This is the first post in a series about OSPF Area Types. Today we’ll go over Stub areas. This one will be somewhat short on config, but should have a good amount of show commands.

Quick refresher, OSPF Stub Areas allow inter- and intra-area routes (Type 2 and Type 3 LSAs). External routes (Type 5 LSAs) are not allowed in stub areas.
(For more detailed information on LSAs and Area Types, check out this post.)

We’ll be using the same topology we used for OSPF Authentication:

I’m not going through the basic OSPF config, so assume everything is configured as the diagram suggests. I’ve also redistributed loopbacks on each router to give us some external routes, and I added 34.34.34.34/32 to Area 34 so we have an intra-area route to look at. Let’s look at some show commands BEFORE we make area 34 a stub:

First we’ll check out “sh ip route ospf” on R4:

R4#sh ip route ospf
     34.0.0.0/32 is subnetted, 1 subnets
O       34.34.34.34 [110/65] via 10.1.34.3, 00:01:17, Serial0/0
     1.0.0.0/32 is subnetted, 1 subnets
O E2    1.1.1.1 [110/20] via 10.1.34.3, 00:01:17, Serial0/0
     2.0.0.0/32 is subnetted, 1 subnets
O E2    2.2.2.2 [110/20] via 10.1.34.3, 00:01:17, Serial0/0
     3.0.0.0/32 is subnetted, 1 subnets
O E2    3.3.3.3 [110/20] via 10.1.34.3, 00:01:17, Serial0/0
     10.0.0.0/24 is subnetted, 2 subnets
O IA    10.1.123.0 [110/74] via 10.1.34.3, 00:01:17, Serial0/0

As expected, we see everything. 34.34.34.34/32 has come through as an intra-area route (O – LSA 2). We see our loopbacks from each router come through as external (O E2 – LSA 5, something to note is E2 routes do not increment cost as they traverse the network, so we see a cost of 20, which will be the same throughout the OSPF domain). Last we see 10.1.123.0/24 as an inter-area route (O IA – LSA 3).

Now let’s check out the OSPF Database on R4:

R4#sh ip ospf d
            OSPF Router with ID (4.4.4.4) (Process ID 100)
 
                Router Link States (Area 34)
Link ID         ADV Router      Age         Seq#       Checksum Link count
3.3.3.3         3.3.3.3         3           0x8000000C 0x00EF87 3
4.4.4.4         4.4.4.4         2           0x8000000D 0x00ABEB 1
 
                Summary Net Link States (Area 34)
Link ID         ADV Router      Age         Seq#       Checksum
10.1.123.0      3.3.3.3         113         0x80000003 0x00B2EB
 
                Summary ASB Link States (Area 34)
Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         3.3.3.3         119         0x80000001 0x0057CA
2.2.2.2         3.3.3.3         119         0x80000001 0x0029F4
 
                Type-5 AS External Link States
Link ID         ADV Router      Age         Seq#       Checksum Tag
1.1.1.1         1.1.1.1         1293        0x80000001 0x009BFC 0
2.2.2.2         2.2.2.2         1303        0x80000001 0x004F41 0
3.3.3.3         3.3.3.3         119         0x80000004 0x00FC88 0
4.4.4.4         4.4.4.4         3           0x80000004 0x00B0CC 0

Lots of output, but nothing crazy. We see our LSAs for area 34, and our redistributed loopbacks as external LSAs.

Now we’ll configure area 34 as a stub:

R3(config)#router ospf 100
R3(config-router)#area 34 stub
*Mar  1 00:13:39.675: %OSPF-5-ADJCHG: Process 100, Nbr 4.4.4.4 on Serial0/0 from FULL to DOWN, Neighbor Down: Adjacency forced to reset
 
R4(config)#router ospf 100
R4(config-router)#area 34 stub
*Mar  1 00:03:25.923: %OSPF-5-ADJCHG: Process 100, Nbr 3.3.3.3 on Serial0/0 from LOADING to FULL, Loading Done

Simple configuration, we configured area 34 as a stub under the OSPF process. Notice that the neighbors go down and reform once they match.

Let’s examine the new RIB on R4:

R4#sh ip route ospf
     34.0.0.0/32 is subnetted, 1 subnets
O       34.34.34.34 [110/65] via 10.1.34.3, 00:01:33, Serial0/0
     10.0.0.0/24 is subnetted, 2 subnets
O IA    10.1.123.0 [110/74] via 10.1.34.3, 00:01:33, Serial0/0
O*IA 0.0.0.0/0 [110/65] via 10.1.34.3, 00:01:33, Serial0/0

Here we see that all the external routes are gone, but intra- and inter-area routes are still in the table. The inter-area route to 10.1.123.0/24 is still there, and we also have a default route, which is showing as an inter-area route as well.

Finally we’ll look at the OSPF Database:

R4#sh ip ospf d
            OSPF Router with ID (4.4.4.4) (Process ID 100)
 
                Router Link States (Area 34)
Link ID         ADV Router      Age         Seq#       Checksum Link count
3.3.3.3         3.3.3.3         259         0x8000000E 0x000475 3
4.4.4.4         4.4.4.4         258         0x80000010 0x008983 2
 
                Summary Net Link States (Area 34)
Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         3.3.3.3         308         0x80000001 0x0057DA
10.1.123.0      3.3.3.3         308         0x80000004 0x00CED0

It is much smaller now. We see the router LSAs and only two inter-area LSAs, the default route and the route to 10.1.123.0/24.

I was planning on putting a debug in here as well, but I didn’t really get anything interesting enough to add.

OSPF Stub Areas are relatively simple, but can be confusing when first digging into OSPF. The key concepts are simply that LSA Type 5s are not allowed in stub areas, and also that a default route is generated by the ABR. I very much expect to see stub areas of some sort on the CCIE lab.

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