OSPF Default Route Advertisement [Configuration & Example]

I- Default route in OSPF Regular area

Topology:

  • via “default-information originate”:

In order to advertise a default route via “default-information originate“, the default route 0.0.0.0/0 must exist in the routing table.

R2(config)#router ospf 1
R2(config-router)#default-information originate


The command default-information originate will generate an LSA-5 inside the OSPF regular area:


If you need to advertise default route into OSPF Regular area even when you don’t have a default route in the routing table of the advertising router, an option “Always”:

R2(config-router)#default-information originate always


  • via route redistribution:

The router R2 has learned a default route via OSPF 2 (from R3) and we will redistribute it to OSPF 1, this will generate LSA-5 default route to R1 :

R2#sh run | sec ospf

router ospf 1
 redistribute ospf 2 subnets

router ospf 2


It’s obvious but also worth to mention that if no default route is learned from OSPF 2, the redistribution of OSPF 2 into OSPF 1 won’t result an advertisement of a default route.

In other word, 0.0.0.0 should exist in routing table (from the redistributed protocol).


II- Default route in OSPF NSSA area

  • via “default-information originate”:

– From NSSA ASBR:

R3#show ip route
S*   0.0.0.0/0 [1/0] via 172.16.34.2

R3#
router ospf 2
 area 10 nssa default-information-originate

This configuration generates a type 7 LSA default route. You can configure this command on any NSSA ASBR with these rules:

  • NSSA ASBR can generate a default only when it has a default route in its routing table.
  • The default route must be known through non-OSPF protocol.


– From NSSA ABR:

R2(config)#router ospf 1
R2(config-router)#area 10 nssa default-information-originate

Note: This configuration generates a type 7 default route. NSSA ABR can generate a default route with or without a default route in its own routing table.


  • via redistribution:
R2#sh run | sec ospf

router ospf 2
 redistribute ospf 10 subnets

router ospf 10

A default should be learned from OSPF 10 (via R4).


III- Default route in OSPF Totally NSSA area

R2(config)#router ospf 1
R2(config-router)#area 10 nssa no-summary

This configuration will generate an LSA-3 default route from the ABR:

R3#show ip route

O*IA 0.0.0.0/0 [110/11] via 172.16.23.1, 00:00:08, Ethernet0/0

If the ABR is also an ASBR, the Type-7 LSA can also be suppressed by using “no-redistribution”


IV- Default route in OSPF STUB area

R2(config)#router ospf 1
R2(config-router)#area 10 stub

This configuration will generate LSA-3 default route from the ABR to the STUB Area.

STUB area allow LSA-3 and LSA-3 default route.


V- Default route in OSPF Totally STUB area

R2(config)#router ospf 1
R2(config-router)#area 10 stub no-summary

This configuration will generate LSA-3 default route from the ABR to the totally STUB Area.

Totally STUB area allow only LSA-3 default route.

Bilel A

Bilel A

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