Implementing OSPF for IPv4

Foundation Topics

Implementing Single-Area OSPFv2

OSPF configuration includes only a few required steps, but it has many optional steps. After an OSPF design has been chosen—a task that can be complex in larger IP internetworks—the configuration can be as simple as enabling OSPF on each router interface and placing that interface in the correct OSPF area.

This section shows several configuration examples, all with a single-area OSPF internetwork. Following those examples, the text goes on to cover several of the additional optional configuration settings. For reference, the following list outlines the configuration steps covered in this first major section of the chapter, as well as a brief reference to the required commands:

• Step 1.Use the router ospf process-id global command to enter OSPF configuration mode for a particular OSPF process.

• Step 2.(Optional) Configure the OSPF router ID by doing the following:

  1. Use the router-id id-value router subcommand to define the router ID

  2. Use the interface loopback number global command, along with an ip address address mask command, to configure an IP address on a loopback interface (chooses the highest IP address of all working loopbacks)

  3. Rely on an interface IP address (chooses the highest IP address of all working nonloopbacks)

• Step 3.Use one or more network ip-address wildcard-mask area area-id router subcommands to enable OSPFv2 on any interfaces matched by the configured address and mask, enabling OSPF on the interface for the listed area.

• Step 4.(Optional) Use the passive-interface type number router subcommand to configure any OSPF interfaces as passive if no neighbors can or should be discovered on the interface.

For a more visual perspective on OSPFv2 configuration, Figure 8-1 shows the relationship between the key OSPF configuration commands. Note that the configuration creates a routing process in one part of the configuration, and then indirectly enables OSPF on each interface. The configuration does not name the interfaces on which OSPF is enabled, instead requiring IOS to apply some logic by comparing the OSPF network command to the interface ip address commands. The upcoming example discusses more about this logic.

Figure 8-1 Organization of OSPFv2 Configuration

OSPF Single-Area Configuration

Figure 8-2 shows a sample network that will be used for the single-area OSPF configuration examples. All links sit in area 0. The design has four routers, each connected to one or two LANs. However, note that Routers R3 and R4, at the top of the figure, connect to the same two VLANs/subnets, so they will form neighbor relationships with each other over each of those VLANs as well. (The two switches at the top of the design are acting as Layer 2 switches.)

Figure 8-2 Sample Network for OSPF Single-Area Configuration

Example 8-1 shows the IPv4 addressing configuration on Router R3, before getting into the OSPF detail. The configuration enables 802.1Q trunking on R3’s G0/0 interface, and assigns an IP address to each subinterface. (Not shown, switch S3 has configured trunking on the other side of that Ethernet link.)

Example 8-1 IPv4 Address Configuration on R3 (Including VLAN Trunking)

interface GigabitEthernet 0/0.341
 encapsulation dot1q 341
 ip address 10.1.3.1 255.255.255.128
!
interface GigabitEthernet 0/0.342
 encapsulation dot1q 342
 ip address 10.1.3.129 255.255.255.128
!
interface serial 0/0/0
 ip address 10.1.13.3 255.255.255.128

The beginning single-area configuration on R3, as shown in Example 8-2, enables OSPF on all the interfaces shown in Figure 8-2. First, the router ospf 1 global command puts the user in OSPF configuration mode, and sets the OSPF process-id. This number just needs to be unique on the local router, allowing the router to support multiple OSPF processes in a single router by using different process IDs. (The router command uses the process-id to distinguish between the processes.) The process-id does not have to match on each router, and it can be any integer between 1 and 65,535.

Example 8-2 OSPF Single-Area Configuration on R3 Using One network Command

router ospf 1
 network 10.0.0.0 0.255.255.255 area 0

Speaking generally rather than about this example, the OSPF network command tells a router to find its local interfaces that match the first two parameters on the network command. Then, for each matched interface, the router enables OSPF on those interfaces, discovers neighbors, creates neighbor relationships, and assigns the interface to the area listed in the network command. (Note that the area can be configured as either an integer or a dotted-decimal number, but this book makes a habit of configuring the area number as an integer. The integer area numbers range from 0 through 4,294,967,295.)

For the specific command in Example 8-2, any matched interfaces are assigned to area 0. However, the first two parameters—the ip_address and wildcard_mask parameter values of 10.0.0.0 and 0.255.255.255—need some explaining. In this case, the command matches all three interfaces shown for Router R3; the next topic explains why.

Matching with the OSPF network Command

The key to understanding the traditional OSPFv2 configuration shown in this first example is to understand the OSPF network command. The OSPF network command compares the first parameter in the command to each interface IP address on the local router, trying to find a match. However, rather than comparing the entire number in the network command to the entire IPv4 address on the interface, the router can compare a subset of the octets, based on the wildcard mask, as follows:

• Wildcard 0.0.0.0: Compare all 4 octets. In other words, the numbers must exactly match.

• Wildcard 0.0.0.255: Compare the first 3 octets only. Ignore the last octet when comparing the numbers.

• Wildcard 0.0.255.255: Compare the first 2 octets only. Ignore the last 2 octets when comparing the numbers.

• Wildcard 0.255.255.255: Compare the first octet only. Ignore the last 3 octets when comparing the numbers.

• Wildcard 255.255.255.255: Compare nothing—this wildcard mask means that all addresses will match the network command.

Basically, a wildcard mask value of 0 in an octet tells IOS to compare to see if the numbers match, and a value of 255 tells IOS to ignore that octet when comparing the numbers.

The network command provides many flexible options because of the wildcard mask. For example, in Router R3, many network commands could be used, with some matching all interfaces, and some matching a subset of interfaces. Table 8-2 shows a sampling of options, with notes.

Table 8-2 Example OSPF network Commands on R3, with Expected Results

The wildcard mask gives the local router its rules for matching its own interfaces. For example, Example 8-2 shows R3 using the network 10.0.0.0 0.255.255.255 area 0 command. However, the wildcard mask allows for many different valid OSPF configurations. For instance, in that same internetwork, Routers R1 and R2 could use the configuration shown in Example 8-3, with two other wildcard masks. In both routers, OSPF is enabled on all the interfaces shown in Figure 8-2.

Example 8-3 OSPF Configuration on Routers R1 and R2

! R1 configuration next - one network command enables OSPF
! on all three interfaces
router ospf 1
 network 10.1.0.0 0.0.255.255 area 0

! R2 configuration next - One network command per interface
router ospf 1
 network 10.1.12.2 0.0.0.0 area 0
 network 10.1.24.2 0.0.0.0 area 0
 network 10.1.2.2 0.0.0.0 area 0

Finally, note that other wildcard mask values can be used as well, as long as the wildcard mask in binary is one unbroken string of 0s and another single string of binary 1s. Basically, that includes all wildcard masks that could be used to match all IP addresses in a subnet, as discussed in the “Finding the Right Wildcard Mask to Match a Subnet” section of Chapter 16, “Basic IPv4 Access Control Lists” (which is Chapter 25 of the ICND1 Cert Guide). For example, a mask of 0.255.255.0 would not be allowed.

Verifying OSPFv2 Single Area

As mentioned in Chapter 7, OSPF routers use a three-step process to eventually add OSPF-learned routes to the IP routing table. First, they create neighbor relationships. Then they build and flood LSAs, so each router in the same area has a copy of the same LSDB. Finally, each router independently computes its own IP routes using the SPF algorithm and adds them to its routing table.

The show ip ospf neighbor, show ip ospf database, and show ip route commands display information for each of these three steps, respectively. To verify OSPF, you can use the same sequence. Or, you can just go look at the IP routing table, and if the routes look correct, OSPF probably worked.

For example, first, examine the list of neighbors known on Router R3 from the configuration in Examples 8-1, 8-2, and 8-3. R3 should have one neighbor relationship with R1, over the serial link. It also has two neighbor relationships with R4, over the two different VLANs to which both routers connect. Example 8-4 shows all three.

Example 8-4 OSPF Neighbors on Router R3 from Figure 8-2

R3# show ip ospf neighbor

Neighbor ID     Pri   State      Dead Time   Address       Interface
1.1.1.1           0   FULL/  -   00:00:33    10.1.13.1     Serial0/0/0
10.1.24.4         1   FULL/DR    00:00:35    10.1.3.130    GigabitEthernet0/0.342
10.1.24.4         1   FULL/DR    00:00:36    10.1.3.4      GigabitEthernet0/0.341

The detail in the output mentions several important facts, and for most people, working right to left works best in this case. For example, looking at the headings:

• Interface: This is the local router’s interface connected to the neighbor. For example, the first neighbor in the list is reachable through R3’s S0/0/0 interface.

• Address: This is the neighbor’s IP address on that link. Again, for this first neighbor, the neighbor, which is R1, uses IP address 10.1.13.1.

• State: While many possible states exist, for the details discussed in this chapter, FULL is the correct and fully working state in this case.

• Neighbor ID: This is the router ID of the neighbor.

Next, Example 8-5 shows the contents of the LSDB on Router R3. Interestingly, when OSPF is working correctly in an internetwork with a single-area design, all the routers will have the same LSDB contents. So, the show ip ospf database command in Example 8-5 should list the same exact information, no matter on which of the four routers it is issued.

Example 8-5 OSPF Database on Router R3 from Figure 8-2

R3# show ip ospf database

            OSPF Router with ID (10.1.13.3) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         498         0x80000006 0x002294 6
2.2.2.2         2.2.2.2         497         0x80000004 0x00E8C6 5
10.1.13.3       10.1.13.3       450         0x80000003 0x001043 4
10.1.24.4       10.1.24.4       451         0x80000003 0x009D7E 4

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.3.4        10.1.24.4       451         0x80000001 0x0045F8
10.1.3.130      10.1.24.4       451         0x80000001 0x00546B

For the purposes of this book, do not be concerned about the specifics in the output of this command. However, for perspective, note that the LSDB should list one “Router Link State” (Type 1 Router LSA) for each of the routers in the same area. In this design, all four routers are in the same area, so there are four highlighted Type 1 LSAs listed.

Next, Example 8-6 shows R3’s IPv4 routing table with the show ip route command. Note that it lists connected routes as well as OSPF routes. Take a moment to look back at Figure 8-2, and look for the subnets that are not locally connected to R3. Then look for those routes in the output in Example 8-5.

Example 8-6 IPv4 Routes Added by OSPF on Router R3 from Figure 8-2

R3# show ip route
Codes: L - local, C - connected, S - static, 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
! Legend lines omitted for brevity


      10.0.0.0/8 is variably subnetted, 11 subnets, 2 masks
O        10.1.1.0/25 [110/65] via 10.1.13.1, 00:13:28, Serial0/0/0
O        10.1.1.128/25 [110/65] via 10.1.13.1, 00:13:28, Serial0/0/0
O        10.1.2.0/25 [110/66] via 10.1.3.130, 00:12:41, GigabitEthernet0/0.342
                     [110/66] via 10.1.3.4, 00:12:41, GigabitEthernet0/0.341
C        10.1.3.0/25 is directly connected, GigabitEthernet0/0.341
L        10.1.3.1/32 is directly connected, GigabitEthernet0/0.341
C        10.1.3.128/25 is directly connected, GigabitEthernet0/0.342
L        10.1.3.129/32 is directly connected, GigabitEthernet0/0.342
O        10.1.12.0/25 [110/128] via 10.1.13.1, 00:13:28, Serial0/0/0
C        10.1.13.0/25 is directly connected, Serial0/0/0
L        10.1.13.3/32 is directly connected, Serial0/0/0
O        10.1.24.0/25
           [110/65] via 10.1.3.130, 00:12:41, GigabitEthernet0/0.342
           [110/65] via 10.1.3.4, 00:12:41, GigabitEthernet0/0.341

First, take a look at the bigger ideas confirmed by this output. The code of “O” on the left identifies a route as being learned by OSPF. The output lists five such IP routes. From Figure 8-2, five subnets exist that are not connected subnets off Router R3. Looking for a quick count of OSPF routes, versus nonconnected routes in the diagram, gives a quick check of whether OSPF learned all routes.

Next, take a look at the first route (to subnet 10.1.1.0/25). It lists the subnet ID and mask, identifying the subnet. It also lists two numbers in brackets. The first, 110, is the administrative distance of the route. All the OSPF routes in this example use the default of 110. The second number, 65, is the OSPF metric for this route.

Additionally, the show ip protocols command is also popular as a quick look at how any routing protocol works. This command lists a group of messages for each IPv4 routing protocol running on a router. Example 8-7 shows a sample, this time taken from Router R3.

Example 8-7 The show ip protocols Command on R3

R3# show ip protocols
*** IP Routing is NSF aware ***

Routing Protocol is "ospf 1"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID 10.1.13.3
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
    10.0.0.0 0.255.255.255 area 0
  Routing Information Sources:
    Gateway         Distance      Last Update
    1.1.1.1              110      06:26:17
    2.2.2.2              110      06:25:30
    10.1.24.4            110      06:25:30
  Distance: (default is 110)

The output shows several interesting facts. The first highlighted line repeats the parameters on the router ospf 1 global configuration command. The second highlighted item points out R3’s router ID, as discussed further in the next section. The third highlighted line repeats more configuration, listing the parameters of the network 10.0.0.0 0.255.255.255 area 0 OSPF subcommand. Finally, the last highlighted item in the example acts as a heading before a list of known OSPF routers, by router ID.

Configuring the OSPF Router ID

While OSPF has many other optional features, most enterprise networks that use OSPF choose to configure each router’s OSPF router ID. OSPF-speaking routers must have a router ID (RID) for proper operation. By default, routers will choose an interface IP address to use as the RID. However, many network engineers prefer to choose each router’s router ID, so command output from commands like show ip ospf neighbor lists more recognizable router IDs.

To choose its RID, a Cisco router uses the following process when the router reloads and brings up the OSPF process. Note that when one of these steps identifies the RID, the process stops.

1. If the router-id rid OSPF subcommand is configured, this value is used as the RID.

2. If any loopback interfaces have an IP address configured, and the interface has an interface status of up, the router picks the highest numeric IP address among these loopback interfaces.

3. The router picks the highest numeric IP address from all other interfaces whose interface status code (first status code) is up. (In other words, an interface in up/down state will be included by OSPF when choosing its router ID.)

The first and third criteria should make some sense right away: the RID is either configured or is taken from a working interface’s IP address. However, this book has not yet explained the concept of a loopback interface, as mentioned in Step 2.

A loopback interface is a virtual interface that can be configured with the interface loopback interface-number command, where interface-number is an integer. Loopback interfaces are always in an “up and up” state unless administratively placed in a shutdown state. For example, a simple configuration of the command interface loopback 0, followed by ip address 2.2.2.2 255.255.255.0, would create a loopback interface and assign it an IP address. Because loopback interfaces do not rely on any hardware, these interfaces can be up/up whenever IOS is running, making them good interfaces on which to base an OSPF RID.

Example 8-8 shows the configuration that existed in Routers R1 and R2 before the creation of the show command output in Examples 8-4, 8-5, and 8-6. R1 set its router ID using the direct method, while R2 used a loopback IP address.

Example 8-8 OSPF Router ID Configuration Examples

! R1 Configuration first
router ospf 1
 router-id 1.1.1.1
 network 10.1.0.0 0.0.255.255 area 0

! R2 Configuration next
!
interface Loopback2
 ip address 2.2.2.2 255.255.255.255

Each router chooses its OSPF RID when OSPF is initialized, which happens when the router boots or when a CLI user stops and restarts the OSPF process (with the clear ip ospf process command). So, if OSPF comes up, and later the configuration changes in a way that would impact the OSPF RID, OSPF does not change the RID immediately. Instead, IOS waits until the next time the OSPF process is restarted.

Example 8-9 shows the output of the show ip ospf command on R1, after the configuration of Example 8-8 was made, and after the router was reloaded, which made the OSPF router ID change.

Example 8-9 Confirming the Current OSPF Router ID

R1# show ip ospf
 Routing Process "ospf 1" with ID 1.1.1.1
! lines omitted for brevity

OSPF Passive Interfaces

Once OSPF has been enabled on an interface, the router tries to discover neighboring OSPF routers and form a neighbor relationship. To do so, the router sends OSPF Hello messages on a regular time interval (called the Hello Interval). The router also listens for incoming Hello messages from potential neighbors.

Sometimes, a router does not need to form neighbor relationships with neighbors on an interface. Often, no other routers exist on a particular link, so the router has no need to keep sending those repetitive OSPF Hello messages.

When a router does not need to discover neighbors off some interface, the engineer has a couple of configuration options. First, by doing nothing, the router keeps sending the messages, wasting some small bit of CPU cycles and effort. Alternately, the engineer can configure the interface as an OSPF passive interface, telling the router to do the following:

• Quit sending OSPF Hellos on the interface.

• Ignore received Hellos on the interface.

• Do not form neighbor relationships over the interface.

By making an interface passive, OSPF does not form neighbor relationships over the interface, but it does still advertise about the subnet connected to that interface. That is, the OSPF configuration enables OSPF on the interface (using the network router subcommand), and then makes the interface passive (using the passive-interface router subcommand).

To configure an interface as passive, two options exist. First, you can add the following command to the configuration of the OSPF process, in router configuration mode:

• passive-interface type number

Alternately, the configuration can change the default setting so that all interfaces are passive by default, and then add a no passive-interface command for all interfaces that need to not be passive:

• passive-interface default

• no passive interface type number

For example, in the sample internetwork in Figure 8-2 (used in the single-area configuration examples), Router R1, at the bottom left of the figure, has a LAN interface configured for VLAN trunking. The only router connected to both VLANs is Router R1, so R1 will never discover an OSPF neighbor on these subnets. Example 8-10 shows two alternative configurations to make the two LAN subinterfaces passive to OSPF.

Example 8-10 Configuring Passive Interfaces on R1 and R2 from Figure 8-2

! First, make each subinterface passive directly
router ospf 1
 passive-interface GigabitEthernet0/0.11
 passive-interface GigabitEthernet0/0.12

! Or, change the default to passive, and make the other interfaces
! not be passive

router ospf 1
 passive-interface default
 no passive-interface serial0/0/0
 no passive-interface serial0/0/1

In real internetworks, the choice of configuration style reduces to which option requires the least number of commands. For example, a router with 20 interfaces, 18 of which are passive to OSPF, has far fewer configuration commands when using the passive-interface default command to change the default to passive. If only two of those 20 interfaces need to be passive, use the default setting, in which all interfaces are not passive, to keep the configuration shorter.

Interestingly, OSPF makes it a bit of a challenge to use show commands to find whether or not an interface is passive. The show running-config command lists the configuration directly, but if you cannot get into enable mode to use this command, note these two facts:

• The show ip ospf interface brief command lists all interfaces on which OSPF is enabled, including passive interfaces.

• The show ip ospf interface command lists a single line that mentions that the interface is passive.

Example 8-11 shows these two commands on Router R1, with the configuration shown in the top of Example 8-10. Note that subinterfaces G0/0.11 and G0/0.12 both show up in the output of show ip ospf interface brief.

Example 8-11 Displaying Passive Interfaces

R1# show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Gi0/0.12     1     0               10.1.1.129/25      1     DR    0/0
Gi0/0.11     1     0               10.1.1.1/25        1     DR    0/0
Se0/0/0      1     0               10.1.12.1/25       64    P2P   0/0
Se0/0/1      1     0               10.1.13.1/25       64    P2P   0/0

R1# show ip ospf interface g0/0.11
GigabitEthernet0/0.11 is up, line protocol is up
  Internet Address 10.1.1.1/25, Area 0, Attached via Network Statement
  Process ID 1, Router ID 10.1.1.129, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 10.1.1.129, Interface address 10.1.1.1
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    No Hellos (Passive interface)
! Lines omitted for brevity

Implementing Multiarea OSPFv2

Configuring the routers in a multiarea design is almost just like configuring OSPFv2 for a single area. The only difference is that the configuration places some interfaces on each ABR in different areas. The differences come in the verification and operation of OSPFv2.

This second major section of the chapter provides a second set of configurations to contrast multiarea configuration with single-area configuration. This new scenario shows the configuration for the routers in the multiarea OSPF design based on Figures 8-3 and 8-4. Figure 8-3 shows the internetwork topology and subnet IDs, and Figure 8-4 shows the area design. Note that Figure 8-3 lists the last octet of each router’s IPv4 address near each interface, rather than the entire IPv4 address, to reduce clutter.

Figure 8-3 Subnets for a Multiarea OSPF Configuration Example

Figure 8-4 Area Design for an Example Multiarea OSPF Configuration

Take a moment to think about the area design shown in Figure 8-4, and look for the ABRs. Only R1 connects to the backbone area at all. The other three routers are internal routers in a single area. So, as it turns out, three of the four routers have single-area configurations, with all interfaces in the same area.

Note that the examples in this section use a variety of configuration options just so you can see those options. The options include different ways to set the OSPF RID, different wildcard masks on OSPF network commands, and the use of passive interfaces where no other OSPF routers should exist off an interface.

Single-Area Configurations

Example 8-12 begins the configuration example by showing the OSPF and IP address configuration on R2. Note that R2 acts as an internal router in area 23, meaning that the configuration will refer to only one area (23). The configuration sets R2’s RID to 2.2.2.2 directly with the router-id command. And, because R2 should find neighbors on both its two interfaces, neither can reasonably be made passive, so R2’s configuration lists no passive interfaces.

Example 8-12 OSPF Configuration on R2, Placing Two Interfaces into Area 23

interface GigabitEthernet0/0
 ip address 10.1.23.2 255.255.255.0
!
interface serial 0/0/1
 ip address 10.1.12.2 255.255.255.0
!
router ospf 1
 network 10.0.0.0 0.255.255.255 area 23
 router-id 2.2.2.2

Example 8-13 continues reviewing a few commands with the configuration for both R3 and R4. R3 puts both its interfaces into area 23, per its network command, sets its RID to 3.3.3.3 by using a loopback interface, and, like R2, cannot make either of its interfaces passive. The R4 configuration is somewhat different, with both interfaces placed into area 4, setting its RID based on a nonloopback interface (G0/0, for OSPF RID 10.1.14.4), and making R4’s G0/1 interface passive, because no other OSPF routers sit on that link. (Note that the choice to use one method over another to set the OSPF RID is simply to show the variety of configuration options.)

Example 8-13 OSPF Single-Area Configuration on R3 and R4

! First, on R3
interface GigabitEthernet0/0
 ip address 10.1.23.3 255.255.255.0
!
interface serial 0/0/0
 ip address 10.1.13.3 255.255.255.0
!
interface loopback 0
 ip address 3.3.3.3 255.255.255.0
!
router ospf 1
 network 10.0.0.0 0.255.255.255 area 23

! Next, on R4
interface GigabitEthernet0/0
 description R4 will use this interface for its OSPF RID
 ip address 10.1.14.4 255.255.255.0
!
interface GigabitEthernet0/1
 ip address 10.1.4.4 255.255.255.0
!
router ospf 1
 network 10.0.0.0 0.255.255.255 area 4
 passive-interface GigabitEthernet0/1

Multiarea Configuration

The only router that has a multiarea config is an ABR, by virtue of the configuration referring to more than one area. In this design (as shown in Figure 8-4), only Router R1 acts as an ABR, with interfaces in three different areas. Example 8-14 shows R1’s OSPF configuration. Note that the configuration does not state anything about R1 being an ABR; instead, it uses multiple network commands, some placing interfaces into area 0, some into area 23, and some into area 4.

Example 8-14 OSPF Multiarea Configuration on Router R1

interface GigabitEthernet0/0.11
 encapsulation dot1q 11
 ip address 10.1.1.1 255.255.255.0
!
interface GigabitEthernet0/0.12
 encapsulation dot1q 12
 ip address 10.1.2.1 255.255.255.0
!
interface GigabitEthernet0/1
 ip address 10.1.14.1 255.255.255.0
!
interface serial 0/0/0
 ip address 10.1.12.1 255.255.255.0
!
interface serial 0/0/1
 ip address 10.1.13.1 255.255.255.0
!
router ospf 1
 network 10.1.1.1 0.0.0.0 area 0
 network 10.1.2.1 0.0.0.0 area 0
 network 10.1.12.1 0.0.0.0 area 23
 network 10.1.13.1 0.0.0.0 area 23
 network 10.1.14.1 0.0.0.0 area 4
 router-id 1.1.1.1
 passive-interface GigabitEthernet0/0.11
 passive-interface GigabitEthernet0/0.12

Focus on the highlighted network commands in the example. All five commands happen to use a wildcard mask of 0.0.0.0, so that each command requires a specific match of the listed IP address. If you compare these network commands to the various interfaces on Router R1, you can see that the configuration enables OSPF, for area 0, on subinterfaces G0/0.11 and G0/0.12, area 23 for the two serial interfaces, and area 4 for R1’s G0/1 interface.

Finally, note that R1’s configuration also sets its RID directly and makes its two LAN subinterfaces passive.

So, what’s the big difference between single-area and multiarea OSPF configuration? Practically nothing. The only difference is that with multiarea, the ABR’s network commands list different areas.

Verifying the Multiarea Configuration

The next few pages look at how to verify a few of the new OSPF features introduced in this chapter. Figure 8-5 summarizes the most important OSPF verification commands for reference.

Figure 8-5 OSPF Verification Commands

This section looks at the following topics:

• Verifying the ABR interfaces are in the correct (multiple) areas

• Finding which router is DR and BDR on multiaccess links

• A brief look at the LSDB

• Displaying IPv4 routes

Verifying the Correct Areas on Each Interface on an ABR

The easiest place to make a configuration oversight with a multiarea configuration is to place an interface into the wrong OSPF area. Several commands mention the OSPF area. The show ip protocols command basically relists the OSPF network configuration commands, which indirectly identify the interfaces and areas. Also, the show ip ospf interface and show ip ospf interface brief commands directly show the area configured for an interface; Example 8-15 shows an example of the briefer version of these commands.

Example 8-15 Listing the OSPF-Enabled Interfaces and the Matching OSPF Areas

R1# show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Gi0/0.12     1     0               10.1.2.1/24        1     DR    0/0
Gi0/0.11     1     0               10.1.1.1/24        1     DR    0/0
Gi0/1        1     4               10.1.14.1/24       1     BDR   1/1
Se0/0/1      1     23              10.1.13.1/24       64    P2P   1/1
Se0/0/0      1     23              10.1.12.1/24       64    P2P   1/1

In the output, to correlate the areas, just look at the interface in the first column and the area in the third column. Also, for this example, double-check this information with Figures 8-3 and 8-4 to confirm that the configuration matches the design.

Verifying Which Router Is DR and BDR

Several show commands identify the DR and BDR in some way, as well. In fact, the show ip ospf interface brief command output, just listed in Example 8-15, lists the local router’s state, showing that R1 is DR on two subinterfaces and BDR on its G0/1 interface.

Example 8-16 shows two other examples that identify the DR and BDR, but with a twist. The show ip ospf interface command lists detailed output about OSPF settings, per interface. Those details include the RID and interface address of the DR and BDR. At the same time, the show ip ospf neighbor command lists shorthand information about the neighbor’s DR or BDR role as well; this command does not say anything about the local router’s role.

Example 8-16 Discovering the DR and BDR on the R1–R4 Ethernet (from R4)

R4# show ip ospf interface gigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up
  Internet Address 10.1.14.4/24, Area 4, Attached via Network Statement
  Process ID 1, Router ID 10.1.14.4, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 10.1.14.4, Interface address 10.1.14.4
  Backup Designated router (ID) 1.1.1.1, Interface address 10.1.14.1
!
! Lines omitted for brevity
R4# show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/BDR        00:00:33    10.1.14.1       GigabitEthernet0/0

First, focus on the highlighted lines from the show ip ospf interface command output. It lists the DR as RID 10.1.14.4, which is R4. It also lists the BDR as 1.1.1.1, which is R1.

The end of the example shows the show ip ospf neighbor command on R4, listing R4’s single neighbor, with Neighbor RID 1.1.1.1 (R1). The command lists R4’s concept of its neighbor state with neighbor 1.1.1.1 (R1), with the current state listed as FULL/BDR. The FULL state means that R4 has fully exchanged its LSDB with R1. BDR means that the neighbor (R1) is acting as the BDR, implying that R4 (the only other router on this link) is acting as the DR.

Example 8-16 also shows the results of an DR/BDR election, with the router using the higher RID winning the election. The rules work like this:

• When a link comes up, if two (or more) routers on the subnet send and hear each other’s Hello messages, they elect a DR and BDR, with the higher OSPF RID becoming DR, and the second highest RID becoming the BDR.

• Once the election has completed, new routers entering the subnet do not take over the DR or BDR role, even if they have better (higher) RID.

In this case, Routers R1 and R4, on the same Ethernet, heard each other’s Hellos. R1, with RID 1.1.1.1, has a lower-value RID than R4’s 10.1.14.1. As a result, R4 (10.1.14.1) won the DR election.

Verifying Interarea OSPF Routes

Finally, all this OSPF theory and all the show commands do not matter if the routers do not learn IPv4 routes. To verify the routes, Example 8-17 shows R4’s IPv4 routing table.

Example 8-17 Verifying OSPF Routes on Router R4

R4# show ip route
Codes: L - local, C - connected, S - static, 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
       i - IS-IS, su - IS-IS summary, 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, H - NHRP, l - LISP
       + - replicated route, % - next hop override

      10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
O IA     10.1.1.0/24 [110/2] via 10.1.14.1, 11:04:43, GigabitEthernet0/0
O IA     10.1.2.0/24 [110/2] via 10.1.14.1, 11:04:43, GigabitEthernet0/0
C        10.1.4.0/24 is directly connected, GigabitEthernet0/1
L        10.1.4.4/32 is directly connected, GigabitEthernet0/1
O IA     10.1.12.0/24 [110/65] via 10.1.14.1, 11:04:43, GigabitEthernet0/0
O IA     10.1.13.0/24 [110/65] via 10.1.14.1, 11:04:43, GigabitEthernet0/0
C        10.1.14.0/24 is directly connected, GigabitEthernet0/0
L        10.1.14.4/32 is directly connected, GigabitEthernet0/0
O IA     10.1.23.0/24 [110/66] via 10.1.14.1, 11:04:43, GigabitEthernet0/0

This example shows a couple of new codes that are particularly interesting for OSPF. As usual, a single character on the left identifies the source of the route, with O meaning OSPF. In addition, IOS notes any interarea routes with an IA code as well. (The example does not list any intra-area OSPF routes, but these routes would simply omit the IA code; earlier Example 8-6 lists some intra-area OSPF routes.) Also, note that R4 has routes to all seven subnets in the topology used in this example: two connected routes and five interarea OSPF routes.

Additional OSPF Features

So far this chapter has focused on the most common OSPF features using the traditional configuration using the OSPF network command. This final of three major sections discusses some very popular but optional OSPFv2 configuration features, as listed here in their order of appearance:

• Default routes

• Metrics

• Load balancing

• OSPF interface configuration

OSPF Default Routes

In some cases, routers benefit from using a default route. The ICND1 Cert Guide showed many of the details, with the configuration of static default routes in Chapter 18, learning default routes with DHCP in Chapter 20, and advertising default routes with RIP in Chapter 19. For those exact same reasons, networks that happen to use OSPFv2 can use OSPF to advertise default routes.

The most classic case for using a routing protocol to advertise a default route has to do with an enterprise’s connection to the Internet. As a strategy, the enterprise engineer uses these design goals:

• All routers learn specific routes for subnets inside the company; a default route is not needed when forwarding packets to these destinations.

• One router connects to the Internet, and it has a default route that points toward the Internet.

• All routers should dynamically learn a default route, used for all traffic going to the Internet, so that all packets destined to locations in the Internet go to the one router connected to the Internet.

Figure 8-6 shows the idea of how OSPF advertises the default route, with the specific OSPF configuration. In this case, a company connects to an ISP with its Router R1. That router has a static default route (destination 0.0.0.0, mask 0.0.0.0) with a next-hop address of the ISP router. Then, the use of the OSPF default-information originate command (Step 2) makes the router advertise a default route using OSPF to the remote routers (B1 and B2).

Figure 8-6 Using OSPF to Create and Flood a Default Route

Figure 8-7 shows the default routes that result from OSPF’s advertisements in Figure 8-6. On the far left, the branch routers all have OSPF-learned default routes, pointing to R1. R1 itself also needs a default route, pointing to the ISP router, so that R1 can forward all Internet-bound traffic to the ISP.

Figure 8-7 Default Routes Resulting from the default-information originate Command

Finally, this feature gives the engineer control over when the router originates this default route. First, R1 needs a default route, either defined as a static default route, learned from the ISP with DHCP, or learned from the ISP with a routing protocol like eBGP. The default-information originate command then tells R1 to advertise a default route when its own default route is working, and to advertise the default route as down when its own default route fails.

Example 8-18 shows details of the default route on both R1 and branch router B01. Beginning with Router R1, in this case, Router R1 used DHCP to learn its IP address on its G0/3 interface from the ISP. R1 then creates a static default route with the ISP router’s IP address of 192.0.2.1 as the next-hop address, as highlighted in the output of the show ip route static command output.

Example 8-18 Default Routes on Routers R1 and B01

! The next command is from Router R1. Note the static code for the default route
R1# show ip route static
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
! Rest of the legend omitted for brevity

Gateway of last resort is 192.0.2.1 to network 0.0.0.0

S*    0.0.0.0/0 [254/0] via 192.0.2.1

! The next command is from router B01; notice the External route code for the default
BO1# show ip route ospf
Codes: L - local, C - connected, S - static, 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
! Rest of the legend omitted for brevity

Gateway of last resort is 10.1.12.1 to network 0.0.0.0

O*E2  0.0.0.0/0 [110/1] via 10.1.12.1, 00:20:51, GigabitEthernet0/1
      10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
O        10.1.3.0/24 [110/3] via 10.1.12.1, 00:20:51, GigabitEthernet0/1
O        10.1.13.0/24 [110/2] via 10.1.12.1, 00:20:51, GigabitEthernet0/1

Keeping the focus on the command on Router R1, note that R1 indeed has a default route, that is, a route to 0.0.0.0/0. The “Gateway of last resort,” which refers to the default route currently used by the router, points to next-hop IP address 192.0.2.1, which is the ISP router’s IP address. (Refer back to Figure 8-7 for the particulars.)

Next look to the bottom half of the example, and router BO1’s OSPF-learned default route. BO1 lists a route for 0.0.0.0/0 as well. The next-hop router in this case is 10.1.12.1, which is Router R1’s IP address on the WAN link. The code on the far left is O*E2, meaning: an OSPF-learned route, which is a default route, and is specifically an external OSPF route. Finally, BO1’s gateway of last resort setting uses that one OSPF-learned default route, with next-hop router 10.1.12.1.

OSPF Metrics (Cost)

Earlier, the Chapter 7 section “Calculating the Best Routes with SPF” discussed how SPF calculates the metric for each route, choosing the route with the best metric for each destination subnet. OSPF routers can influence that choice by changing the OSPF interface cost on any and all interfaces.

Cisco routers allow two different ways to change the OSPF interface cost. The one straightforward way is to set the cost directly, with an interface subcommand: ip ospf cost x. The other method is to let IOS choose default costs, based on a formula, but to change the inputs to the formula. This second method requires a little more thought and care and is the focus of this next topic.

Setting the Cost Based on Interface Bandwidth

The default OSPF cost values can actually cause a little confusion, for a couple of reasons. So, to get through some of the potential confusion, this section begins with some examples.

First, IOS uses the following formula to choose an interface’s OSPF cost. IOS puts the interface’s bandwidth in the denominator, and a settable OSPF value called the reference bandwidth in the numerator:

Reference_bandwidth / Interface_bandwidth

With this formula, the following sequence of logic happens:

1. A higher interface bandwidth—that is, a faster bandwidth—results in a lower number in the calculation.

2. A lower number in the calculation gives the interface a lower cost.

3. An interface with a lower cost is more likely to be used by OSPF when calculating the best routes.

Now for some examples. Assume a default reference bandwidth, set to 100 Mbps, which is the same as 100,000 Kbps. (The upcoming examples will use a unit of Kbps just to avoid math with fractions.) Assume defaults for interface bandwidth on serial, Ethernet, and Fast Ethernet interfaces, as shown in the output of the show interfaces command, respectively, of 1544 Kbps, 10,000 Kbps (meaning 10 Mbps), and 100,000 Kbps (meaning 100 Mbps). Table 8-3 shows the results of how IOS calculates the OSPF cost for some interface examples.

Table 8-3 OSPF Cost Calculation Examples with Default Bandwidth Settings

Example 8-19 shows the cost settings on R1’s OSPF interfaces, all based on default OSPF (reference bandwidth) and default interface bandwidth settings.

Example 8-19 Confirming OSPF Interface Costs

R1# show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Gi0/0.12     1     0               10.1.2.1/24        1     DR    0/0
Gi0/0.11     1     0               10.1.1.1/24        1     DR    0/0
Gi0/1        1     4               10.1.14.1/24       1     BDR   1/1
Se0/0/1      1     23              10.1.13.1/24       64    P2P   1/1
Se0/0/0      1     23              10.1.12.1/24       64    P2P   1/1

To change the OSPF cost on these interfaces, the engineer simply needs to use the bandwidth speed interface subcommand to set the bandwidth on an interface. The interface bandwidth does not change the Layer 1 transmission speed at all; instead, it is used for other purposes, including routing protocol metric calculations. For instance, if you add the bandwidth 10000 command to a serial interface, with a default reference bandwidth, the serial interface’s OSPF cost could be calculated as 100,000 / 10,000 = 10.

Note that if the calculation of the default metric results in a fraction, OSPF rounds down to the nearest integer. For instance, the example shows the cost for interface S0/0/0 as 64. The calculation used the default serial interface bandwidth of 1.544 Mbps, with reference bandwidth 100 (Mbps), with the 100 / 1.544 calculation resulting in 64.7668394. OSPF rounds down to 64.

The Need for a Higher Reference Bandwidth

This default calculation works nicely as long as the fastest link in the network runs at 100 Mbps. The default reference bandwidth is set to 100, meaning 100 Mbps, the equivalent of 100,000 Kbps. As a result, with default settings, faster router interfaces end up with the same OSPF cost, as shown in Table 8-4, because the lowest allowed OSPF cost is 1.

Table 8-4 Faster Interfaces with Equal OSPF Costs

To avoid this issue, and change the default cost calculation, you can change the reference bandwidth with the auto-cost reference-bandwidth speed OSPF mode subcommand. This command sets a value in a unit of megabits per second (Mbps). To avoid the issue shown in Table 8-4, set the reference bandwidth value to match the fastest link speed in the network. For instance, auto-cost reference-bandwidth 10000 accommodates links up to 10 Gbps in speed.

For convenient study, the following list summarizes the rules for how a router sets its OSPF interface costs:

1. Set the cost explicitly, using the ip ospf cost x interface subcommand, to a value between 1 and 65,535, inclusive.

2. Change the interface bandwidth with the bandwidth speed command, with speed being a number in kilobits per second (Kbps).

3. Change the reference bandwidth, using router OSPF subcommand auto-cost reference-bandwidth ref-bw, with a unit of megabits per second (Mbps).

OSPF Load Balancing

When a router uses SPF to calculate the metric for each of several routes to reach one subnet, one route may have the lowest metric, so OSPF puts that route in the routing table. However, when the metrics tie for multiple routes to the same subnet, the router can put multiple equal-cost routes in the routing table (the default is four different routes) based on the setting of the maximum-paths number router subcommand. For example, if an internetwork has six possible paths between some parts of the network, and the engineer wants all routes to be used, the routers can be configured with the maximum-paths 6 subcommand under router ospf.

The more challenging concept relates to how the routers use those multiple routes. A router could load balance the packets on a per-packet basis. For example, if the router has three equal-cost OSPF routes for the same subnet in the routing table, the router could send the one packet over the first route, the next packet over the second route, the next packet over the third route, and then start over with the first route for the next packet. Alternatively, the load balancing could be on a per-destination IP address basis.

Note that the default setting of maximum-paths varies by router platform.

OSPFv2 Interface Configuration

The newer interface-style OSPF configuration works mostly like the old style, for almost all features, with one important exception. The interface configuration enables OSPF directly on the interface with the ip ospf interface subcommand, while the traditional OSPFv2 configuration enables OSPFv2 on an interface, but indirectly, using the network command in OSPF configuration mode. The rest of the OSPF features discussed throughout this chapter are not changed by the use of OSPFv2 interface configuration.

Basically, instead of matching interfaces with indirect logic using network commands, you directly enable OSPFv2 on interfaces by configuring an interface subcommand on each interface.

OSPFv2 Interface Configuration Example

To show how OSPF interface configuration works, this example basically repeats the example shown earlier in the book using the traditional OSPFv2 configuration with network commands. So, before looking at the OSPFv2 interface configuration, take a moment to look back at Figures 8-3 and 8-4, along with Examples 8-12, 8-13, and 8-14. Once reviewed, for easier reference, Figure 8-8 repeats Figure 8-4 for reference in the upcoming interface configuration examples.

To convert from the old-style configuration in Examples 8-12, 8-13, and 8-14, simply do the following:

• Step 1. Use the no network network-id area area-id subcommands in OSPF configuration mode to remove the network commands.

• Step 2. Add one ip ospf process-id area area-id command in interface configuration mode under each interface on which OSPF should operate, with the correct OSPF process (process-id) and the correct OSPF area number.

For example, Example 8-12 had a single network command that enabled OSPF on two interfaces on Router R2, putting both in area 23. Example 8-20 shows the replacement newer style of configuration.

Figure 8-8 Area Design Used in the Upcoming OSPF Example

Example 8-20 New-Style Configuration on Router R2

interface GigabitEthernet0/0
 ip address 10.1.23.2 255.255.255.0
 ip ospf 1 area 23
!
interface serial 0/0/1
 ip address 10.1.12.2 255.255.255.0
 ip ospf 1 area 23

router ospf 1
 router-id 2.2.2.2
! Notice – no network commands here!

Verifying OSPFv2 Interface Configuration

OSPF operates the same way whether you use the new style or old style of configuration. The OSPF area design works the same, neighbor relationships form the same way, routers negotiate to become the DR and BDR the same way, and so on. However, you can see a few small differences in command output when using the newer OSPFv2 configuration if you look closely.

The show ip protocols command relists most of the routing protocol configuration, just in slightly different format, as shown in Example 8-21. With the newer-style configuration, the output lists the phrase “Interfaces Configured Explicitly,” with the list of interfaces configured with the new ip ospf process-id area area-id commands, as highlighted in the example. With the old configuration, the output lists the contents of all the network commands, just leaving out the “network” word itself. Note that in the next two examples, R2 has been reconfigured to use OSPF interface configuration as shown in the previous example (Example 8-20), while Router R3 still uses the older-style network commands per earlier configuration Example 8-13.

Example 8-21 Differences in show ip protocols Output: Old- and New-Style OSPFv2 Configuration

R2# show ip protocols
*** IP Routing is NSF aware ***

Routing Protocol is "ospf 1"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID 2.2.2.2
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
  Routing on Interfaces Configured Explicitly (Area 23):
    Serial0/0/1
    GigabitEthernet0/0
  Routing Information Sources:
    Gateway        Distance      Last Update
    3.3.3.3              110      00:04:59
    1.1.1.1              110      00:04:43
  Distance: (default is 110)

! Below, showing only the part that differs on R3:
R3# show ip protocols
! ... beginning lines omitted for brevity
  Routing for Networks:
    10.0.0.0 0.255.255.255 area 23
! ... ending line omitted for brevity

Basically, the show ip protocols command output differs depending on the style of configuration, either relisting the interfaces when using interface configuration or relisting the network commands if using network commands.

Next, the show ip ospf interface [interface] command lists details about OSPF settings for the interface(s) on which OSPF is enabled. The output also makes a subtle reference to whether that interface was enabled for OSPF with the old or new configuration style. As seen in Example 8-22, R2’s new-style interface configuration results in the highlighted text, “Attached via Interface Enable,” whereas R3’s old-style configuration lists “Attached via Network Statement.”

Example 8-22 Differences in show ip ospf interface Output with OSPFv2 Interface Configuration

R2# show ip ospf interface g0/0
GigabitEthernet0/0 is up, line protocol is up
  Internet Address 10.1.23.2/24, Area 23, Attached via Interface Enable
  Process ID 1, Router ID 22.2.2.2, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0          1        no          no            Base
  Enabled by interface config, including secondary ip addresses
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 10.1.23.2
  Backup Designated router (ID) 3.3.3.3, Interface address 10.1.23.3

! Showing only the part that differs on R3:
R3# show ip ospf interface g0/0
GigabitEthernet0/0 is up, line protocol is up
  Internet Address 10.1.23.3/24, Area 23, Attached via Network Statement
! ... ending line omitted for brevity

Note that the briefer version of this command, the show ip ospf interface brief command, does not change whether the configuration uses traditional network commands or the alternative interface configuration with the ip ospf interface subcommand.