This is the first post in a series about OSPF Area Types. Today we’ll go over Totally Stubby areas. We’ll be using the same topology as the Stub post. I’m also reposting the first portion of that here since it will be the same.

Quick refresher, OSPF Totally Stubby Areas allow only intra-area routes and a default route generated by the ABR (Type 2 LSAs – the default route comes through as a Type 3 LSA, but no other Type 3s are allowed). Inter-area and External routes (Type 5 LSAs) are not allowed in totally stubby areas.
(For more detailed information on LSAs and Area Types, check out this post.)

Here’s the topology:

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 (“redistribute connected subnets” under the OSPF process) 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 totally stubby:

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 totally stubby:

R3(config)#router ospf 100
R3(config-router)#area 34 stub no-summary
 
R4(config)#router ospf 100
R4(config-router)#area 34 stub

There isn’t much to the config at all, as we can see. The command is “area n stub no-summary”, this tells the ABR not to send Type 3s into the area. On the non-ABR(s) we simply specify the area as a stub, the “no summary” keyword is only needed on the ABR.

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, 00:02:04, Serial0/0
O*IA 0.0.0.0/0 [110/65] via 10.1.34.3, 00:02:04, Serial0/0

Very small table. Here we see that all the external routes are gone, but intra-area route to 34.34.34.34 is still in the table. Our only other OSPF route is the default generated by R3.

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         665         0x8000000B 0x000A72 3
4.4.4.4         4.4.4.4         300         0x8000000A 0x00957D 2
 
                Summary Net Link States (Area 34)
Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         3.3.3.3         1214        0x80000001 0x0057DA

It is much smaller now. We see the router LSAs and a single inter-area LSA, the default route from R3.

Totally Stubby areas are pretty basic once you understand Stub areas and LSAs in general. The key concepts are simply that LSA Type 3s and Type 5s are not allowed in totally stubby areas, and also that a default route is generated by the ABR.

Colby

Colby Glass has been in IT since 2002. He is currently a Systems Engineer (presales) with a Cisco Gold partner and holds the CCNP R/S, CCNP DC, CCDP, CCIP, JNCIA-ER.

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