OSPF Interface Config 1

From the first days of OSPF in Cisco routers, OSPF configuration has included a network command. The network command enables OSPF on that router’s interfaces, but it uses indirect logic. In fact, the network command does not list interface numbers. Cisco added an alternative OSPF configuration method a while back – a method that does directly enable OSPF on the interface, and avoids using the OSPF network command. I’m sure you guessed by now: today’s lab gives you a chance to practice configuring OSPF with this alternative method.

Requirements

Configure OSPF for the lab network shown in the figure. However, do not use the traditional configuration with network commands in OSPF configuration mode. Instead, use OSPF interface configuration. (This lab does not tell you the name of the command, in the spirit of letting you exercise your knowledge.)

The specific rules for this lab are:

• Configure so each router uses a router-id of x.x.x.x where x equals the router number.
• Do not rely on interface IP addresses for the setting of the router IDs.
• Use OSPF area 0 for all interfaces
• Enable OSPF directly on each interface, rather than using the indirect method and the OSPF network command
• Assume all interfaces shown in the lab are up and working

Figure 1: Three Router Triangle with IP Subnets

Initial Configuration

Example 1, 2 and 3 show the beginning configuration state of R1, R2 and R3.

Example 1: R1 Config

hostname R1
!
interface GigabitEthernet0/1
 no shutdown
 ip address 172.30.1.1 255.255.255.252
!
interface GigabitEthernet0/2
 no shutdown
 ip address 172.30.1.10 255.255.255.252
!
interface GigabitEthernet0/3
 no shutdown
 ip address 172.21.1.1 255.255.255.128

Example 2: R2 Config

interface GigabitEthernet0/1
 no shutdown
 ip address 172.30.1.5 255.255.255.252
!
interface GigabitEthernet0/2
 no shutdown
 ip address 172.30.1.2 255.255.255.252
!
interface GigabitEthernet0/3
 no shutdown
 ip address 172.22.2.2 255.255.255.192

Example 3: R3 Config

interface GigabitEthernet0/1
 no shutdown
 ip address 172.30.1.9 255.255.255.252
!
interface GigabitEthernet0/2
 no shutdown
 ip address 172.30.1.6 255.255.255.252
!
interface GigabitEthernet0/3
 no shutdown
 ip address 172.23.3.3 255.255.255.224

Answer on Paper, or Maybe Test in Lab

Next, write your answer on paper. Or if you have some real gear, or other tools, configure the lab with those tools.

To test your solution if you happen to try it with VIRL or real gear, you can check a few obvious items with show commands. First, each router should have two OSPF neighbors (show ip ospf neighbor). Each router show list three OSPF-learned routes (show ip route). If you view the links between routers as WAN links, the three OSPF routes will be for: the two remote LANs, plus the WAN link between the other two routers. Finally, you can check the interfaces enabled for OSPF with show ip ospf interface brief, and the OSPF router ID with show ip ospf database.