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OSPF External Routes: N1 & N2 Routes

March 27, 2024 , , , ,

 OSPF External Routes: N1 & N2 Routes

Generally if an OSPF area is connected to the external, it receives OE2 routes as external routes on ASBR router. OE2 (External Type 2) routes maintain the original cost they had in the other routing protocol or static configuration. OSPF doesn't modify the cost during advertisement.

But if the OSPF area which is a NSSA area and connected to External and LSA type 5 is not allowed which means OE2 is not allowed in that area, so how can NSSA area and other areas beyond it learn about the external routes ? 

⭐Related : OSPF External Routes: E1 Vs E2 Routes

Here comes the ON2 routes which is an NSSA Type 2 and is similar to E2, N2 routes retain the original cost from the external source. No additional cost is added within the NSSA area.

Lets check the scenario here to understand more in depth, you will see two images where top one shows in case of normal area and below one is the NSSA area, you will see the type of routes injected into the OSPF areas.

⭐Related : Top OSPF Protocol Interview Questions and Answers
⭐Related : OSPF protocol : OSPF Packet Types

OSPF External Routes: N1 & N2 Routes
Fig 1.1- OSPF External Routes: N1 & N2 Routes

So in case of NSSA area, Router R2 received external routes as a ON2 route and pushed that to the Area 2 which is a NSSA area, once it reaches to Router R3 ( ABR) it converts back to OE2 external route and pushed to area 0 and further.

As many of you aware that Not-So-Stubby Areas (NSSA) are a special type of OSPF area designed to simplify routing and prevent routing loops. They achieve this by suppressing the advertisement of certain Link-State Advertisements (LSAs) within the area.

So now you understand that the ON1 and ON2 routes are specific to NSSA areas and function similarly to OE1 and OE2 routes

⭐Related : Introduction to OSPF LSA Types
⭐Related : Introduction to OSPF Area Types

Here is the configuration on Router R2 for NSSA Area, 

R2> enable
R2# config t
R2(config)# router ospf 1
R2(config-router)# area 2 nssa
R2(config-router)# redistribute eigrp 100 subnets metric 10
R2(config-router)# area 2 nssa default-information-originate
R2(config-router)# exit
R2#
Router R2 which is an ASBR router connected to the External EIGRP as well has above OSPF configuration

Check what routes you are receiving on router R2

R2#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
O IA    10.10.10.0/24 [110/2] via 10.3.0.1, 00:19:27, GigabitEthernet0/1
C       10.10.20.0/24 is directly connected, GigabitEthernet0/1
O N2    20.20.20.5/32 [110/20] via 10.20.10.2, 00:19:42, GigabitEthernet0/0
Here you see the external routes are learned as ON2 routes in an NSSA  OSPF Area. Further if you check the specific route on the router R3, R4 & R5, you will see the external route will be learned as OE2.

So now if we are going to redistribute the routes with the metric type 1 as shown below

R2> enable
R2# config t
R2(config)# router ospf 1
R2(config-router)# area 2 nssa
R2(config-router)# redistribute eigrp 100  subnets metric 10 metric-type 1
R2(config-router)# area 2 nssa default-information-originate
R2(config-router)# exit
R2#
Check what routes you are receiving on router R2 now 

R2#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
O IA    10.10.10.0/24 [110/2] via 10.3.0.1, 00:19:27, GigabitEthernet0/1
C       10.10.20.0/24 is directly connected, GigabitEthernet0/1
O N1   20.20.20.5/32 [110/20] via 10.20.10.2, 00:10:12, GigabitEthernet0/0
Further if you check the specific route on the router R3, R4 & R5, you will see the external route will be learned as OE1.

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