This is the last post in a series about OSPF Area Types. Today we’ll go over Not So Totally Stubby Areas. We’ll be using the same topology as the NSSA post, but this time we will inject a specific route (40.40.40.0/24) from the ASBR (R4) instead of a default.

Quick refresher, OSPF Not So Totally Stubby Areas have intra-area routes (Type 2 LSAs) and also external routes in the form of Type 7 LSAs, which are converted to Type 5 LSAs by the ABR. No inter-area routes (Type 3 LSAs) are permitted in a Not So Totally Stubby Area and a default route will be injected by the ABR.
(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 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. I’ve also added a static route on R4 (40.40.40.0/24) which I’m injecting into OSPF with the “redistribute static subnets” command. Let’s look at some show commands BEFORE we make area 34 a Not So Totally Stubby Area:

First we’ll check out “sh ip route ospf” on R3:

R3#sh ip route ospf
     1.0.0.0/32 is subnetted, 1 subnets
O E2    1.1.1.1 [110/20] via 10.1.123.1, 00:04:20, FastEthernet0/0
     2.0.0.0/32 is subnetted, 1 subnets
O E2    2.2.2.2 [110/20] via 10.1.123.2, 00:04:20, FastEthernet0/0
     4.0.0.0/32 is subnetted, 1 subnets
O E2    4.4.4.4 [110/20] via 10.1.34.4, 00:03:33, Serial0/0
     40.0.0.0/24 is subnetted, 1 subnets
O E2    40.40.40.0 [110/20] via 10.1.34.4, 00:02:57, Serial0/0

Here we see all the loopbacks and the static (40.40.40.0/24) come through as external type 2, which is the default.

Now let’s check out the RIB on R4:

R4#sh ip route
...
Gateway of last resort is not set
     34.0.0.0/32 is subnetted, 1 subnets
O       34.34.34.34 [110/65] via 10.1.34.3, 00:04:51, Serial0/0
     1.0.0.0/32 is subnetted, 1 subnets
O E2    1.1.1.1 [110/20] via 10.1.34.3, 00:04:51, Serial0/0
     2.0.0.0/32 is subnetted, 1 subnets
O E2    2.2.2.2 [110/20] via 10.1.34.3, 00:04:51, Serial0/0
     3.0.0.0/32 is subnetted, 1 subnets
O E2    3.3.3.3 [110/20] via 10.1.34.3, 00:04:51, Serial0/0
     4.0.0.0/32 is subnetted, 1 subnets
C       4.4.4.4 is directly connected, Loopback0
     40.0.0.0/24 is subnetted, 1 subnets
S       40.40.40.0 is directly connected, Null0
     10.0.0.0/24 is subnetted, 2 subnets
C       10.1.34.0 is directly connected, Serial0/0
O IA    10.1.123.0 [110/74] via 10.1.34.3, 00:04:51, Serial0/0

We see one intra-area route (O – LSA 2) to 34.34.34.34/32, one inter-area route (O IA – LSA 3) to 10.1.123.0/23 and three external type 2 (O E2 – LSA 5) routes to the respective loopbacks. Also notice the static (40.40.40.0/24) to null0, which we’re injecting into the OSPF domain.

Now we’ll configure area 34 as not so totally stubby:

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

Easy stuff, we configure area 34 with “nssa no-summary” on R3 (ABR), then we configure R4 (ASBR) with “nssa” for area 34.

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, 00:01:50, Serial0/0
     3.0.0.0/32 is subnetted, 1 subnets
O N2    3.3.3.3 [110/20] via 10.1.34.3, 00:01:50, Serial0/0
O*IA 0.0.0.0/0 [110/65] via 10.1.34.3, 00:01:50, Serial0/0

We now have only three OSPF routes, our O (LSA 2) for the 34.34.34.34/32 network, our O*IA default route, which is injected from R3 (ABR) and the N2 (LSA 7) route for R3′s loopback, which is being redistributed with the “redistribute connected subnets” command on R3.

Here’s R4′s 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         255         0x80000006 0x00A1D5 3
4.4.4.4         4.4.4.4         252         0x80000009 0x0025E4 2
 
                Summary Net Link States (Area 34)
 
Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         3.3.3.3         272         0x80000001 0x00DE4B
 
                Type-7 AS External Link States (Area 34)
 
Link ID         ADV Router      Age         Seq#       Checksum Tag
3.3.3.3         3.3.3.3         271         0x80000001 0x00E69F 0
4.4.4.4         4.4.4.4         256         0x80000001 0x0090B4 0
40.40.40.0      4.4.4.4         256         0x80000001 0x00A339 0

We see the router LSAs for R3 and R4, which are normal. Next we see the Type 3 LSA for the default route R3 is injecting and finally we see three Type 7 LSAs, one for each external network injected into area 34.

Last we’ll look at R1′s RIB so we can see how Not So Totally Stubby Areas affect the OSPF domain:

R1#sh ip route ospf
     34.0.0.0/32 is subnetted, 1 subnets
O IA    34.34.34.34 [110/11] via 10.1.123.3, 00:14:31, FastEthernet0/0
     2.0.0.0/32 is subnetted, 1 subnets
O E2    2.2.2.2 [110/20] via 10.1.123.2, 00:15:24, FastEthernet0/0
     3.0.0.0/32 is subnetted, 1 subnets
O E2    3.3.3.3 [110/20] via 10.1.123.3, 00:14:31, FastEthernet0/0
     4.0.0.0/32 is subnetted, 1 subnets
O E2    4.4.4.4 [110/20] via 10.1.123.3, 00:05:40, FastEthernet0/0
     40.0.0.0/24 is subnetted, 1 subnets
O E2    40.40.40.0 [110/20] via 10.1.123.3, 00:05:40, FastEthernet0/0
     10.0.0.0/24 is subnetted, 2 subnets
O IA    10.1.34.0 [110/74] via 10.1.123.3, 00:14:31, FastEthernet0/0

The table looks normal. The important thing to note here is that the external routes from R4 are showing up as O E2 (LSA 5s), which we know is caused by R3 converting them from the Type 7s which only exist in NSSAs to Type 5s, which are allowed in normal areas.

Not So Totally Stubby Areas sound odd, and I’ve never seen them in the real world, but they are fair game on the CCIE lab, and you may come across them in a real network. The key concepts are simply that LSA Type 5s are not allowed in Not So Totally Stubby Areas, external routes will show as N (LSA 7s) in the RIB and are converted to Type 5s on the ABR before leaving the area. Also remember that the ABR injects a default route like a normal Totally Stubby Area.

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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