OSPF Path Selection explained

As you might have learned in CCNA or CCNP, OSPF will use cost as the metric to choose the shortest path for each destination, this is true but it’s not entirely correct. OSPF will first look at the “type of path” to make a decision and secondly look at the metric. This is the prefered path list that OSPF uses:

• Intra-Area (O)
• Inter-Area (O IA)
• External Type 1 (E1)
• NSSA Type 1 (N1)
• External Type 2 (E2)
• NSSA Type 2 (N2)

After the path selection it will look at the lowest cost path. To give a quick example…when Prefix “X” is learned as an intra-area route (O) and as an inter-area route (O IA) then OSPF will always select the intra-area route, even if the inter-area route has a lower cost.

I will demonstrate this behavior to you using the following topology:

We will create a loopback0 interface on R2 – R7 using the same prefix 1.1.1.1/32 and advertise it in OSPF as following:

• R2: Intra-Area (O)
• R3: Inter-Area (O IA)
• R4: External Type 1 (E1)
• R5: External Type 2 (E2)
• R6: NSSA Type 1 (N1)
• R7: NSSA Type 2 (N2)

We will check R1 to see what path it will prefer, let’s configure OSPF first:

R1(config)#router ospf 1
R1(config-router)#router-id 11.11.11.11
R1(config-router)#network 192.168.12.0 0.0.0.255 area 0
R1(config-router)#network 192.168.13.0 0.0.0.255 area 0
R1(config-router)#network 192.168.14.0 0.0.0.255 area 0
R1(config-router)#network 192.168.15.0 0.0.0.255 area 0
R1(config-router)#network 192.168.16.0 0.0.0.255 area 167
R1(config-router)#network 192.168.17.0 0.0.0.255 area 167
R1(config-router)#area 167 nssa

First we’ll advertise the correct areas on R1. Don’t forget to make area 167 the NSSA area. Let’s continue with the other routers:

R2(config)#router ospf 1
R2(config-router)#router-id 22.22.22.22
R2(config-router)#network 192.168.12.0 0.0.0.255 area 0
R2(config-router)#network 1.1.1.1 0.0.0.0 area 0

On R2 we will advertise 1.1.1.1/32 as an intra-area route.

R3(config)#router ospf 1
R3(config-router)#router-id 33.33.33.33
R3(config-router)#network 192.168.13.0 0.0.0.255 area 0
R3(config-router)#network 1.1.1.1 0.0.0.0 area 3

R3 will advertise 1.1.1.1/32 in area 3 to make it an inter-area route.

R4(config)#router ospf 1
R4(config-router)#router-id 44.44.44.44
R4(config-router)#network 192.168.14.0 0.0.0.255 area 0       
R4(config-router)#redistribute connected subnets metric-type 1

R4 will redistribute prefix 1.1.1.1/32 as an external type 1 route.

R5(config-if)#router ospf 1
R5(config-router)#router-id 55.55.55.55
R5(config-router)#network 192.168.15.0 0.0.0.255 area 0
R5(config-router)#redistribute connected subnets metric-type 2

R5 will redistribute prefix 1.1.1.1/32 as an external type 2 route.

R6(config)#router ospf 1
R6(config-router)#router-id 66.66.66.66
R6(config-router)#network 192.168.16.0 0.0.0.255 area 167       
R6(config-router)#redistribute connected subnets metric-type 1
R6(config-router)#area 167 nssa 

R6 is a NSSA ABR and will advertise 1.1.1.1/32 as a N1 route.

R7(config)#router ospf 1
R7(config-router)#router-id 77.77.77.77
R7(config-router)#network 192.168.17.0 0.0.0.255 area 167
R7(config-router)#redistribute connected subnets metric-type 2
R7(config-router)#area 167 nssa

Last but not least, R7 will redistribute 1.1.1.1/32 so it shows up as a N2 route.

Let’s verify our configuration:

R1#show ip ospf neighbor 

Neighbor ID     Pri   State           Dead Time   Address         Interface
55.55.55.55       1   FULL/BDR        00:00:38    192.168.15.5    FastEthernet0/3
44.44.44.44       1   FULL/BDR        00:00:38    192.168.14.4    FastEthernet0/2
33.33.33.33       1   FULL/BDR        00:00:38    192.168.13.3    FastEthernet0/1
22.22.22.22       1   FULL/BDR        00:00:37    192.168.12.2    FastEthernet0/0
77.77.77.77       1   FULL/BDR        00:00:30    192.168.17.7    FastEthernet0/5
66.66.66.66       1   FULL/BDR        00:00:39    192.168.16.6    FastEthernet0/4

All OSPF neighbor adjacencies are working. Let’s take a look at the routing table to see what path OSPF has decided to use: