In this scenario, you configure two Cisco routers for OSPF routing using a Class B (/16) network (131.108.0.0) with a Class C subnet mask (255.255.255.0, or /24 mask). You build a small network and an OSPF routing table.
You must also configure a number of loopback interfaces to populate the IP routing table. Figure 3-1 displays two routers named R1 and R2 connected through Ethernet.
Configure the routers of OSPF area 1 and place the loopbacks in area 1 also.
Figure 3-1. Basic OSPF
Figure 3-1 displays the IP addressing and area assignments for Routers R1 and R2.
Configure R1 for OSPF first. Assign all interfaces with the area assignment 1. Also note that this scenario uses VLSM. Use the network command and match the IP subnet exactly. Example 3-2 displays the OSPF configuration performed on R1.
NOTE
Routers R1 and R2 reside in one area; so, in fact, you could apply the one IOS command to enable all interfaces configured with an IP address in the range 131.108.0.0 through 131.108.255.255 with the command network 131.108.0.0 0.0.255.255 area 1.
Example 3-2 R1 OSPF Configuration
router ospf 1
network 131.108.1.0 0.0.0.255 area 1 network 131.108.4.0 0.0.0.127 area 1 network 131.108.4.128 0.0.0.127 area 1 network 131.108.5.0 0.0.0.31 area 1
Example 3-3 displays the OSPF configuration performed on R2.
Example 3-3 R2 OSPF Configuration
router ospf 2
network 131.108.1.0 0.0.0.255 area 1 network 131.108.5.32 0.0.0.31 area 1 network 131.108.6.1 0.0.0.0 area 1 network 131.108.6.2 0.0.0.0 area 1
NOTE
R1 has a process ID of 1 and R2 has a process ID of 2. The process ID is locally significant only and doesn't need to match between routers. The process ID can be any number between 1–65535. Also, because R2 has host (or /32 subnets) masks on loopbacks 2 and 3, the inverse mask is 0.0.0.0, or an exact match.
Example 3-4 displays the three remote networks reachable through OSPF with a cost metric of 11 for all three. The next hop address is 131.108.1.2 through Ethernet 0/0.
You might ask yourself why some of the remote networks are displayed as a /32 route when you used a /27 mask.
To discover why loopbacks appear as /32 host routers, examine Example 3-4, which displays the IP routing table on R1.
Example 3-4 R1's IP Routing Table
R1#show ip route
131.108.0.0/16 is variably subnetted, 7 subnets, 4 masks C 131.108.4.128/25 is directly connected, Loopback1
O 131.108.5.33/32 [110/11] via 131.108.1.2, 00:02:22, Ethernet0/0 O 131.108.6.1/32 [110/11] via 131.108.1.2, 00:02:22, Ethernet0/0 C 131.108.5.0/27 is directly connected, Loopback2
O 131.108.6.2/32 [110/11] via 131.108.1.2, 00:02:22, Ethernet0/0 C 131.108.4.0/25 is directly connected, Loopback0
C 131.108.1.0/24 is directly connected, Ethernet0/0 R1#
The remote network is displayed as a /32 route when a /27 mask is used because, by default, OSPF advertises loopbacks as host addresses, or as /32 routes. Change this default configuration and make the routes appear as /27 with the configuration on R2, as displayed in Example 3-5. To make things a little more interesting, modify the cost as well to 1000.
Example 3-5 Advertising Loopbacks as /27 on R2 and Changing the Default Cost to 1000
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#int loopback0
R2(config-if)#ip ospf cost 1000
R2(config-if)#ip ospf network point-to-point
The command ip ospf cost 1000 changes the cost to 1000. The command ip ospf network point-to-point changes the route advertisement to /27. Example 3-6 displays R1's routing table after these changes.
Example 3-6 R1 Routing Table
R1#show ip route
131.108.0.0/16 is variably subnetted, 7 subnets, 4 masks C 131.108.4.128/25 is directly connected, Loopback1 O 131.108.5.32/27 [110/1010] via 131.108.1.2, 00:01:19, Ethernet0/0
O 131.108.6.1/32 [110/11] via 131.108.1.2, 00:01:19, Ethernet0/0 C 131.108.5.0/27 is directly connected, Loopback2
O 131.108.6.2/32 [110/11] via 131.108.1.2, 00:01:19, Ethernet0/0 C 131.108.4.0/25 is directly connected, Loopback0
C 131.108.1.0/24 is directly connected, Ethernet0/0 R1#
In Example 3-6, the subnet 131.108.5.32 displayed is 27 bits. The remaining loopbacks are still /32, so you need to modify them also.
The associated cost of the remote network 131.108.5.32/27 is 1010. To figure out why, remember that OSPF calculates the total cost from source to destination. The 1000 is the cost R2 assigns and advertises to R1. When R1 receives the update, it makes a calculation on total cost. The path taken to the remote network
131.108.5.32 is through Ethernet 0/0. Find out the cost associated with R1 Ethernet 0/0 by using the show ip ospf interface ethernet 0/0 command as displayed in Example 3-7.
Example 3-7 show ip ospf interface ethernet 0/0 on R1
R1#show ip ospf interface ethernet 0/0 Ethernet0/0 is up, line protocol is up Internet Address 131.108.1.1/24, Area 1
Process ID 1, Router ID 131.108.5.1, Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 131.108.5.1, Interface address 131.108.1.1 Backup Designated router (ID) 131.108.1.2, Interface address 131.108.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:06
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 131.108.1.2 (Backup Designated Router) Suppress hello for 0 neighbor(s)
R1#
The cost associated with the path on the Ethernet segment is 10. Therefore, the total cost is 1000 (as advertised by R2) plus 10, which equals 1010. Another method you can use to determine the cost with an Ethernet segment is to use the cost
calculation, cost = 108 / Bandwidth = 108 / 107 = 10. Example 3-8 displays the full routing configuration on R1.
Example 3-8 R1 Full Configuration
version 12.0
!
hostname R1
!
enable password cisco
!
no ip domain-lookup interface Loopback0
ip address 131.108.4.1 255.255.255.128
!
interface Loopback1
ip address 131.108.4.129 255.255.255.128
!
interface Loopback2
ip address 131.108.5.1 255.255.255.224
!
interface Ethernet0/0
ip address 131.108.1.1 255.255.255.0
!
interface Serial0/0 shutdown
!
interface Serial0/1 shutdown
router ospf 1
network 131.108.1.0 0.0.0.255 area 1 network 131.108.4.0 0.0.0.127 area 1 network 131.108.4.128 0.0.0.127 area 1 network 131.108.5.0 0.0.0.31 area 1
!
line con 0 line aux 0 line vty 0 4
! end
Example 3-9 displays the full routing configuration on R2.
Example 3-9 R2 Full Configuration
version 12.0
!
hostname R2
!
enable password cisco
!
no ip domain-lookup
!
interface Loopback0
ip address 131.108.5.33 255.255.255.224 ip ospf network point-to-point
ip ospf cost 1000
!
interface Loopback1
ip address 131.108.6.1 255.255.255.255
!
interface Loopback2
ip address 131.108.6.2 255.255.255.255
!
interface Ethernet0/0
ip address 131.108.1.2 255.255.255.0
!
interface Serial1/0 shutdown
!
interface Serial1/1 shutdown
!
interface Serial1/2
shutdown
!
interface Serial1/3 shutdown
!
router ospf 2
network 131.108.1.0 0.0.0.255 area 1 network 131.108.5.32 0.0.0.31 area 1 network 131.108.6.1 0.0.0.0 area 1 network 131.108.6.2 0.0.0.0 area 1
!
line con 0 line aux 0 line vty 0 4 end
Now, apply the OSPF principles to a larger, more complex network in Scenario 3-2.