Home Network Layer OSPF Version 2 OSPF Default Route Explained: Cost, Metric Type and More

OSPF Default Route Explained: Cost, Metric Type and More

Open Shortest Path First allows network engineers to inject the OSPF default route (route 0.0.0.0/0) into the routing domain in different ways, either implicitly or explicitly. The default route gets advertised by ABR implicitly into stubby areas, while it can be injected into normal only if the router has been configured manually to do so. Routers use the OSPF gateway of last resort whenever there is no matching entry in the routing table for a destination.

In this post, you learn how a router originates and injects the default route into the OSPF autonomous system, and how to configure the OSPF default route on Cisco IOS and Cisco IOS-XE.

What is OSPF Default Route?

The OSPF default route is the 0.0.0.0/0 route that is originated by OSPF nodes and advertised into normal OSPF areas; routers use it to forward a packet with a destination address that doesn’t have a matching entry in the routing table. Injecting an OSPF default route into stub, totally stubby, NSSA, and totally NSSA areas is done by ABRs automatically, and you do not need to configure the route manually. However, the default route cannot get propagated to normal areas, including the backbone area.

OSPF Default Route Injection

On Cisco IOS, advertising the default route into an OSPF normal area can be accomplished manually using a particular command, and you cannot rely on redistributing a 0.0.0.0 route that is already installed in the routing table. Moreover, the route will get propagated to all areas in the OSPF autonomous system, except stub, totally stubby, NSSA, and totally NSSA areas.

Note that throughout this post, I will be using the following network topology to demonstrate aspects related to how the OSPF default route works.

Figure 1 – Network diagram consisting of two routing domains

Here are the links to download the initial configurations applied to the routers according to the network diagram in Figure 1.

Router R1 Router R2 Router R3 Router R4
Router R5 Router R6 Router R7 Router R8

How Does the OSPF Default Route Get Originated?

In the rest of this tutorial, we focus on the OSFP default route injected into normal areas.

You can configure OSPF to advertise the 0.0.0.0/0 route using the default-information originate command only. In this case, the router creates an LSA Type 5 for the route (Example 1), and thus it becomes ASBR (Autonomous System Boundary Router) whether it is connected to area 0 or not.

The router must have a non-OSPF default route in the routing table for the command to work. Otherwise, there is a turnaround to instruct the router to flood the 0.0.0.0/0 route. Besides, the router should be at least connected to one normal OSPF area in order to be able to generate the default route.

R4# show ip ospf database external

            OSPF Router with ID (4.4.4.4) (Process ID 1)

                Type-5 AS External Link States

  LS age: 62
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 0.0.0.0 (External Network Number )
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000001
  Checksum: 0xC2DF
  Length: 36
  Network Mask: /0
        Metric Type: 2 (Larger than any link state path)
        MTID: 0 
        Metric: 1 
        Forward Address: 0.0.0.0
        External Route Tag: 1

Example 1 – LSA Type 5 for the OSPF default route injected by router R4

By default, the OSPF default route is advertised and flooded to OSPF nodes with metric type 2 and a cost of 1 (Example 2).

R8# 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
       i - IS-IS, su - IS-IS summary, 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, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is 10.0.28.2 to network 0.0.0.0

O*E2  0.0.0.0/0 [110/1] via 10.0.28.2, 00:12:26, GigabitEthernet0/2
      10.0.0.0/8 is variably subnetted, 10 subnets, 2 masks
O IA     10.0.12.0/24 [110/2] via 10.0.28.2, 00:12:26, GigabitEthernet0/2
O IA     10.0.14.0/24 [110/3] via 10.0.28.2, 00:12:26, GigabitEthernet0/2
O IA     10.0.15.0/24 [110/3] via 10.0.28.2, 00:12:26, GigabitEthernet0/2
O IA     10.0.16.0/24 [110/3] via 10.0.28.2, 00:12:26, GigabitEthernet0/2
O IA     10.0.23.0/24 [110/2] via 10.0.28.2, 00:12:26, GigabitEthernet0/2
C        10.0.28.0/24 is directly connected, GigabitEthernet0/2
L        10.0.28.8/32 is directly connected, GigabitEthernet0/2
D        10.0.67.0/24 [90/3072] via 10.0.78.7, 00:18:45, GigabitEthernet0/7
C        10.0.78.0/24 is directly connected, GigabitEthernet0/7
L        10.0.78.8/32 is directly connected, GigabitEthernet0/7

Example 2 – Displaying R8’s routing table

OSPF Default Route Advertisement and Flooding

When you instruct an OSPF-enabled router to inject the default route into the current OSPF autonomous system, the router creates an LSA Type 5 for the 0.0.0.0/0 IP prefix, and floods to all neighbors, except those in areas that don’t allow LSAs Type 5.

When the default route reaches an ABR, it gets advertised in all normal areas containing the ABR. In our network topology, all areas are connected to area 0. In this way, no matter the router we instruct to advertise the default route, the route gets flooded between all normal areas via the ABR routers.

At this point, let’s remove area 0 by placing the link between routers R1 and R2 in area 12 (Examples 3 and 4).

Router R1

R1(config)# router ospf 1
R1(config-router)# no network 10.0.12.1 0.0.0.0 area 0
R1(config-router)# network 10.0.12.1 0.0.0.0 area 12

Example 3  – Removing R1’s G0/2 interface from OSPF area 0 and placing it in area 12

Router R2

R2(config)# router ospf 1
R2(config-router)# no network 10.0.12.2 0.0.0.0 area 0
R2(config-router)# network 10.0.12.2 0.0.0.0 area 12

Example 4  – Removing R2’s G0/1 interface from OSPF area 0 and placing it in area 12

Since router R1 is no longer in the backbone area, it won’t flood the LSA Type 5, describing the default route generated by router R4, into areas 12, 15, and 16. Thus, the default route won’t get flooded to routers R2, R3, R5, R6, and R8. In other words, even LSAs Type 5 do not get associated with a specific area, they get flooded between areas via ABRs only.

Finally, when the default route reaches an internal router within a normal OSPF area, the route gets flooded to the router’s neighbors.

The 0.0.0.0/0 Prefix Cannot Be Redistributed to Inject The Default Route Into OSPF

As mentioned before, you cannot advertise the default route in OSPF by redistributing the 0.0.0.0/0 prefix added through static or dynamic routing. To illustrate this, we create a static default route on router R7 and inject it into EIGRP (Example 5).

R7(config)# ip route 0.0.0.0 0.0.0.0 null 0
R7(config)# router eigrp 100
R7(config-router)# redistribute static metric 1 1 1 1 1

Example 5 – Advertising a static default route into EIGRP

As you can see in examples 6 and 7, the 0.0.0.0/0 route has been successfully installed in routers R6 and R8’s routing tables.

R6# show ip route eigrp
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
       i - IS-IS, su - IS-IS summary, 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, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is 10.0.67.7 to network 0.0.0.0

D*EX  0.0.0.0/0 [170/2560000512] via 10.0.67.7, 00:22:12, GigabitEthernet0/7
      10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
D        10.0.78.0/24 [90/3072] via 10.0.67.7, 02:08:57, GigabitEthernet0/7

Example 6 – Displaying EIGRP routes on router R6

R8# show ip route eigrp
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
       i - IS-IS, su - IS-IS summary, 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, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is 10.0.78.7 to network 0.0.0.0

D*EX  0.0.0.0/0 [170/2560000512] via 10.0.78.7, 00:22:26, GigabitEthernet0/7
      10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
D        10.0.67.0/24 [90/3072] via 10.0.78.7, 02:08:51, GigabitEthernet0/7

Example 7 – Displaying EIGRP routes on router R8

Next, we redistribute EIGRP routes into OSPF on router R8 using the following commands.

R8(config)# router ospf 1
R8(config-router)# redistribute eigrp 100 subnets

However, the configuration did not work. Router R8 does not generate a type 5 LSA for prefix 0.0.0.0/0 and does not advertise this prefix to router R2 (example 8).

R2# show ip route ospf
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
       i - IS-IS, su - IS-IS summary, 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, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
O E2     10.0.67.0/24 [110/20] via 10.0.28.8, 00:26:48, GigabitEthernet0/8
O E2     10.0.78.0/24 [110/20] via 10.0.28.8, 00:26:48, GigabitEthernet0/8

Example 8 – Displaying OSPF routes on router R2

OSPF Default Route Configuration on Cisco IOS and IOS-XE

On Cisco IOS, you cannot advertise a default route by redistributing the 0.0.0.0/0 route configured using the ip route command or learned via a dynamic routing protocol. However, Cisco IOS and IOS-XE supply the default-information originate command, which is the only command you can use to advertise an OSPF default route.

OSPF default-information originate Command

The default-information originate command requires the router to include a default route in the routing table. Otherwise, you should use the always keyword. Moreover, the command allows you to set the metric type and cost of the default route.

Here is the full syntax of the command:

default-information originate [always] [metric metric-value] [metric-type type-value] [route-map map-name]

First of all, always, metric, metric-type, and route-map keywords are optional parameters.

When the always keyword is used, the router advertises the default route despite the fact that the OSPF node doesn’t have a default route.

R4(config)# router ospf 1
R4(config-router)# default-information originate always

The metric keyword allows you to set the cost of the default route. If you do not specify a value, the router uses the value configured with the default-metric router configuration command; otherwise, the router sets the metric of the default route to 10.

R4(config)# router ospf 1
R4(config-router)# default-information originate metric 100

The metric-type keyword allows you to set the metric type of the default route. You have two options to choose from: Type 1 external route, and Type 2 external route.

R4(config)# router ospf 1
R4(config-router)# default-information originate metric-type 1

Finally, the route-map keyword instructs the router to originate the default route if a route map’s conditions are valid.

Conditional OSPF Default Route Configuration Using The Cisco default-information originate route-map Command

The default-information originate route-map rm_name command advertises a default route if the conditions in route map rm_name are met.

In Example 9, we can figure router R8 to inject the default route in OSPF as long as the  10.0.67.0/24 prefix is in its routing table.

R8(config)# access-list 1 permit 10.0.67.0 0.0.0.255
R8(config)# 
R8(config)# route-map default_route_condition permit 10
R8(config-route-map)# match ip address 1
R8(config-route-map)# exit
R8(config)# 
R8(config)# router ospf 1
R8(config-router)# default-information originate route-map default_route_condition

Example 9 – Configuring conditional OSPF default routing on router R8

Finally, note that when the always keyword is applied, the router ignores any route map used with the default-information originate command.

OSPF Default Route Troubleshooting

The Cisco default-information originate Command is not Working or The OSPF Default Route is not Advertised

The default-information originate command requires the routing table to include an entry for the 0.0.0.0 route. Otherwise, it won’t work unless you use the always keyword. To check whether the OSPF node uses a gateway of last resort, use the show ip route command in enable mode. Additionally, if the router are connected to stub/NSSA areas only, it cannot advertise a default route.

OSPF Default Route not in The Routing Table

The default-information originate command creates an LSA Type 5 for the 0.0.0.0 prefix, which gets flooded through the OSPF routing domain. Moreover, this Cisco OSPF command does not create a default route and place it in the routing table. However, when a router receives an LSAs adverting the 0.0.0.0 IP address, it may install the OSPF default route in the routing table.

If you found that the OSPF default route is not the routing table, this may be the result of one of the following:

  • You checked the default route on the router that is originating it.
  • The routing table has a default route with a better administrative distance.
  • The default route stops getting propagated at a particular node.

Related Lessons to OSPF Default Route

Conclusion

I hope this blog post helps you learn something.
Now I’d like to turn it over to you:
What did you like about this tutorial?
Or maybe you have an excellent idea that you think I need to add.
Either way, let me know by leaving a comment below right now.

Mohamed Ouamer
Mohamed Ouamer is a computer science teacher and a self-published author. He taught networking technologies and programming for more than fifteen years. While he loves to share knowledge and write, Mohamed's best passions include spending time with his family, visiting his parents, and learning new things.
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