OSPF Authentication

Configuring OSPF

Unlike other routing protocols, OSPF requires a certain amount of predesign before implementation. Careful consideration must be applied to the OSPF network as a whole, not just a single area. The following list contains some design considerations that you should take into account when deploying OSPF:

• Area deployment ” Area 0 must be continuous and should be located in the most stable part of the network. This usually consists of the core routers.

• Router IDs ” Statically configure router ID (RIDs). Use the private subnet of 192.168.0.0 to accomplish this. Remember, the highest addresses become the DR and the BDR. Cisco IOS Level 12.0 and above allow for static RIDs without having to use loopback interfaces.

• RIDs and priority used to "hard-code" DR and BDR ” Where the OSPF network type requires the use of a DR and BDR, use OSPF priority or router IDs to force the election of the DR and BDR. The DR on Frame Relay networks should be the router with a direct PVC to all the neighbors in that area. On LANs, the DR and BDR should be the higher-end routers.

• Continuous IP addressing within an area ” When possible, all addresses in an area should be continuous. This leads toward route summarization and a natural hierarchical design.

• Various forms of Stub areas ” Many edge routers and Frame Relay networks lead nicely toward the use of stub areas. Use the different forms of stub areas whenever possible.

• Virtual links avoided ” Even though we will discuss virtual links, their deployment and presence is an indication of poor network design. There are a few cases in which backup links might not be directly connected to Area 0, and a virtual link will be needed. Overall, they should be avoided in production networks.

With these design considerations in mind, you will use the following seven-step task list to configure OSPF:

Step 1. Divide the OSPF network into various areas by using the design guidelines listed previously. Make a network diagram, and highlight Area 0, other areas, and the area types. If the network will be using a DR/BDR, mark which routers will serve that function.

Step 2. (Optional) Assign permanent router IDs to the network. To accomplish this, configure loopback interfaces for use as static RIDs, if on Cisco IOS Software earlier than Release 12.0. The loopback interfaces should be in a high private address space and do not need to be advertised by OSPF. We recommend using the range 192.168. x.x. on loopback interface 0. If the OSPF network type will elect a DR/BDR, such as in an Ethernet network, assign the routers to be the DR and BDR the higher IP addresses, such as 192.168.250.251 and 192.168.250.250. RIDs can be assigned in Cisco IOS Software 12.0 and above with the OSPF router command router-id ip_address . Note: OSPF must be enabled before the router ID can be assigned.

Step 3. Enable OSPF on the router and configure RIDs. This is accomplished with the router ospf process_id global command. I like to think of the process_id as an Autonomous System ID. This number should be the same on all routers within the autonomous system. This is not a technical requirement, but a suggestion for network documentation. The process ID is only used internally by the router. At this time, also configure the RIDs that you assigned in Step 2. Use the router command router-id ip_address.

Step 4. Configure interfaces to participate in OSPF. The OSPF uses a network statement that is followed by a wildcard mask and an area ID.

  network   a.b.c.d wildcard_mask   area   X  

A wildcard mask can be considered an inverse bit mask, where the 0 bit is a "care" bit, and a 1 bit is a "don't care" bit. For example, to run OSPF only on networks 128.10.1.0/24 to 128.10.255.0/24 in Area 0, the syntax would resemble this:

  network 128.10.0.0 0.0.255.255 area 0  

For another example, to run OSPF only on the network 172.16.128.4/30 in Area 100, the syntax would resemble this:

  network 172.16.128.4 0.0.0.3 area 100  

Use a wildcard mask of 0.0.0.0 to enable OSPF on a single interface. Use the tightest wildcard mask possible when configuring OSPF. This will prevent OSPF from advertising networks that you might be unaware of. It also prevents unnecessary OSPF hellos from entering those segments.

Step 5. Configure OSPF neighbor support. The OSPF network type might require additional configuration for OSPF to build an adjacency . On NBMA networks, such as Frame Relay, the OSPF configuration will be interface-dependant. Table 12-3 highlights some common network types and tells whether any additional configuration is needed.

Table 12-3. OSPF Network Configuration Table

Physical Interface Type	Default OSPF Network Type	Static Neighbor Needed	DR/BDR Elected	Desired Neighbor State	Priority Recommended
Broadcast media Ethernet, Token Ring, and so on	Broadcast [*]	No	Yes	FULL/DR FULL/BDR FULL/ ”	Yes
Frame Relay natural or multipoint	NMBA	Yes	Yes	FULL/ DROTHER	No
Frame Relay point-to-point	Point-to-point	No	No	FULL/ ”	No
Frame Relay multipoint	Point-to- multipoint [*]	No	No	FULL/ ”	No

^[*] If the OSPF network type on a Frame Relay network is changed to BROADCAST, it will have a DR and BDR election and priority should be set.

^[*] The OSPF network type of point-to-multipoint is not the default network type of any interface. OSPF point-to-multipoint networks must be statically configured.

When configuring OSPF over Frame Relay multipoint interfaces, it is necessary to configure static neighbors. Without the neighbor statements, the router's neighbor will be in a constant state of waiting and will never form an adjacency. To configure the static neighbor, use this OSPF router command:

  Router(config-route)neighbor   ip_address_of_neighbor  

The hub router of the multipoint network, or the router that has a PVC to each site, should be statically configured as the DR. To accomplish this, set the priority of the spoke or remote routers to 0. A priority of 0 tells OSPF that this interface or neighbor will not participate in the DR/BDR election process. The DR choice can be further influenced by configuring the priority of the link on the router of the DR to the high number, such as 255. The priority can be configured with this interface command:

  Router(config-if)ip ospf priority   0-255  

The default OSPF priority is 1.

Another way to force an adjacency creation is simply to change the OSPF network type to something more desirable. Changing a Frame Relay multipoint network to a point-to-multipoint will make OSPF treat the multipoint network like many point-to-point networks. Changing the network type to broadcast also forces adjacency creation and DR/BDR election. To change the OSPF network type, use the following interface command:

  Router(config-if)ip ospf network  [  broadcast   non-broadcast   point-to-multipoint   point-to-point  ] 

Step 6. (Optional) Configure OSPF special area types. To configure OSPF areas as stubs, NSSAs, and totally stubby areas, use the following OSPF router command:

  Router(config-route)area   x  [  nssa   stub   virtual-link  ] [  no-summary  ] 

To configure an area as a stub or an NSSA area, simply add the area command followed by this parameter. For example, to configure Area 10 as a stub, the syntax would resemble this:

  Router(config-route)area 10 stub  

A totally stubby area is configured by configuring a normal stub area followed by the no-summary argument to block LS Types 3 and 4.

  Router(config-route)area 10 stub no-summary  

Step 7. (Optional) Configure any optional OSPF parameters. These include such things as hello timers, route summarization, authentication, and so on. These and other OSPF optional parameters are discussed in upcoming sections.

Practical Example: Configuring Multiple OSPF Area Types over Frame Relay

To better understand this rather lengthy process, let's apply it to an practical example. Figure 12-7 illustrates an OSPF network that is in Step 1 of the configuration process. This model of OSPF has a Frame Relay multipoint and point-to-point network. You will be configuring a totally stubby area, Area 100, for the luke router. The paul router will reside in Area 0, the backbone area, along with the Ethernet interface of the router john. The mark and mathew routers will be in OSPF Area 10. We have marked john to be the DR for the WAN and LAN networks.

Figure 12-7. OSPF Multiple-Area Network

Step 2 calls for defining or configuring RIDs. In this particular network, only the mathew router is running a version of Cisco IOS Software prior to Release 12.0, so it will have to use a loopback address to configure a RID. Recall that OSPF will use the highest loopback address for a RID; this is why this step precedes enabling OSPF. On the mathew router, create a static RID with the following commands:

 mathew(config)#  int loop 0  mathew(config-if)#  ip address 172.16.250.1 255.255.255.0  

Figure 12-8 illustrates the RIDs used in this model. It should be noted that this step is optional but recommended for stability reasons.

Figure 12-8. OSPF Static RIDs

TIP

When configuring OSPF in a lab scenario, we assign our RIDs so that the last octet of the RID is the router number. For example, if you have the routers R1, R2 and R3, you would assign the RIDs to be 192.168.250.1, 192.168.250.2, and 192.168.250.3, respectively. The DR in the lab has an unusually high RID, 250, so it stands out from the rest. When viewing the OSPF database, having RIDs that somewhat "self-document" the network can be a great benefit.

Step 3 involves enabling the OSPF process and configuring RIDs. The OSPF autonomous system is 7, so you will use 7 as the OSPF process ID. During this step, you also will configure the RIDs with the router command router-id ip_address. Example 12-3 demonstrates this step on the mark and john routers.

Example 12-3 Enabling OSPF on mark and john

 mark(config)#  router ospf 7  mark(config-router)#  router-id 192.168.250.6  john(config)#  router ospf 7  john(config-router)#  router-id 192.168.250.250  

After the OSPF process is enabled on all the routers, Step 4 requires that you configure the interfaces or networks participating in OSPF. Using the router command network ip_address wildcard_mask area x, you define the networks to run OSPF and also specify the area that those networks will reside in. Example 12-4 demonstrates the configuration of network statements on the router john.

Example 12-4 Configuring OSPF on the Router john

 john(config)#  router ospf 7  john(config-router)#  network 172.16.9.0 0.0.0.255 area 0  john(config-router)#  network 172.16.1.0 0.0.0.255 area 10  john(config-router)#  network 172.16.2.4 0.0.0.3 area 100  

You can configure the network statements on the remaining routers in many ways. We personally prefer limiting the network statement with a specific wildcard mask to one network per statement, or to a single interface with the 0.0.0.0 as the wildcard mask. In large networks, this type of configuration might be less desirable, and you might want to use a single network statement to group multiple interfaces into the same OSPF area. However, if you add new interfaces and they must go into different areas, you will have to remove the network statement and add it again for the new network statement to take effect.

At this point during the configuration, OSPF builds adjacencies on the Frame Relay point-to-point network between john and luke. Another adjacency is built on the Ethernet network between john and paul. OSPF, however, will not be capable of building an adjacency on the Frame Relay multipoint network between john, mark, and mathew without additional configuration. You can verify adjacency formation by performing a show ip ospf neighbor command on the router john, as in Example 12-5. The show and debug commands for OSPF are discussed in the next section.

Example 12-5 show ip ospf neighbor Performed on the Router john

 john#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 192.168.250.9     1   FULL/BDR        00:00:34    172.16.9.9      Ethernet0 192.168.250.3     1   FULL/  -        00:00:38    172.16.2.6      Serial0.2 john# 

Notice how clearly the router IDs show up in the previous command. This can significantly help troubleshooting OSPF on any size network.

Step 5 calls for the configuration of additional neighbor support to remedy the adjacency situation between john, mark, and mathew. Because you took the extra time to hard-code RIDs on all the routers, the john router will be DR for the connected LAN and WAN interfaces. You will take this a step further when defining neighbor statements. For the router john to build an adjacency with mark and mathew, you will need to add neighbor statements to all the routers. By setting the OSPF priority to 0 on the interface, that router will never participate in DR/BDR election. On multipoint networks, only the routers that have PVCs to all remote locations should be eligible for DR/BDR election. The default priority of 1 will be sufficient on the neighbor statements of the mark and mathew routers that point toward john. Example 12-6 illustrates the relevant portions of the OSPF configuration, to this point, on john, mark, and mathew, respectively.

Example 12-6 OSPF Configuration of john, mark, and mathew Routers

  hostname john   !   interface Serial0.1 multipoint   ip address 172.16.1.5 255.255.255.0   no ip directed-broadcast    ip ospf priority 255  Set this routers priority to 255, forcing the DR    frame-relay map ip 172.16.1.6 121 broadcast   frame-relay map ip 172.16.1.1 111 broadcast   !   interface Serial0.2 point-to-point   ip address 172.16.2.5 255.255.255.252   no ip directed-broadcast   frame-relay interface-dlci 150   !   router ospf 7   router-id 192.168.250.250   network 172.16.1.0 0.0.0.255 area 10   network 172.16.2.4 0.0.0.3 area 100   network 172.16.9.0 0.0.0.255 area 0    neighbor 172.16.1.1  A neighbor priority of 0 will not     neighbor 172.16.1.6  be listed in the configuration    !  ________________________________________________________________  hostname mark   !   interface Serial0   ip address 172.16.1.6 255.255.255.0   no ip directed-broadcast   encapsulation frame-relay    ip ospf priority 0  This router will not participate in DR/BDR election    no ip mroute-cache   frame-relay map ip 172.16.1.5 102 broadcast   frame-relay map ip 172.16.1.1 102 broadcast   !   router ospf 7   router-id 192.168.250.6   network 172.16.1.0 0.0.0.255 area 10   network 172.16.5.0 0.0.0.255 area 10   neighbor 172.16.1.5   !  ________________________________________________________________  hostname mathew   !   interface Serial0   ip address 172.16.1.1 255.255.255.0   encapsulation frame-relay    ip ospf priority 0  This router will not participate in DR/BDR election    no ip mroute-cache   frame-relay map ip 172.16.1.5 110 broadcast   frame-relay map ip 172.16.1.6 110 broadcast   !   router ospf 7   network 172.16.1.0 0.0.0.255 area 10   network 172.16.10.0 0.0.0.255 area 10   neighbor 172.16.1.5   !  

By adding the neighbor statements, OSPF now builds adjacencies over the Frame Relay multipoint network. This can be verified by performing the show ip ospf neighbor command on the router john. Example 12-7 demonstrates this command on john.

Example 12-7 show ip ospf neighbor Command Output for Router john

 john#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 192.168.250.9     1   FULL/BDR        00:00:32    172.16.9.9      Ethernet0 172.16.250.1      0   FULL/DROTHER    00:01:55    172.16.1.1      Serial0.1 192.168.250.6     0   FULL/DROTHER    00:01:46    172.16.1.6      Serial0.1 192.168.250.3     1   FULL/  -        00:00:37    172.16.2.6      Serial0.2 john# 

The mathew and mark routers will not become adjacent with each other because there is not a direct connection between them. Example 12-8 shows the active neighbor on the mathew router. The adjacency is in a FULL state, with john being the DR for the link.

Example 12-8 show ip ospf neighbor Command Output for Router mathew

 mathew#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 192.168.250.250  255   FULL/DR         00:01:48    172.16.1.5      Serial0 mathew# 

You could accomplish this in other ways without using neighbor commands. Another way is to change the OSPF network type to either broadcast or point-to-multipoint. Technically, a point-to-multipoint would be a more accurate network type to use. Configuring the same network model, you will now use the network type of point-to-multipoint to create adjacencies between mathew, mark, and john. Example 12-9 lists the relevant configuration of the john and mathew routers, using the ip osfp network type command in place of neighbor statements. The configuration of mark would be identical to that of mathew.

Example 12-9 Configuration of john and mark Using Network Types

  hostname john   !   interface Serial0   no ip address   no ip directed-broadcast   encapsulation frame-relay   no ip mroute-cache   !   interface Serial0.1 multipoint   ip address 172.16.1.5 255.255.255.0   no ip directed-broadcast    ip ospf network point-to-multipoint  Change the default OSPF network type     to PTM      Priority is not needed, no DR/BDR on PTM    frame-relay map ip 172.16.1.6 121 broadcast   frame-relay map ip 172.16.1.1 111 broadcast   !   router ospf 7   router-id 192.168.250.250   network 172.16.1.0 0.0.0.255 area 10   network 172.16.2.4 0.0.0.3 area 100    network 172.16.9.0 0.0.0.255 area 0  No neighbors    !  ________________________________________________________________  hostname mathew   !   interface Serial0   ip address 172.16.1.1 255.255.255.0   encapsulation frame-relay    ip ospf network point-to-multipoint    no ip mroute-cache   frame-relay map ip 172.16.1.5 110 broadcast   frame-relay map ip 172.16.1.6 110 broadcast   !   router ospf 7   network 172.16.1.0 0.0.0.255 area 10   network 172.16.10.0 0.0.0.255 area 10   !  

Example 12-10 lists how the neighbors look on the mathew and john routers.

Example 12-10 OSPF Neighbors on mathew and john

 mathew#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 192.168.250.250   1   FULL/  -        00:01:35    172.16.1.5      Serial0 mathew# ________________________________________________________________ john#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 192.168.250.9     1   FULL/BDR        00:00:36    172.16.9.9      Ethernet0 172.16.250.1      1   FULL/  -        00:01:58    172.16.1.1      Serial0.1 192.168.250.6     1   FULL/  -        00:01:58    172.16.1.6      Serial0.1 192.168.250.3     1   FULL/  -        00:00:38    172.16.2.6      Serial0.2 john# 

Steps 6 and 7 in the OSPF configuration process are optional; they involve configuring any OSPF area types and any other OSPF enhancements. In this model, you need only to configure one more step, and that is to configure the router luke to be in a totally stubby area. To configure a totally stubby area, the area needs to first be configured as a stub area, and then the argument no-summary must be used . The syntax will resemble the following:

  area 100 stub no-summary  

These commands should be added to the john and luke routers. Example 12-11 lists the output of the show ip ospf and show ip route command on the router luke.

Example 12-11 Verifying the Totally Stubby Area on luke

 luke#  show ip ospf 7  Routing Process "ospf 7" with ID 192.168.250.3  Supports only single TOS(TOS0) routes  Supports opaque LSA  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs  Number of external LSA 0. Checksum Sum 0x0  Number of opaque AS LSA 0. Checksum Sum 0x0  Number of DCbitless external and opaque AS LSA 0  Number of DoNotAge external and opaque AS LSA 0  Number of areas in this router is 1. 0 normal 1 stub 0 nssa  External flood list length 0     Area 100         Number of interfaces in this area is 2         It is a stub area, no summary LSA in this area         Area has no authentication         SPF algorithm executed 46 times         Area ranges are         Number of LSA 3. Checksum Sum 0x16F14         Number of opaque link LSA 0. Checksum Sum 0x0         Number of DCbitless LSA 0         Number of indication LSA 0         Number of DoNotAge LSA 0         Flood list length 0 luke#  show ip route  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 - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o - ODR        P - periodic downloaded static route Gateway of last resort is 172.16.2.5 to network 0.0.0.0      172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks C       172.16.2.4/30 is directly connected, Serial0.1 C       172.16.3.0/24 is directly connected, TokenRing0 O*IA 0.0.0.0/0 [110/65] via 172.16.2.5, 00:02:23, Serial0.1 luke# 

Instead of a full routing table, the luke router will receive only a default route from john, as shown in Example 12-12.

Example 12-12 shows the relevant portions of the configurations and the route tables used in this model.

Example 12-12 Configurations and Route Table of john, mark, mathew, luke, and paul

  !   hostname john   !   interface Ethernet0   ip address 172.16.9.5 255.255.255.0   no ip directed-broadcast   !   interface Serial0   no ip address   no ip directed-broadcast   encapsulation frame-relay   no ip mroute-cache   !   interface Serial0.1 multipoint   ip address 172.16.1.5 255.255.255.0   no ip directed-broadcast   ip ospf network point-to-multipoint   frame-relay map ip 172.16.1.6 121 broadcast   frame-relay map ip 172.16.1.1 111 broadcast   !   interface Serial0.2 point-to-point   ip address 172.16.2.5 255.255.255.252   no ip directed-broadcast   frame-relay interface-dlci 150   !   router ospf 7   router-id 192.168.250.250   area 100 stub no-summary   network 172.16.1.0 0.0.0.255 area 10   network 172.16.2.4 0.0.0.3 area 100   network 172.16.9.0 0.0.0.255 area 0  john# john#  show ip route  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 - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o - ODR        P - periodic downloaded static route Gateway of last resort is not set      172.16.0.0/16 is variably subnetted, 8 subnets, 4 masks C       172.16.9.0/24 is directly connected, Ethernet0 O       172.16.10.0/29 [110/74] via 172.16.1.1, 00:16:06, Serial0.1 O       172.16.5.0/24 [110/74] via 172.16.1.6, 00:16:06, Serial0.1 C       172.16.2.4/30 is directly connected, Serial0.2 O       172.16.1.6/32 [110/64] via 172.16.1.6, 00:16:06, Serial0.1 O       172.16.1.1/32 [110/64] via 172.16.1.1, 00:16:06, Serial0.1 C       172.16.1.0/24 is directly connected, Serial0.1 O       172.16.3.0/24 [110/70] via 172.16.2.6, 00:13:47, Serial0.2 john# ________________________________________________________________  !   hostname mark   !   interface Ethernet0   ip address 172.16.5.5 255.255.255.0   no ip directed-broadcast   !   interface Serial0   ip address 172.16.1.6 255.255.255.0   no ip directed-broadcast   encapsulation frame-relay   ip ospf network point-to-multipoint   no ip mroute-cache   frame-relay map ip 172.16.1.5 102 broadcast   frame-relay map ip 172.16.1.1 102 broadcast   !   router ospf 7   router-id 192.168.250.6   network 172.16.1.0 0.0.0.255 area 10   network 172.16.5.0 0.0.0.255 area 10   !  mark# mark#  show ip route  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 - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o - ODR        P - periodic downloaded static route Gateway of last resort is not set      172.16.0.0/16 is variably subnetted, 8 subnets, 4 masks O IA    172.16.9.0/24 [110/74] via 172.16.1.5, 00:29:30, Serial0 O       172.16.10.0/29 [110/138] via 172.16.1.5, 00:29:30, Serial0 O       172.16.1.5/32 [110/64] via 172.16.1.5, 00:29:30, Serial0 C       172.16.5.0/24 is directly connected, Ethernet0 O IA    172.16.2.4/30 [110/128] via 172.16.1.5, 00:29:30, Serial0 O       172.16.1.1/32 [110/128] via 172.16.1.5, 00:29:30, Serial0 C       172.16.1.0/24 is directly connected, Serial0 O IA    172.16.3.0/24 [110/134] via 172.16.1.5, 00:15:36, Serial0 mark# mark# ________________________________________________________________  !   hostname mathew   !   interface Loopback0   ip address 172.16.250.1 255.255.255.0   !   interface Ethernet2   ip address 172.16.10.1 255.255.255.248   media-type 10BaseT   !   interface Serial0   ip address 172.16.1.1 255.255.255.0   encapsulation frame-relay   ip ospf network point-to-multipoint   no ip mroute-cache   frame-relay map ip 172.16.1.5 110 broadcast   frame-relay map ip 172.16.1.6 110 broadcast   !   router ospf 7   network 172.16.1.0 0.0.0.255 area 10   network 172.16.10.0 0.0.0.255 area 10   !  mathew# mathew#  show ip route  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 - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      172.16.0.0/16 is variably subnetted, 9 subnets, 4 masks C       172.16.250.0/24 is directly connected, Loopback0 O IA    172.16.9.0/24 [110/74] via 172.16.1.5, 00:29:44, Serial0 C       172.16.10.0/29 is directly connected, Ethernet2 O       172.16.1.5/32 [110/64] via 172.16.1.5, 00:29:44, Serial0 O       172.16.5.0/24 [110/138] via 172.16.1.5, 00:29:44, Serial0 O IA    172.16.2.4/30 [110/128] via 172.16.1.5, 00:29:44, Serial0 O       172.16.1.6/32 [110/128] via 172.16.1.5, 00:29:44, Serial0 C       172.16.1.0/24 is directly connected, Serial0 O IA    172.16.3.0/24 [110/134] via 172.16.1.5, 00:15:54, Serial0 mathew# mathew# ________________________________________________________________  !   hostname luke   !   interface Serial0   no ip address   no ip directed-broadcast   encapsulation frame-relay   no ip mroute-cache   frame-relay lmi-type cisco   !   interface Serial0.1 point-to-point   ip address 172.16.2.6 255.255.255.252   no ip directed-broadcast   frame-relay interface-dlci 151   !   router ospf 7   router-id 192.168.250.3   area 100 stub no-summary   network 172.16.2.4 0.0.0.3 area 100   network 172.16.3.0 0.0.0.255 area 100   !  luke# luke#  show ip route  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 - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area        * - candidate default, U - per-user static route, o - ODR        P - periodic downloaded static route Gateway of last resort is 172.16.2.5 to network 0.0.0.0      172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks C       172.16.2.4/30 is directly connected, Serial0.1 C       172.16.3.0/24 is directly connected, TokenRing0 O*IA 0.0.0.0/0 [110/65] via 172.16.2.5, 00:14:56, Serial0.1 luke# luke# ________________________________________________________________  !   hostname paul   !   interface Ethernet0/0   ip address 172.16.9.9 255.255.255.0   no ip directed-broadcast   !   router ospf 7   router-id 192.168.250.9   network 172.16.9.0 0.0.0.255 area 0   !  paul# paul# paul#  show ip route  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 - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      172.16.0.0/16 is variably subnetted, 8 subnets, 4 masks C       172.16.9.0/24 is directly connected, Ethernet0/0 O IA    172.16.10.0/29 [110/84] via 172.16.9.5, 00:30:32, Ethernet0/0 O IA    172.16.1.5/32 [110/10] via 172.16.9.5, 00:31:27, Ethernet0/0 O IA    172.16.5.0/24 [110/84] via 172.16.9.5, 00:30:42, Ethernet0/0 O IA    172.16.2.4/30 [110/74] via 172.16.9.5, 00:31:27, Ethernet0/0 O IA    172.16.1.6/32 [110/74] via 172.16.9.5, 00:30:42, Ethernet0/0 O IA    172.16.1.1/32 [110/74] via 172.16.9.5, 00:30:32, Ethernet0/0 O IA    172.16.3.0/24 [110/80] via 172.16.9.5, 00:16:41, Ethernet0/0 paul# 

Before further discussing these and other OSPF configuration options, let's take a closer look at the "Big show " and "Big D" commands for OSPF.