OSPF Stub Areas

In a multi-area OSPF network, stub areas are basically used to provide scalability.   OSPF scalability is achieved by reducing the flooding domain using areas and reducing the size of the routing table doing summarization.

Now, summarization and filtering are functions of the ABR.  This is due the hierarchical design of OSPF.   A way to reduce the routing information in an area is converting the area into Stub.

When configuring an area as Stub, the ABR will filter whatever LSA (Type-3, Type-4 and/or Type-5) which enter the area depending on the type of Stub Area that is configured.    The filtered LSAs in Stub areas are then replaced with a Type-3 default route (LSA Summarization).  Another consideration when configuring stub areas is that all routers within the stub area must be also configured as stub. Otherwise, they won’t form the adjacency.

OSPF Stub Area Types

There are four stub area types:

  • Stub Area – Filters Type-5 LSAs.
  • Totally Stubby Area – Filters Type-3 and Type-5 LSAs. These LSAs are summarized into a single Type-3 LSA which is the default route.
  • Not-So-Stubby Area (NSSA) – Filter Type-5 LSAs. Allows local redistribution in the area. The router doing redistribution in the NSSA area generates LSA Type-7 for the redistributed prefixes. The ABR translates the Type-7 LSA into Type-5 LSA when advertised to the backbone Area.  In NSSA, the ABR does not automatically originate a default route.
  • Totally-Not-So-Stubby-Area – Like Totally Stubby Area, filters Type-3, Type-4, and Type-5 LSAs and also allows local redistribution. It is, in essence, a combination of the Totally Stubby Area and NSSA.  It also generates Type-7 LSAs for the redistributed prefixes, the ABR translate this Type-7 into Type-5 LSAs when entering the backbone area. In Totally-Not-So-Stubby-Area, the ABR automatically originates the default route.

Today we will focus in the first two Stub Area Types.

To configure an area as Stub, use the area {area-id} stub [no-summary] OSPF process command.   The stub area must be defined in all routers participating in the stubby area.  The no-summary keyword is used to change from stub area to Totally Stubby Area.

Let’s use the following topology for the examples:

OSPF-STUB-01

As can be seen in the topology, the area 234 is defined as a stub area.  R1 and R5 are doing mutual redistribution between RIP and OSPF.

The initial configuration files are as follows:

R1:

!
interface Loopback0
ip address 1.1.1.1 255.255.255.255
ip ospf 1 area 0
!
interface Ethernet0/0
ip address 192.168.12.1 255.255.255.0
ip ospf 1 area 0
!
interface Ethernet0/1
ip address 192.168.15.1 255.255.255.0
!
router ospf 1
router-id 0.0.0.1
redistribute rip subnets
!
router rip
redistribute ospf 1 metric 6
network 192.168.15.0
no auto-summary
!

R2:

!
interface Loopback0
ip address 2.2.2.2 255.255.255.255
ip ospf 1 area 0
!
interface Ethernet0/0
ip address 192.168.12.2 255.255.255.0
ip ospf 1 area 0
!
router ospf 1
router-id 0.0.0.2
!

R3:

!
interface Loopback0
ip address 3.3.3.3 255.255.255.255
ip ospf 1 area 234
!
interface Ethernet0/0
ip address 192.168.34.3 255.255.255.0
ip ospf 1 area 234
!
router ospf 1
router-id 0.0.0.3
!

R4:

!
interface Loopback0
ip address 4.4.4.4 255.255.255.255
ip ospf 1 area 234
!
interface Ethernet0/0
ip address 192.168.34.4 255.255.255.0
ip ospf 1 area 234
!
router ospf 1
router-id 0.0.0.4
!

R5:

!
interface Loopback0
ip address 5.5.5.5 255.255.255.255
!
interface Ethernet0/1
ip address 192.168.15.5 255.255.255.0
!
router rip
network 5.0.0.0
network 192.168.15.0
no auto-summary
!

At this moment area 234 is a normal area.

Let’s take a look at the routing tables in R5 and R3 and test the reachability between them.

OSPF-STUB-02

OSPF-STUB-03

As can be seen in the above outputs, R5 is learning the redistributed OSPF routes from R1 with a metric of 6.   When doing redistribution between OSPF and RIP the seed metric is required.   RIP has a max-metric of 15.   Also, note R3 is learning redistributed routes from R5.   These routes are being advertised as “O E2” or OSPF External type 2 with a default metric of 20.

Now let’s take a look at the link-state database in R3:

OSPF-STUB-04

As can be seen in the above link-state database output, R3 has Type-1, Type-2, Type-3, Type-4 and Type-5 LSAs.   The Type-3 Summary information is coming from the ABR, detailing the summary information of the networks in Area 0. The Type-5 External LSAs details the redistributed routes, they were generated by R1, the Type-4 LSA details how to reach the ASBR (R1 – RID: 0.0.0.1) from where the Type-5 LSAs were learned.   Also, note the Type-5 External don’t have an explicit area (Area X) attached to the description in the database.   This is because the flooding scope of External Type-5 LSAs is the entire OSPF domain.

Let’s configure the area 234 as stub and let’s see what happens:

R2:

!
router ospf 1
area 234 stub
!

Let’s stop here for a moment and take a look on the adjacency between R2 and R4:

OSPF-STUB-05

As soon we changed the area 234 to stub, the adjacency with R4 went down.   What is the problem?   Let’s debug the hello packets and see what’s going on.

OSPF-STUB-06

According to the above output, R2 has received a hello packet with mismatched Stub/Transit area option bit.   When configuring stub areas, all participating routers must be configured as stub.   Thus, changing from normal to stub areas will tear down your adjacencies.

Let’s continue with the configuration.

R3:

!
router ospf 1
area 234 stub
!

R4:

!
router ospf 1
area 234 stub
!

Let’s take a look to the routing table and the link-state database on R3:

OSPF-STUB-07

OSPF-STUB-08

As can be seen in the above output, the routing table has changed.   Now the “O E2” OSPF external routes have disappeared.   Now the routing table has an Inter-Area default route (O*IA).

Also, note the “O IA” OSPF inter-area routes corresponding to the prefixes in area 0 are still present.

Now, let’s take a look at the link-state database:

OSPF-STUB-09

As can be seen in the above output, the Type-5 LSAs were filtered, thus, the Type-4 LSA was not generated by the ABR.

Totally Stubby Area

The Totally Stubby Area what it does is increase the filtering even more.  This type of areas filters inter-area (O IA) and external routes (O E1|E2).   To configure the area as totally Stub just adds the keyword no-summary at the end of the stub area configuration.  Because the Totally Stubby Area is an extension of stub areas, the no-summary keyword is only required in the ABR connecting the stub area.

Considering that the link between R2 and R4 is the only transit link between the backbone and the area 234, it would make sense to convert it into a totally stubby area.

So let’s make the change:

R2:

!
router ospf 1
area 234 stub no-summary
!

Finally, let’s take a look to the routing table and the link-state database:

OSPF-STUB-10

OSPF-STUB-11

As can be seen in the above outputs, now the inter-area (O IA) and the External (O E1|E2) routes were replaced with a single Inter-Area default route (O*IA).  The Type-3, Type-4 and Type-5 LSAs were filtered from the link-state database.   The full topology is still reachable because the inter-area and external routes fall into the default route.

OSPF-STUB-12

Despite the Stub and Totally Stub Areas seems attractive because of their low overhead, they also have important limitations of which I want to emphasize:

  1. Stub and Totally Stub areas do not support redistribution.
  2. Stub areas, in general, do not support Virtual-Links.

It is time to close this long post.

Thank you for visiting.

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