Implementing OSPFv2 Using network Commands
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 first major section of the chapter focuses on the required configuration using the traditional OSPFv2 network command along with one optional configuration setting: how to set the OSPF router-id. Additionally, this section works through how to show the various lists and tables that confirm how OSPF is working.
For reference and study, the following list outlines the configuration steps covered in this first major section of the chapter:
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:
Use the router-id id-value router subcommand to define the router ID, or
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), or
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.
Figure 22-1 shows the relationship between the OSPF configuration commands, with the idea 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.
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Figure 22.1 Organization of OSPFv2 Configuration with the network Command
OSPF Single-Area Configuration
Figure 22-2 shows a sample network that will be used for most examples throughout this chapter. All links reside in area 0, making the area design a single-area design, with four routers. You can think of Router R1 as a router at a central site, with WAN links to each remote site. Routers R2 and R3 might be at one large remote site that needs two WAN links and two routers for WAN redundancy, with both routers connected to the LAN at that remote site. Router R4 might be a typical smaller remote site with a single router needed for that site.
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Figure 22.2 Sample Network for OSPF Single-Area Configuration
Example 22-1 shows the IPv4 addressing configuration on Router R1, before getting into the OSPF detail.
Example 22-1 IPv4 Address Configuration on R1
interface GigabitEthernet0/0/0 ip address 10.1.1.1 255.255.255.0 ! interface GigabitEthernet0/0/1 ip address 10.1.12.1 255.255.255.0 ! interface GigabitEthernet0/0/2 ip address 10.1.13.1 255.255.255.0 ! interface GigabitEthernet0/0/3 ip address 10.1.14.1 255.255.255.0
The OSPF configuration begins with the router ospf process-id global command, which puts the user in OSPF configuration mode, and sets the OSPF process-id value. The process-id number just needs to be unique on the local router, matching between various commands in a router. The process-id does not need to match between neighboring routers or other routers in the same area. The value can be any integer between 1 and 65,535.
Second, the configuration needs one or more network commands in OSPF mode. These commands tell the 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.)
Example 22-2 shows an example configuration on Router R1 from Figure 22-2. The router ospf 1 command enables OSPF process 1, and the single network command enables OSPF on all interfaces shown in the figure.
Example 22-2 OSPF Single-Area Configuration on R1 Using One network Command
router ospf 1 network 10.0.0.0 0.255.255.255 area 0
For the specific network command in Example 22-2, 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 interfaces shown for Router R1; the next topic explains why.
Wildcard Matching with the 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 four octets. In other words, the numbers must exactly match.
Wildcard 0.0.0.255: Compare the first three octets only. Ignore the last octet when comparing the numbers.
Wildcard 0.0.255.255: Compare the first two octets only. Ignore the last two octets when comparing the numbers.
Wildcard 0.255.255.255: Compare the first octet only. Ignore the last three 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 decimal 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 R1, many network commands could be used, with some matching all interfaces, and some matching a subset of interfaces. Table 22-2 shows a sampling of options, with notes.
Table 22-2 Example OSPF network Commands on R1, with Expected Results
Command |
Logic in Command |
Matched Interfaces |
|---|---|---|
network 10.1.0.0 0.0.255.255 |
Match addresses that begin with 10.1 |
G0/0/0 G0/0/1 G0/0/1 G0/0/2 |
network 10.0.0.0 0.255.255.255 |
Match addresses that begin with 10 |
G0/0/0 G0/0/1 G0/0/1 G0/0/2 |
network 0.0.0.0 255.255.255.255 |
Match all addresses |
G0/0/0 G0/0/1 G0/0/1 G0/0/2 |
network 10.1.13.0 0.0.0.255 |
Match addresses that begin with 10.1.13 |
G0/0/2 |
network 10.1.13.1 0.0.0.0 |
Match one address: 10.1.13.1 |
G0/0/2 |
The wildcard mask gives the local router its rules for matching its own interfaces. To show examples of the different options, Example 22-3 shows the configuration on routers R2, R3, and R4, each using different wildcard masks. Note that all three routers (R2, R3, and R4) enable OSPF on all the interfaces shown in Figure 22-2.
Example 22-3 OSPF Configuration on Routers R2, R3, and R4
! R2 configuration next - one network command enables OSPF on both interfaces interface GigabitEthernet0/0/0 ip address 10.1.23.2 255.255.255.0 ! interface GigabitEthernet0/0/1 ip address 10.1.12.2 255.255.255.0 ! router ospf 1 network 10.0.0.0 0.255.255.255 area 0
! R3 configuration next - One network command per interface interface GigabitEthernet0/0/0 ip address 10.1.23.3 255.255.255.0 ! interface GigabitEthernet0/0/1 ip address 10.1.13.3 255.255.255.0 ! router ospf 1 network 10.1.13.3 0.0.0.0 area 0 network 10.1.23.3 0.0.0.0 area 0
! R4 configuration next - One network command per interface with wildcard 0.0.0.255 interface GigabitEthernet0/0/0 ip address 10.1.4.4 255.255.255.0 ! interface GigabitEthernet0/0/1 ip address 10.1.14.4 255.255.255.0 ! router ospf 1 network 10.1.14.0 0.0.0.255 area 0 network 10.1.4.0 0.0.0.255 area 0
Finally, note that OSPF uses the same wildcard mask logic as defined by Cisco IOS access control lists. The section titled “Finding the Right Wildcard Mask to Match a Subnet” section in Chapter 6 of the CCNA 200-301 Official Cert Guide, Volume 2, Second Edition, provides more detail about wildcard masks.
Verifying OSPF Operation
As mentioned in Chapter 21, “Understanding OSPF Concepts,” 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 between those neighbors 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. This next topic works through how to display the results of each of those steps, which lets you confirm whether OSPF has worked correctly or not.
The show ip ospf neighbor, show ip ospf database, and show ip route commands display information to match each of these three steps, respectively. Figure 22-3 summarizes the commands you can use (and others) when verifying OSPF.
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Figure 22.3 OSPF Verification Commands
Many engineers begin OSPF verification by looking at the output of the show ip ospf neighbor command. For instance, Example 22-4 shows a sample from Router R1, which should have one neighbor relationship each with routers R2, R3, and R4. Example 22-4 shows all three.
Example 22-4 OSPF Neighbors on Router R1 from Figure 22-2
R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/DR 00:00:37 10.1.12.2 GigabitEthernet0/0/1 3.3.3.3 1 FULL/DR 00:00:37 10.1.13.3 GigabitEthernet0/0/2 4.4.4.4 1 FULL/BDR 00:00:34 10.1.14.4 GigabitEthernet0/0/3
The detail in the output mentions several important facts, and for most people, working right to left works best in this case. For example, look 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 R1’s G0/0/1 interface.
Address: This is the neighbor’s IP address on that link. Again, this first neighbor, the neighbor, which is R2, uses IP address 10.1.12.2.
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.
Once OSPF convergence has completed, a router should list each neighbor. On links that use a designated router (DR), the state will also list the role of the neighboring router after the / (DR, BDR, or DROther). As a result, the normal working states will be:
FULL/ -: The neighbor state is full, with the “-” instead of letters meaning that the link does not use a DR/BDR.
FULL/DR: The neighbor state is full, and the neighbor is the DR.
FULL/BDR: The neighbor state is full, and the neighbor is the backup DR (BDR).
FULL/DROTHER: The neighbor state is full, and the neighbor is neither the DR nor BDR. (It also implies that the local router is a DR or BDR because the state is FULL.)
2WAY/DROTHER: The neighbor state is 2-way, and the neighbor is neither the DR nor BDR—that is, a DROther router. (It also implies that the local router is also a DROther router because otherwise the state would reach a full state.)
Once a router’s OSPF process forms a working neighbor relationship, the routers exchange the contents of their LSDBs, either directly or through the DR on the subnet. Example 22-5 shows the contents of the LSDB on Router R1. Interestingly, with a single-area design, all the routers will have the same LSDB contents once all neighbors are up and all LSAs have been exchanged. So, the show ip ospf database command in Example 22-5 should list the same exact information, no matter on which of the four routers it is issued.
Example 22-5 OSPF Database on Router R1 from Figure 22-2
R1# show ip ospf database
OSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 431 0x8000008F 0x00DCCA 5
2.2.2.2 2.2.2.2 1167 0x8000007F 0x009DA1 2
3.3.3.3 3.3.3.3 441 0x80000005 0x002FB1 1
4.4.4.4 4.4.4.4 530 0x80000004 0x007F39 2
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.12.2 2.2.2.2 1167 0x8000007C 0x00BBD5
10.1.13.3 3.3.3.3 453 0x80000001 0x00A161
10.1.14.1 4.4.4.4 745 0x8000007B 0x004449
10.1.23.3 3.3.3.3 8 0x80000001 0x00658F
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, so with the design based on Figure 22-2, the output lists four Type 1 LSAs. Also, with all default settings in this design, the routers will create a total of four Type 2 Network LSAs as shown, one each for the subnets that have a DR and contain at least two routers in that subnet (the three WAN links plus the LAN to which both R2 and R3 connect).
Next, Example 22-6 shows R4’s IPv4 routing table with the show ip route command. As configured, with all links working, R4 has connected routes to two of those subnets and should learn OSPF routes to the other subnets.
Example 22-6 IPv4 Routes Added by OSPF on Router R4 from Figure 22-2
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
! Additional legend lines omitted for brevity
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
O 10.1.1.0/24 [110/2] via 10.1.14.1, 00:27:24, GigabitEthernet0/0/1
C 10.1.4.0/24 is directly connected, GigabitEthernet0/0/0
L 10.1.4.4/32 is directly connected, GigabitEthernet0/0/0
O 10.1.12.0/24 [110/2] via 10.1.14.1, 00:27:24, GigabitEthernet0/0/1
O 10.1.13.0/24 [110/2] via 10.1.14.1, 00:25:15, GigabitEthernet0/0/1
C 10.1.14.0/24 is directly connected, GigabitEthernet0/0/1
L 10.1.14.4/32 is directly connected, GigabitEthernet0/0/1
O 10.1.23.0/24 [110/3] via 10.1.14.1, 00:27:24, GigabitEthernet0/0/1
Any time you want to check OSPF on a router in a small design like the ones in the book, you can count all the subnets, then count the subnets connected to the local router, and know that OSPF should learn routes to the rest of the subnets. Then just use the show ip route command and add up how many connected and OSPF routes exist as a quick check of whether all the routes have been learned or not.
In this case, Router R4 has two connected subnets, but six subnets exist per the figure, so Router R4 should learn four OSPF routes. Next look for the code of “O” on the left, which identifies a route as being learned by OSPF. The output lists four such IP routes: one for the LAN subnet off Router R1, one for the LAN subnet connected to both R2 and R3, and one each for the WAN subnets from R1 to R2 and R1 to R3.
Next, examine the first route (to subnet 10.1.1.0/24). 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 (see Table 24-4 in Chapter 24, “OSPF Neighbors and Route Selection,” for the list of administrative distance values). The second number, 2, is the OSPF metric for this route. The route also lists the forwarding instructions: the next-hop IP address (10.1.14.1) and R4’s outgoing interface (G0/0/1).
Verifying OSPF Configuration
Once you can configure OSPF with confidence, you will likely verify OSPF focusing on OSPF neighbors and the IP routing table as just discussed. However, if OSPF does not work immediately, you may need to circle back and check the configuration. To do so, you can use these steps:
If you have enable mode access, use the show running-config command to examine the configuration.
If you have only user mode access, use the show ip protocols command to re-create the OSPF configuration.
Use the show ip ospf interface [brief] command to determine whether the router enabled OSPF on the correct interfaces or not based on the configuration.
The best way to verify the configuration begins with the show running-config command, of course. However, the show ip protocols command repeats the details of the OSPFv2 configuration and does not require enable mode access. Example 22-7 does just that for Router R3.
Example 22-7 Router R3 Configuration and the show ip protocols Command
R3# show running-config | section router ospf 1
router ospf 1
network 10.1.13.3 0.0.0.0 area 0
network 10.1.23.3 0.0.0.0 area 0
router-id 3.3.3.3
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 3.3.3.3
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
10.1.13.3 0.0.0.0 area 0
10.1.23.3 0.0.0.0 area 0
Routing Information Sources:
Gateway Distance Last Update
1.1.1.1 110 02:05:26
4.4.4.4 110 02:05:26
2.2.2.2 110 01:51:16
Distance: (default is 110)
The highlighted output emphasizes some of the configuration. The first highlighted line repeats the parameters on the router ospf 1 global configuration command. (The second highlighted item points out the router’s router ID, which will be discussed in the next section.) The third set of highlighted lines begins with a heading of “Routing for Networks:” followed by two lines that closely resemble the parameters on the configured network commands. In fact, closely compare those last two highlighted lines with the network configuration commands at the top of the example, and you will see that they mirror each other, but the show command just leaves out the word network. For instance:
Configuration: network 10.1.13.3 0.0.0.0 area 0
show Command: 10.1.13.3 0.0.0.0 area 0
IOS interprets the network commands to choose interfaces on which to run OSPF, so it could be that IOS chooses a different set of interfaces than you predicted. To check the list of interfaces chosen by IOS, use the show ip ospf interface brief command, which lists all interfaces that have been enabled for OSPF processing. Verifying the interfaces can be a useful step if you have issues with OSPF neighbors because OSPF must first be enabled on an interface before a router will attempt to discover neighbors on that interface. Example 22-8 shows a sample from Router R1.
Example 22-8 Router R1 show ip ospf interface brief Command
R1# show ip ospf interface brief Interface PID Area IP Address/Mask Cost State Nbrs F/C Gi0/0/0 1 0 10.1.1.1/24 1 DR 0/0 Gi0/0/1 1 0 10.1.12.1/24 1 BDR 1/1 Gi0/0/2 1 0 10.1.13.1/24 1 BDR 1/1 Gi0/0/3 1 0 10.1.14.1/24 1 DR 1/1
The show ip ospf interface brief command lists one line per interface, showing all the interfaces on which OSPF has been enabled. Each line identifies the OSPF process ID (per the router ospf process-id command), the area, the interface IP address, and the number of neighbors found via each interface.
You may use the command in Example 22-8 quite often, but the show ip ospf interface command (without the brief keyword) gives much more detail about OSPF per-interface settings. Example 23-4 in Chapter 23, “Implementing Optional OSPF Features,” shows an example of the entire output of that command.
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 the router stops looking for a router ID to use once one of the steps identifies a value to use.
If the router-id rid OSPF subcommand is configured, this value is used as the RID.
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.
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 22-9 shows the configuration that existed in Routers R1 and R2 before the creation of the show command output earlier in this chapter. R1 set its router ID using the direct method, while R2 used a loopback IP address. Example 22-10 that follows shows the output of the show ip ospf command on R1, which identifies the OSPF RID used by R1.
Example 22-9 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
Example 22-10 Confirming the Current OSPF Router ID
R1# show ip ospf Routing Process "ospf 1" with ID 1.1.1.1 ! lines omitted for brevity
Routers need a stable OSPF RID because any change to the OSPF RID causes a router to close existing neighbor relationships and remove all routes learned through those neighbors. To keep the RID stable, a router chooses its RID when the router first initializes (at power-on or per the reload command). So the RID might change at the next reload when the router re-evaluates the RID choice rules based on the current conditions.
However, routers do support one scenario to update their RID without a reload, which can be useful for testing in lab. To do so, configure the OSPF router-id OSPF subcommand followed by the clear ip ospf process EXEC command.
Implementing Multiarea OSPF
Even though the current CCNA 200-301 V1.1 exam blueprint mentions single area but not multiarea OSPF, you only need to learn one more idea to know how to configure multiarea OSPF. So, this chapter takes a brief page to show how.
For example, consider a multiarea OSPF design as shown in Figure 22-4. It uses the same routers and IP addresses as shown earlier in Figure 22-2, on which all the examples in this chapter have been based so far. However, the design shows three areas instead of the single-area design shown in Figure 22-2.
Configuring the routers in a multiarea design is almost just like configuring OSPFv2 for a single area. To configure multiarea OSPF, all you need is a valid OSPF area design (for instance, like Figure 22-4) and a configuration that places each router interface into the correct area per that design. For example, both of R4’s interfaces connect to links in area 4, making R4 an internal router, so any network commands on Router R4 will list area 4.
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Figure 22.4 Area Design for an Example Multiarea OSPF Configuration
Example 22-11 shows a sample configuration for Router R1. To make the configuration clear, it uses network commands with a wildcard mask of 0.0.0.0, meaning each network command matches a single interface. Each interface will be placed into either area 0, 23, or 4 to match the figure.
Example 22-11 OSPF Configuration on R1, Placing Interfaces into Different Areas
router ospf 1 network 10.1.1.1 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