It is time to take a look to configuring OSPF ( Open shortest path first ) with an OSPF configuration example. In this article we will see “how to configure OSPF” step by step. In our example, we will examine the figure below deeply.
1 Configure OSPF for the network above. Router 1 should be a ASBR which redistributes the static routes. These routes shouldn’t increase their metrics while they are passing over the network and should have a beginning OSPF cost of 200. All routers should have a router – id that is similar to their names and all router- id’s should be pinged from all over the OSPF network.
2 After configuring first step, which router is DR and which router is BDR in Area 0 ?
3 Define Router 1 DR in Area 0 and Router 2 and Router 3 is not a BDR .rea 0’da R1’i DR yapın. After this configuration, which kind of neighborship established between Router 1 – Router 2 and Router 2 – Router 3 ?
4 Configure summarization on ABR routers.
5 Configure summarization on ASBR routers. Summary route should have the same properties as its original and should redistribute the individual routes.
6 Which kind of configuration changes are made if you update the links as gigabitethernet in this network ?
OSPF CONFIGURATION EXAMPLE
R1(conf)#router ospf 1 R1(conf-router) # network 172.30.0.1 0.0.0.0 area 0 (A) R1(conf-router)# router-id 1.1.1.1
Router – id value should be configured first when we configure OSPF . If we configure router-id after neighborship established, router-id is determined as “172.30.0.1” and this value can be changed by resetting all OSPF process” . OSPF configuration is not so hard as you can see
R1(conf)#redistribute static subnets metric 200 metric-type 2
OSPF external routes are advertised inside as E1 or E2 type routes. E1 routes increase their metrics while E1 routes metrics are constant. So in the example metric-type 2 was used.
R2(conf)#router ospf 1 R2(config-router)#network 172.30.0.2 0.0.0.0 area 0
Router – id value is set to 172.30.0.2 and neighborship is established if we configure like this.
R2(config-router)#router-id 2.2.2.2 (B)
We can set the router-id to 2.2.2.2 after resetting all OSPF process by using the command “clear ip ospf process” . Or we should configure the router-id first as i mentioned before.
R2(config-router)#network 172.30.10.2 0.0.0.0 area 10
After (A) and (B) steps :
R1#show ip route S 172.16.0.0 directly connected null 0 S 172.16.1.0 directly connected null 0 S 172.16.2.0 directly connected null 0 S 172.16.3.0 directly connected null 0 C 172.30.0.0 is directly connected f 0/0 O IA 172.30.10.0 via 172.30.0.2 // IA is interarea route, which means the route learnt from another area. //
R2#show ip route O E2 172.16.0.0 [110/200] via 172.30.0.1 // Type E2 route with cost of 200 O E2 172.16.1.0 via 172.30.0.1 O E2 172.16.2.0 via 172.30.0.1 O E2 172.16.3.0 via 172.30.0.1 C 172.30.0.0 directly connected C 172.30.10.0. directly connected
R3(config)#router ospf 1 R3(config-router)#router-id 3.3.3.3 R3(config-router)#network 172.30.0.3 0.0.0.0 area 0 R3(config-router)#network 172.30.20.3 0.0.0.0 area 2
R4(config)# router ospf 1 R4(config-router)#router-id 4.4.4.4 (C) R4(config-router)#network 172.30.20.4 0.0.0.0 area 20 R4(config-router)#network 10.0.0.0 0.255.255.255 area 20 R5(config)#router ospf 1 R5(config-router)#router-id 5.5.5.5 R5(config-router)#network 10.10.0.0 0.0.255.255 area 10 R5(config-router)#network 172.30.10.5 0.0.0.0 area 10
After (A) (B) and (C) configuration steps ;
R5#show ip route
…
O IA 10.20.3.1/32 via 172.30.10.2
10.20.2.1/32 ….
10.20.1.1/32 ….
10.20.0.1/32
Even if loopbacks of router 4 is configured as 10.20.3.1 255.255.255.0 , OSPF sets the mask to /32 because of this is a loopback interface. If we want to see as /24 in the routing table, we should make these config too :
R4(config)# interface loopback 1 R4(config-if)#ip ospf network point-to-point
This is not a well-known trick of configuring OSPF.
R1#show ip ospf interface (*)state is seen as DR.
Router 1 is DR , Router 2 is BDR in this situation because we firstly configured Router 1 , then we configured Router 2. If we run the “R1(config)# clear ip ospf process” command then we can see the state as DR OTHER which means it is not a DR or BDR
R1(config)#int f 0/0 R1(config-router)#ip ospf priority 200 R2(config)#int f 0/0 R2(config-router)#ip ospf priority 0 R3(config)#int f 0/0 R3(config-router)#ip ospf priority 0
If we don’t want to make a router become a DR or a BDR , we change the priority value of that router’s interface to 0. Priority value is configured under the related interface. This is a very important point for this OSPF configuration example.
R1 # sh ip ospf neigh 2.2.2.2 FULL/DROTHER 3.3.3.3 FULL/DROTHER
R2 # sh ip ospf neigh 1.1.1.1 FULL/DR 3.3.3.3 2WAY/DROTHER 5.5.5.5 FULL/- // No DR or BDR selection is made on the point-to point links. //
R2(config)# router ospf 1
R2(config-router)#area 10 range 10.10.0.0 255.255.252.0
After this configuration:
R1#sh ip route 10.10.0.0/22 via 172.30.0.2
R3(config)# router ospf 1 R3(config-router)#area 20 range 10.20.0.0 255.255.252.0 // Auto summarization on ABR routers are made with "range" commands as you can see above. //
R1(config) # router ospf 1 R1(config-router)#summary-adress 172.16.0.0 255.255.252.0
R2(config)# auto-cost reference-bandwidth 1000
Auto summarization on ASBR routers are made with “summary-address” command as you can see above.
Same config should be made for R2# R3# R4# R5# too..
In this article we made an OSPF configuration example for a better understanding of OSPF protocol.