domain reachability is enabled with hierarchical LSPs using BGP-labeled unicast as per RFC 3107 procedures, where iBGP is used to distribute labels in addition to remote prefixes, and LDP is used to reach the labeled BGP next hop. Two options for extending the seamless MPLSLSP into the mobile RAN access domain to accommodate different operator preferences are presented below.

OSPF

OSPF is an open-standard dynamic routing protocol used to exchange routing information in large to very large networks. Compared to RIP, OSPF is more difficult to configure and administer but it tends to be much more efficient than RIP even in very large networks. OSPF requires very little network overhead, even in complex networks.

OSPF uses the shortest path first (SPF) algorithm to determine routes that should be added to the routing table. OSPF routers maintain a map of the internetwork called the link state database. This database is synchronized by all OSPF routers and the information contained in the link state database is used to compute routing table entries. Each OSPF router forms an adjacency with its neighboring routers. Any time a change occurs in the internetwork, information about the change is flooded to the entire network.

Every time updated information about the link state database is received, the routes in the routing table are recalculated. This presents a problem in that the larger the network, the more system resources are required to maintain and calculate route information. OSPF allows for routing areas as a means to cut down on routing changes flooded to the network and also to reduce the resource requirements for OSPF routers.AU routing areas must connect to the backbone area to facilitate proper route advertisements. An area is a group of contiguous networks and the hosts that are attached to them. OSPF uses Autonomous System (AS) numbers to label the areas. AS numbers are used to determine the authority for a network. In other words, an organization will be provided with a specific AS number or set of AS numbers to use for their network. This AS number provides accountability for routing information.

Each routing area maintains link state information only about itself, with the exception of the backbone. The backbone must maintain information about all other areas, including. itself. This provides a hierarchical network infrastructure that, when designed correctly, promotes scalability.

Everywhere that a border exists between areas, there must be at least one area border router (ABR). This is a router with multiple interfaces that attach to multiple areas. The ABR maintains routing information for each area that it belongs to, with a separate database for each area. The backbone is made up of the ABRs and any networks that aren’t completely within any of the areas (and their routers).

Also, OSPF has a special-case area in which only one entry and exit point exists for the network area. This area, known as a stub area, provides a default route for the area. A default route is a route listed with all zeros, providing a route for all external traffic where a specific route is not known. The stub area does not advertise external routes and, through the use of default route advertisement, significantly reduces the amount of link state advertisements and resource utilization within the stub area.

OSPF uses two different types of routes: inter-area and intra-area routes. Inter-area routes are advertisements passed between routing areas. Intra-area routes are routes advertised within a routing area. By design, all inter-area routes will eventually reach the backbone area in an OSPF network. The backbone area controls route advertisements to all surrounding areas. Figure 8.79 illustrates some of the OSPF terminology.

OSPF has the following advantages over RIP:

Network topology changes converge faster.

OSPF provides loop-free routes.

OSPF scales to large or very large internetworks.

The Microsoft Windows Server 2003 implementation of OSPF has the following features:

Coexistence with RIP

Dynamic addition and deletion of interfaces

Dynamic reconfiguration of all OSPF settings

Route filters for controlling interaction with other routing protocols

Test Day Tip

There is a trend in IT that seems to be a reflection of the popularity of the Internet itself. The Internet is based on the popular open standard TCP/IP. Companies have used proprietary methods for sharing information between their systems in the past. You will see in Window Server 2003 that most of the protocols reviewed here are open standard protocols. This ensures better interoperability between different vendor solutions and between partnering companies. These open standards are topics that are likely to be covered on the test. Understand the benefits of open standard protocols and review the open standard protocols supported in Windows Server 2003 (in particular, Microsoft’s implementation of them) for this exam. RIP, OSPF, RADIUS (IAS), and PEAP are just a few of the open standard protocols that we have discussed.

So far, we have been analyzing routing protocols that provide dynamic routing for unicast IP traffic. Next we will take a look at Microsoft’s support for multicast IP traffic.

Exercise 8.09

Configuring OSPF on a Windows 2003 Network

In this exercise, we will configure two Windows Server 2003 routers to advertise OSPF routes. As mentioned earlier in this section, it is preferable to use RIP v2 instead of RIP v1 for routing because it supports CIDR, a method capable of carrying subnet information within routing updates. In the previous example, if we used networks with non-default subnet masks (for example, a 10.0.0.10/24 address instead of 10.0.0.10/8), RIP version 1 will not properly advertise the routes because it does not understand nonclassful addressing. We would use RIP v1 only for compatibility reasons. In other words, use RIP v1 only if you have equipment that does not support RIP v2 routing.

For the final portion of this exercise, we will change our routing protocol to OSPF, as shown in Figure 8.80. To demonstrate some of the features of OSPF, we will configure the common connection between the routers as the backbone area or Area 0. Each of the respective LAN connections on our routers will be configured as Area 10 for the 192.168.1.0/24 network and Area 30 for the 192.168.3.0/24 network. Because each of these networks has only a single source for external traffic, we will configure these areas as stub areas. Let’s begin by disabling RIP and enabling OSPF for our Router.

Open the Routing and Remote Access management console by selecting Start | Programs | Administrative Tools | Routing and Remote Access.

Disable RIR Select RIP under IP Routing in the left pane of the management console. Right-click RIP and select Delete. Select Yes. When the message box asks Are you sure you want to remove RIP Version 2 for Internet Protocol?

Right-click the General tab and select Open Shortest Path First (OSPF) and select OK to enable OSPF on the server.

Now that OSPF is enabled, we need to specify interfaces for OSPF participation. From the left pane of the Routing and Remote Access management console, below IP Routing, right-click OSPF and select New Interface…. Select the common interface between the routers first (our WAN) interface. The OSPF Properties dialog box will be displayed as shown in Figure 8.81.

Router priority is used to determine which router will be the designated router on a broadcast network. We will go with the default settings here so select OK. Please note the other tabs on the OSPF Properties dialog box. The NBMA Neighbors tab provides configuration options for Non-Broadcast Multi-Access Neighbors as seen in multi-access environments like ATM and Frame Relay. The Advanced tab provides configuration options for OSPF timers and packet limitations.

Now that we have created our backbone area, we have to configure it. Right-click OSPF in the left pane of the Routing and Remote Access management console and select Properties. Select the Areas tab, select the backbone area (0.0.0.0), and select Edit as shown in Figure 8.82.

We will not use OSPF authentication for this scenario so our first configuration change should be to remove the check box from Enable plaintext password under the General tab as shown in Figure 8.83.

Under the Ranges tab, we will tell OSPF which interface range to advertise. Enter the network address for our backbone network (192.168.2.0) into the Destination: text box and the subnet mask (255.255.255.0) into the Network mask: text box and select Add. The Ranges tab should look like the one shown in Figure 8.84.

Select OK to commit the changes to the OSPF backbone.

Repeat the process outlined in steps 4 through 9 to configure Area 10.

Add the LAN interface to the OSPF process by right-clicking OSPF in the left pane of the management console and selecting New Interface…. Select the LAN interface that will be used for Area 10 and then select OK to accept the default settings.

Right-click OSPF in the left pane below IP Routing and select Properties. Click the Areas tab.

From the Areas tab, add Area 10 by selecting Add as shown in Figure 8.85.

From the General tab of the OSPF Area Configuration dialog box, set the Area ID: to 0.0.0.10 to correspond to Area 10 and remove the Enable plaintext password check box to disable authentication for our scenario and select Stub area as shown in Figure 8.86.

To add the network range for Area 10, select the Ranges tab and enter the network address for the Area 10 network (192.168.1.0) into the Destination: text box and the subnet mask (255.255.255.0) into the Network mask: text box. Select. Add. The Ranges tab should look like the one shown in Figure 8.87.

Select OK to complete the configuration of the OSPF Area Configuration and select OK to complete this overall configuration for OSPF.

Click OSPF in the left pane under IP Routing and then from the right pane of the management console right-click on the LAN interface that will be in Area 10; select Properties.

In the LAN Properties dialog box, select 0.0.0.10 from the Area ID: drop-down box on the General tab as shown in Figure 8.88.

Select OK to complete the configuration.

The second Windows Server 2003 router will be configured using almost identical configuration options. The only difference is in Area 30. Since the first configured router was connected to Area 10 and Area 0, the 192.168.1.0/24 interface was assigned to Area 10. On the next router, we will assign the 192.168.3.0/24 network to Area 30.

To verify OSPF routing click OSPF under IP Routing in the left pane of the management console. You should see both of the interfaces that are advertising OSPF routes in the right pane of the management console. The State should be listed as Designated-router or Backup designated-router. If the state is Waiting and more than a few minutes have passed since you completed your configurations, check the OSPF configuration settings to ensure both routers are communicating properly.

Right-click OSPF in the left pane of the management console and select Show Areas… to verify Area advertisement for the router as shown in Figure 8.89.

Verify neighbor adjacencies by right-clicking on OSPF in the left pane of the management console and selecting Show Neighbors… as shown in Figure 8.90.

Again, right-click OSPF in the left pane of the management console and select Show Link-state Database… to view the Link-state Database information as shown in Figure 8.91.

Each of the previous verification steps provided information about the operation of OSPF. The final steps to verify proper operation should include a check of the routing table and a ping to the remote network. First, click General under IP Routing in the left pane of the management console. Next, right-click any interface listed in the right pane and select Show IP Routing Table to view the routing table as shown in Figure 8.92.

Ping the remote network. Open a command prompt Start | Run | type cmd and click OK. From the command prompt on the machine connected to Area 10 or any machine on the Area 10 LAN, type ping 192.168.3.2 and press Enter to test connectivity between the networks as shown in Figure 8.93.

Self Test

A Quick Answer Key follows the Self Test questions. For complete questions, answers, and explanations to the Self Test questions in this chapter as well as the other chapters in this book, see the Self Test Appendix.