Basic Multi-area Configuration 173 RouterB interface Ethernet0 ip address 1.1.3.1 255.255.255.0 ! interface Ethernet1 ip address 1.1.2.2 255.255.255.0 ! router ospf 70 network 1.1.2.0 0.0.0.255 area 0 network 1.1.3.0 0.0.0.255 area 1 RouterC interface Ethernet0 ip address 1.1.4.1 255.255.255.0 ! interface Ethernet1 ip address 1.1.3.2 255.255.255.0 ! router ospf 70 network 1.1.3.0 0.0.0.255 area 1 network 1.1.4.0 0.0.0.255 area 1 Let’s examine the syntax to configure OSPF on RouterA. First, we need to enable the OSPF process on the router: RouterA (config)#router ospf 70 where 70 is the Process ID. Next, we need to identify each of the networks connected to the router that we want to participate in the OSPF process. In this example, we have two networks connected to RouterA (1.1.1.0/24 and 1.1.2.0/24): RouterA(config-router)#network 1.1.1.0 0.0.0.255 area 0 where 1.1.1.0 0.0.0.255 is the network and wildcard mask of a network connected to RouterA and where 0 is the area that network 1.1.1.0/24 is a member of. RouterA(config-router)#network 1.1.2.0 0.0.0.255 area 0 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 174 Chapter 5 Interconnecting OSPF Areas The syntax for RouterB is similar to that used for RouterA. The primary difference is that RouterB is connected to two areas: RouterB(config)#router ospf 70 RouterB(config-router)#network 1.1.2.0 0.0.0.255 area 0 RouterB(config-router)#network 1.1.3.0 0.0.0.255 area 1 The syntax for RouterC is very similar to that of RouterA. The difference is that RouterA is internal to Area 0, thereby classifying it as a backbone router: RouterC(config)#router ospf 70 RouterC(config-router)#network 1.1.3.0 0.0.0.255 area 1 RouterC(config-router)#network 1.1.4.0 0.0.0.255 area 1 Stub Area Configuration Since the main purpose of having stub areas (and totally stubby areas) is to keep such areas from carrying external routes, we need to review some design guidelines before configuring a stub area or a totally stubby area: Do not make the backbone area (Area 0) a stub area. Since external routes are injected by autonomous system boundary routers, do not make any area containing an ASBR a stub area. Since routers within a stub area use a default route to get out of the stub area, typically there is only one route out of the stub area. There- fore, a stub area should usually only contain a single area border router. Keep in mind that since a default route is being used, if a stub area contains more than one ABR, a non-optimal path may be used. If you decide to make a particular area a stub area, be sure to configure all the routers in the area as stubby. If a router within a stub area has not been configured as stubby, it will not be able to correctly form adjacencies and exchange OSPF routes. With these guidelines in mind, let’s examine a sample configuration for a stub area. Consider the network shown in Figure 5.4. We’re going to make Area 25 a stub area. In this example, we won’t be concerned with the con- figuration of RouterA, since it does not participate in Area 25. We will then examine the syntax for RouterB, RouterC, and RouterD. Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Stub Area Configuration 175 FIGURE 5.4 OPSF configuration example continued—stub area configuration RouterB interface Ethernet0 ip address 1.1.1.1 255.255.255.0 ! interface Ethernet1 ip address 10.1.1.1 255.255.255.0 ! interface Ethernet2 ip address 10.1.2.1 255.255.255.0 ! router ospf 10 network 1.0.0.0 0.255.255.255 area 0 network 10.0.0.0 0.255.255.255 area 25 area 25 stub RouterC RouterA RouterD Area 0 Area 25 Summary Route Information 10.1.1.2/24 e0 e0 e0 10.1.1.1/24 e1 e2 1.1.1.1/24 10.1.2.1/24 Summary Route Information RouterB External Route Information Default Route Information Stub Area 10.1.2.2/24 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 176 Chapter 5 Interconnecting OSPF Areas RouterC interface Ethernet0 ip address 10.1.2.2 255.255.255.0 ! router ospf 10 network 10.0.0.0.0 255.255.255 area 25 area 25 stub RouterD interface Ethernet0 ip adress 10.1.1.2 255.255.255.0 ! router ospf 10 network 10.0.0.0 0.255.255.255 area 25 area 25 stub First, we’ll configure RouterB. Notice that RouterB is an ABR and that it is the only ABR in Area 25, as recommended in our stub area design guide- lines. When configuring an ABR that is a member of a stub area, be cautious to only configure the stub area as stubby: RouterB(config)#router ospf 10 where 10 is the Process ID. RouterB(config-router)#network 1.0.0.0 0.255.255.255 area 0 where 1.0.0.0 0.255.255.255 is the network and wildcard mask of a net- work connected to RouterB and where 0 is the area that network 1.1.1.0/24 is a member of. RouterB(config-router)#network 10.0.0.0 0.255.255.255 area 25 where 10.0.0.0 0.255.255.255 is a summary network and wildcard mask of networks connected to RouterB and where 25 is the area that net- works 10.1.1.0/24 and 10.1.2.0/24 are members of. RouterB(config-router)#area 25 stub where 25 is the area that we have designated as stubby. Notice that instead of using two network statements to represent net- works 10.1.1.0/24 and 10.1.2.0/24, we used a single network statement specifying network 10.0.0.0/8, which includes, or summarizes, these two Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Totally Stubby Area Configuration 177 networks. By using these summary routes where possible, we can reduce the size of a router’s routing tables, thus lowering memory and processor overhead. We will also use the 10.0.0.0/8 summary when we configure RouterC and RouterD. Remember that it is critical that all routers that are members of a stub area be configured as stubby for that area. Therefore, RouterC and RouterD will have identical OSPF configurations: RouterC(config)#router ospf 10 RouterC(config-router)#network 10.0.0.0 0.255.255.255 area 25 RouterC(config-router)#area 25 stub RouterD(config)#router ospf 10 RouterD(config-router)#network 10.0.0.0 0.255.255.255 area 25 RouterD(config-router)#area 25 stub Let’s review some key elements of our stub area configuration example: The syntax to make a router stubby is area area-id stub. All routers that are part of Area 25 are configured as stubby. Area 25 has only one ABR (i.e., only one path out of the area). The ABR used the area area-id stub command only for Area 25, not for Area 0, which is not stubby. Totally Stubby Area Configuration Using the same network topology as we had for the stub area config- uration, let’s examine how to make Area 25 a totally stubby area. Remem- bering that the difference between a stub area and a totally stubby area is that a totally stubby area doesn’t have summary routes injected into it, we only need to change the configuration of RouterB. Since RouterB is the ABR, it is the router that will have the responsibility for blocking summary routes from entering the stub area. So, again consider our network, as illustrated in Figure 5.5. Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 178 Chapter 5 Interconnecting OSPF Areas FIGURE 5.5 OPSF configuration example continued—totally stubby area configuration RouterB interface Ethernet0 ip address 1.1.1.1 255.255.255.0 ! interface Ethernet1 ip address 10.1.1.1 255.255.255.0 ! interface Ethernet2 ip address 10.1.2.1 255.255.255.0 ! router ospf 10 network 1.0.0.0 0.255.255.255 area 0 network 10.0.0.0 0.255.255.255 area 25 area 25 stub no-summary RouterC RouterA RouterD Area 0 Area 25 Default Route Information 10.1.1.2/24 e0 e0 e0 10.1.1.1/24 e1 e2 1.1.1.1/24 Summary Route Information RouterB External Route Information Default Route Information Totally Stubby Area 10.1.2.1/24 10.1.2.2/24 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Totally Stubby Area Configuration 179 RouterC interface Ethernet0 ip address 10.1.2.2 255.255.255.0 ! router ospf 10 network 10.0.0.0 0.255.255.255 area 25 area 25 stub RouterD interface Ethernet0 ip address 10.1.1.2 255.255.255.0 ! router ospf 10 network 10.0.0.0 0.255.255.255 area 25 area 25 stub Notice that we only have to change, from the previous example, the con- figuration of RouterB. We simply add the no-summary argument to the area area-id stub command: RouterB(config)#router ospf 10 where 10 is the Process ID. RouterB(config-router)#network 1.0.0.0 0.255.255.255 area 0 where 1.0.0.0 0.255.255.255 is the network and wildcard mask of a net- work connected to RouterB and where 0 is the area that network 1.1.1.0/24 is a member of. RouterB(config-router)#network 10.0.0.0 0.255.255.255 area 25 where 10.0.0.0 0.255.255.255 is a summary network and wildcard mask of networks connected to RouterB and where 25 is the area that networks 10.1.1.0/24 and 10.1.2.0/24 are members of. RouterB(config-router)#area 25 stub no-summary where the no-summary argument makes Area 25 totally stubby. Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 180 Chapter 5 Interconnecting OSPF Areas Not-So-Stubby Area Configuration Recall that a not-so-stubby area (NSSA) is useful when we have an area that requires the injection of external routes, although we still want to eliminate the injection of Type 5 LSAs. Figure 5.6 presents such a scenario. In Area 1, we want to prevent Area 0 from injecting Type 5 LSAs, yet we still need external routes from the RIP routing process to be injected into Area 1. The solution to these requirements is to make Area 1 an NSSA. FIGURE 5.6 OPSF configuration example continued—not-so-stubby area configuration RouterA interface Ethernet0 ip address 10.1.1.1 255.255.255.0 ! interface Ethernet1 ip address 10.1.2.1 255.255.255.0 ! router ospf 24 network 10.0.0.0 0.255.255.255 area 0 RouterB interface Ethernet0 ip address 10.1.2.2 255.255.255.0 ! RouterD RouterC RouterB RouterA e0 e0 e1 e0 e1 e0 Area 0Area 1 Ethernet e1 e1 Ethernet RIP OSPFOSPF 172.16.2.1/24 172.16.1.2/24 172.16.1.1/24 1.1.1.2/24 1.1.1.1/24 10.1.2.2/24 10.1.2.1/24 10.1.1.1/24 Not-So-Stubby Area Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Not-So-Stubby Area Configuration 181 interface Ethernet1 ip address 1.1.1.1 255.255.255.0 ! router ospf 24 network 10.0.0.0 0.255.255.255 area 0 network 1.0.0.0 0.255.255.255 area 1 area 0 range 10.0.0.0 255.255.0.0 area 1 nssa RouterC interface Ethernet0 ip address 1.1.1.2 255.255.255.0 ! interface Ethernet1 ip address 172.16.1.1 255.255.255.0 ! router ospf 24 redistribute rip network 1.0.0.0 0.255.255.255 area 1 default-metric 128 area 1 nssa ! router rip redistribute ospf 24 network 172.16.0.0 default-metric 3 RouterD interface Ethernet0 ip address 172.16.1.2 255.255.255.0 ! interface Ethernet1 ip address 172.16.2.1 255.255.255.0 ! router rip network 172.16.0.0 Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 182 Chapter 5 Interconnecting OSPF Areas Let’s examine the configuration of each of these routers, beginning with RouterA. RouterA is a backbone router (and an internal router), which does not participate in our NSSA (Area 1). Therefore, RouterA doesn’t need any special NSSA configuration. However, by way of review, we will still exam- ine its syntax: RouterA(config)#router ospf 24 where 24 is the Process ID. RouterA(config-router)#network 10.0.0.0 0.255.255.255 area 0 where 10.0.0.0 0.255.255.255 is a network and wildcard mask summa- rization of the networks connected to RouterA and where 0 is the area that networks 10.1.1.0/24 and 10.1.2.0/24 are members of. RouterB does participate in the NSSA. Therefore, it will require a special configuration: RouterB(config)#router ospf 24 RouterB(config-router)#network 10.0.0.0 0.255.255.255 area 0 RouterB(config-router)#network 1.0.0.0 0.255.255.255 area 1 RouterB(config-router)#area 0 range 10.0.0.0 255.0.0.0 where 10.0.0.0 255.0.0.0 is the network number and subnet mask of a network that summarizes the individual networks within Area 0, thus reduc- ing the number of a router’s routing table entries. RouterB(config-router)#area 1 nssa where 1 is the area that is being designated as a not-so-stubby area. Notice that the configuration for RouterB included the command area area-id range network_address network_mask, which can be used on area border routers to summarize the IP address space being used by routers within a given area. Also notice the area area-id nssa command. This command tells the router that the specified area the router is connected to is a not-so-stubby area. As we saw when configuring stub areas, all routers within a not-so-stubby area must agree that they are connected to a NSSA (i.e., be configured with the area area-id nssa command). Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com [...]... metric value to be assigned to routes being redistributed into the OSPF routing process RouterC(config-router)#area 1 nssa RouterC(config-router)#router rip This enables the RIP routing process on the router RouterC(config-router)#redistribute ospf 24 where ospf 24 is the routing process whose routes are being injected into the RIP routing process RouterC(config-router)#network 172.16.0.0 RouterC(config-router)#default-metric... part of an NSSA, it also participates in a RIP routing process In order to exchange its OSPF and RIP routes, RouterC must perform route redistribution (route redistribution is the focus of Chapter 10): RouterC(config)#router ospf 24 RouterC(config-router)#redistribute rip where rip is the routing protocol whose routes are being injected into the OSPF routing process RouterC(config-router)#network 1.0.0.0... RouterC(config-router)#network 172.16.0.0 RouterC(config-router)#default-metric 3 where 3 is the RIP metric value (hop count) to be assigned to routes being redistributed into the RIP routing process RouterD is internal to the RIP routing process Therefore, RouterD does not require any NSSA-specific configuration: RouterD(config)#router rip RouterD(config-router)#network 172.16.0.0 Copyright ©2001 SYBEX ,... of the following are scalability issues with single area OSPF networks? (Choose all that apply.) A Size of the routing table B Size of the OSPF database C Maximum hop count limitation D Recalculation of the OSPF database 2 Which of the following describes a router that connects to an external routing process (e.g., EIGRP)? A ABR B ASBR C Type 2 LSA D Stub router 3 Which of the following makes use of... SYBEX , Inc., Alameda, CA www.sybex.com Review Questions 197 9 What is an autonomous system boundary router (ASBR)? A Any OSPF router that is connected to an external routing process B Any OSPF router that is connected to an internal routing process C Any router that is connected to multiple OSPF areas D Any router that is connected to single OSPF areas 10 Which of the following can be described as... the size of a single area OSPF network grows, so does the size of the routing table and OSPF database that has to be maintained Also, if there is a change in network topology, the OSPF algorithm has to be rerun for the entire network 2 B An autonomous system boundary router (ASBR) is any OSPF router that is connected to an external routing process 3 D A not-so-stubby area (NSSA) imports external routes... an external routing process Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com Answers to Review Questions 201 10 B The definition of an internal router is a router that has all of its interfaces within the same OSPF area Keep in mind that a router can fall into multiple categories For example, if an internal router is in Area 0, it would also be considered a backbone router 11 B The routing table... Chapter 5 Interconnecting OSPF Areas 13 Which of the following are design guidelines for setting up stub areas? (Choose all that apply.) A Use only when a router within an area connects to an external routing process (e.g., EIGRP) B Do not make the backbone area a stub area C If you decide to make a particular area a stub area, be sure to con- figure all the routers in the area as stubby D Configure... designated router and the formation of router adjacencies debug ip ospf packet Shows information contained in each OSPF packet, such as Router ID and Area ID show ip ospf border-routers Shows an ABR’s internal routing table show ip ospf virtual-links Shows the status of a router’s virtual link show ip ospf neighbor Shows neighbor router information, such as Neighbor ID and the state of adjacency with the neighboring... of its interfaces in the same area? A Area border router B Internal router C Autonomous System Boundary Router D Designated router 11 Which of the following IOS commands shows an ABR’s internal router routing table? A debug ospf events B show ip ospf border-routers C show ip ospf neighbor D show ip ospf database 12 As compared to an OSPF stub area, what is unique about an OSPF totally stubby area? (Choose . ospf 24 RouterC(config-router)#redistribute rip where rip is the routing protocol whose routes are being injected into the OSPF routing process. RouterC(config-router)#network 1.0.0.0 0.255.255.255. routes being redistrib- uted into the OSPF routing process. RouterC(config-router)#area 1 nssa RouterC(config-router)#router rip This enables the RIP routing process on the router. RouterC(config-router)#redistribute. router. RouterC(config-router)#redistribute ospf 24 where ospf 24 is the routing process whose routes are being injected into the RIP routing process. RouterC(config-router)#network 172.16.0.0 RouterC(config-router)#default-metric