On the previous article I’ve discuss about OSPF Cost Concept. and in this article I’ve discuss about OSPF Cost Manipulation config on Cisco. Here’s the topology :

First, make sure the ospf already running on your lab. in this article I’m use the EVE-NG and all of routers use 10Mb/s interface speed.

Configure IP Address on R1 :

R1(config)#int fa0/0
R1(config-if)#ip add 12.12.12.1 255.255.255.0
R1(config-if)#no shut
R1(config-if)#exit
R1(config)#int fa0/1
R1(config-if)#ip add 13.13.13.1 255.255.255.0
R1(config-if)#no shut
R1(config-if)#exit
R1(config)#int lo1
R1(config-if)#ip add 192.168.10.1 255.255.255.0
R1(config-if)#exit
R1(config)#

Configure IP Address on R2:

R2(config)#int fa0/0
R2(config-if)#ip add 12.12.12.2 255.255.255.0
R2(config-if)#no shut
R2(config-if)#exit
R2(config)#int fa0/1
R2(config-if)#ip add 23.23.23.1 255.255.255.0
R2(config-if)#no shut
R2(config-if)#exit
R2(config)#

Configure IP Address on R3 :

R3(config)#int fa0/0
R3(config-if)#ip add 13.13.13.2 255.255.255.0
R3(config-if)#no shut
R3(config-if)#exit
R3(config)#int fa0/1
R3(config-if)#ip add 23.2.23.2 255.255.255.0
R3(config-if)#no shut
R3(config-if)#exit
R3(config)#int fa1/0
R3(config-if)#ip add 192.168.100.1 255.255.255.0
R3(config-if)#no shut
R3(config-if)#exit
R3(config)#

Configure PC that connect to R3 :

VPCS> show ip

NAME : VPCS[1]
IP/MASK : 192.168.100.2/24
GATEWAY : 192.168.100.1
DNS :
MAC : 00:50:79:66:68:04
LPORT : 20000
RHOST:PORT : 127.0.0.1:30000
MTU : 1500

VPCS>

Now configure OSPF on all routers.

Configure OSPF on R1 :

R1(config)#router ospf 1
R1(config-router)#router
R1(config-router)#router-id 1.1.1.1
R1(config-router)#network 12.12.12.0 0.0.0.255 area 0
R1(config-router)#network 13.13.13.0 0.0.0.255 area 0
R1(config-router)#network 192.168.10.0 0.0.0.255 area 0
R1(config-router)#exit

Configure OSPF on R2 :

R2(config)#router ospf 1
R2(config-router)#router
R2(config-router)#router-id 2.2.2.2
R2(config-router)#network 12.12.12.0 0.0.0.255 area 0
R2(config-router)#network 23.23.23.0 0.0.0.255 area 0
R2(config-router)#exit

Configure OSPF on R3 :

R3(config)#router ospf 1
R3(config-router)#router-id 3.3.3.3
R3(config-router)#network 23.23.23.0 0.0.0.255 area 0
R3(config-router)#network 13.13.13.0 0.0.0.255 area 0
R3(config-router)#network 192.168.100.0 0.0.0.255 area 0
R3(config-router)#exit
R3(config)#

After that, show ospf routing table on R1 :

R1#show ip route ospf
      23.0.0.0/24 is subnetted, 1 subnets
O         23.23.23.0 [110/20] via 13.13.13.2, 00:14:29, FastEthernet0/1
                     [110/20] via 12.12.12.2, 00:15:51, FastEthernet0/0
O     192.168.100.0/24 [110/11] via 13.13.13.2, 00:14:11, FastEthernet0/1
R1#

Look at the routing table R1, route through 192.168.100.0 use 13.13.13.2 (R3) as gateway.

Because the interface router is 10Mb/s, definitely the cost value is 10.

• R1 -> R3 : costvalue + metric (10+1) = 11
• R1 -> R2 -> R3 : 10+10+1 = 21

Based on ospf cost calculation, router will select the R3 as a best path through 192.168.100.0 network.

Now, how if we need to change the best path or do a loadbalance routing so both path will be used by R1 ?  based on cost calculation above, we need to increase the cost value through R2 as the following example we need to equal the cost value so route will be loadbalance.

Increate cost value on interface fa0/1 :

R1(config)#int fa0/1
R1(config-if)#ip osp
R1(config-if)#ip ospf co
R1(config-if)#ip ospf cost 20
R1(config-if)#end

See the routing table on R1 :

R1#show ip route ospf
      23.0.0.0/24 is subnetted, 1 subnets
O        23.23.23.0 [110/20] via 12.12.12.2, 00:26:30, FastEthernet0/0
O        192.168.100.0/24 [110/21] via 13.13.13.2, 00:00:01, FastEthernet0/1
                          [110/21] via 12.12.12.2, 00:00:01, FastEthernet0/0
R1#

Look at the routing table above, routing through 192.168.100.0 network already loadbalance with the cost calculation bellow :

• R1 -> R3 : 20+1 = 21
• R1 -> R2 -> R3 : 10+10+1 = 21