OSPF
Stub
Not-So-Stub-Area (NSSA)
Totally Stub
Totally Not-So-Stubby-Area (Totally NSSA)
Since the goal of most network engineers is to have the routing table and small as
possible sometimes these OSPF area types may help.
Stub
The stub is simple. Basically any OSPF inter-area route (outside the current area) will be
replaced with a default route pointing to the ABR (Area Border Router) of that area.
As an example we have R4 as the ABR (or hub in this Figure) and R4_1, R4_2, and R4_3
as the spokes.
The configuration on R4_1 is as follows
R4_1(config)#do sh run | sec router ospf
router ospf 1
router-id 41.41.41.41
log-adjacency-changes
network 41.41.41.41 0.0.0.0 area 1
network 172.16.4.0 0.0.0.255 area 1
Our configured loopback is 4.4.4.4 and the 172.16.4.0/24 network exists between R4,
R4_1, R4_2, and R4_3. The network 192.168.45.0/24 exists only between R4 and R5.
Let us take a look at the routing table on R4_1
R4_1#sh ip route ospf
51.0.0.0/32 is subnetted, 1 subnets
O IA
51.51.51.51 [110/4] via 172.16.4.123, 00:03:57, FastEthernet1/0
O IA 192.168.45.0/24 [110/2] via 172.16.4.123, 00:03:57,
FastEthernet1/0
4.0.0.0/32 is subnetted, 1 subnets
O
4.4.4.4 [110/2] via 172.16.4.123, 00:03:57, FastEthernet1/0
53.0.0.0/32 is subnetted, 1 subnets
O IA
53.53.53.53 [110/4] via 172.16.4.123, 00:03:57, FastEthernet1/0
172.16.0.0/24 is subnetted, 2 subnets
O IA
172.16.5.0 [110/3] via 172.16.4.123, 00:03:57, FastEthernet1/0
52.0.0.0/32 is subnetted, 1 subnets
O IA
52.52.52.52 [110/4] via 172.16.4.123, 00:03:57, FastEthernet1/0
9.0.0.0/32 is subnetted, 1 subnets
O E2
9.9.9.9 [110/20] via 172.16.4.123, 00:03:57, FastEthernet1/0
Currently we see 5 routes that are outside of this current area. These routes originate on
the other side of R5.
The routes beginning with 51, 52, and 53 are all loopbacks brought into this network by
the network 5x.5x.5x.5x 0.0.0.0 area 2 command. Pay close attention to the O E2
route of 9.9.9.9 (a static route on R5_1) which is being redistributed into OSPF from
R5_1. This makes R5_1 (172.16.5.1) an Autonomous System Border Router.
Take a look at the R4_1 OSPF database.
R4_1#sh ip ospf database
OSPF Router with ID (41.41.41.41) (Process ID 1)
Router Link States (Area 1)
Link ID
count
41.41.41.41
43.43.43.43
172.16.4.2
192.168.45.4
ADV Router
Age
Seq#
Checksum Link
41.41.41.41
43.43.43.43
172.16.4.2
192.168.45.4
365
992
991
365
0x8000002E
0x80000004
0x8000002D
0x8000002F
0x00D830
0x002BEF
0x00ECDD
0x0068CC
Net Link States (Area 1)
Link ID
172.16.4.123
ADV Router
192.168.45.4
Age
365
Seq#
Checksum
0x8000003A 0x009B4F
Summary Net Link States (Area 1)
Link ID
51.51.51.51
52.52.52.52
53.53.53.53
172.16.5.0
192.168.45.0
ADV Router
192.168.45.4
192.168.45.4
192.168.45.4
192.168.45.4
192.168.45.4
Age
378
378
378
378
378
Seq#
0x80000004
0x80000004
0x80000004
0x80000006
0x8000002A
Checksum
0x000EC1
0x00DFEB
0x00B116
0x007663
0x003EA3
2
2
2
2
Summary ASB Link States (Area 1)
Link ID
172.16.5.1
ADV Router
192.168.45.4
Age
379
Seq#
Checksum
0x80000001 0x006874
Type-5 AS External Link States
Link ID
9.9.9.9
R4_1#
ADV Router
172.16.5.1
Age
478
Seq#
Checksum Tag
0x80000001 0x008C2D 0
You can see we are using both LSA (type 4 and type 5). Let us say that R5_1 is
redistributing 10 routes and it is also our internet connection. We want to make our
routing table smaller. What we can do in that case is make the area (area 1) a stub area.
router ospf 1
router-id 41.41.41.41
log-adjacency-changes
area 1 stub
network 41.41.41.41 0.0.0.0 area 1
network 172.16.4.0 0.0.0.255 area 1
R4_1(config-router)#do sh ip route
Codes: 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
Gateway of last resort is 172.16.4.123 to network 0.0.0.0
51.0.0.0/32 is subnetted, 1 subnets
O IA
51.51.51.51 [110/4] via 172.16.4.123, 00:00:07, FastEthernet1/0
O IA 192.168.45.0/24 [110/2] via 172.16.4.123, 00:00:07,
FastEthernet1/0
4.0.0.0/32 is subnetted, 1 subnets
O
4.4.4.4 [110/2] via 172.16.4.123, 00:00:07, FastEthernet1/0
53.0.0.0/32 is subnetted, 1 subnets
O IA
53.53.53.53 [110/4] via 172.16.4.123, 00:00:07, FastEthernet1/0
172.16.0.0/24 is subnetted, 2 subnets
C
172.16.4.0 is directly connected, FastEthernet1/0
O IA
172.16.5.0 [110/3] via 172.16.4.123, 00:00:07, FastEthernet1/0
52.0.0.0/32 is subnetted, 1 subnets
O IA
52.52.52.52 [110/4] via 172.16.4.123, 00:00:07, FastEthernet1/0
41.0.0.0/32 is subnetted, 1 subnets
C
41.41.41.41 is directly connected, Loopback0
O*IA 0.0.0.0/0 [110/2] via 172.16.4.123, 00:00:09, FastEthernet1/0
Looking at the OSPF database we can see that both Type 5 and Type 4 LSA’s (LSAs that
help to describe the ASBRs) have been removed replaced with just the default route/Link
ID of 0.0.0.0.
R4_1(config-router)#do sh ip ospf database
OSPF Router with ID (41.41.41.41) (Process ID 1)
Router Link States (Area 1)
Link ID
count
41.41.41.41
43.43.43.43
172.16.4.2
192.168.45.4
ADV Router
Age
Seq#
Checksum Link
41.41.41.41
43.43.43.43
172.16.4.2
192.168.45.4
75
74
74
76
0x80000030
0x80000006
0x8000002F
0x80000031
0x00F216
0x0027F1
0x00E8DF
0x0082B2
2
2
2
2
Net Link States (Area 1)
Link ID
172.16.4.123
ADV Router
192.168.45.4
Age
71
Seq#
Checksum
0x8000003D 0x0077FA
Summary Net Link States (Area 1)
Link ID
0.0.0.0
51.51.51.51
52.52.52.52
53.53.53.53
172.16.5.0
192.168.45.0
ADV Router
192.168.45.4
192.168.45.4
192.168.45.4
192.168.45.4
192.168.45.4
192.168.45.4
Age
83
83
83
83
83
84
Seq#
0x80000001
0x80000005
0x80000005
0x80000005
0x80000007
0x8000002B
Checksum
0x005152
0x002AA6
0x00FBD0
0x00CDFA
0x009248
0x005A88
Not-So-Stubby-Area (NSSA)
Let us say that router R4_3 will have a EIGRP router R10 attached to it and R4_3 will
redistribute the EIGRP routes into OSPF like the following diagram.
The first thing that you will notice is the following warning.
R4_3(config-router)#redistribute eigrp 1043 subnets
Warning: Router is currently an ASBR while having only one area which
is a stub area
Previously, we configured the OSPF domain to be a totally stub area. Remember that the
only place we added the no-summary option was to the originator (hub) of our routes,
namely R4 (172.16.4.123).
With a totally stub area we only saw Type 1, 2, and 3 LSAs in the OSPF database.
As you can see we have a route for the 10.10.10.10 route, which is the loopback of R10.
R4_3(config-router)#do sh ip route | inc ^O|^D
O IA
51.51.51.51 [110/4] via 172.16.4.123, 00:00:26, FastEthernet1/0
O IA 192.168.45.0/24 [110/2] via 172.16.4.123, 00:00:26,
FastEthernet1/0
O
4.4.4.4 [110/2] via 172.16.4.123, 00:00:26, FastEthernet1/0
O IA
53.53.53.53 [110/4] via 172.16.4.123, 00:00:26, FastEthernet1/0
O IA
172.16.5.0 [110/3] via 172.16.4.123, 00:00:26, FastEthernet1/0
O IA
52.52.52.52 [110/4] via 172.16.4.123, 00:00:26, FastEthernet1/0
O
42.42.42.42 [110/2] via 172.16.4.2, 00:00:26, FastEthernet1/0
D
10.10.10.10/32 [90/156160] via 10.10.43.10, 00:19:58,
FastEthernet1/1
O
41.41.41.41 [110/2] via 172.16.4.1, 00:00:26, FastEthernet1/0
O*IA 0.0.0.0/0 [110/2] via 172.16.4.123, 00:00:26, FastEthernet1/0
We are also redistributing this route from EIGRP into OSPF on router R4_3.
router ospf 1
router-id 43.43.43.43
log-adjacency-changes
area 1 stub
redistribute eigrp 1043 subnets
network 43.43.43.43 0.0.0.0 area 1
network 172.16.4.0 0.0.0.255 area 1
If this is the case then our R4 neighbor should be seeing the 10.10.10.10/32 route.
R4(config-router)#do sh ip route | inc 10.10
R4(config-router)#
We do not see it since it appears that the router R4_3 is not actually advertising the AS
route of 10.10.10.10/32 and why should it since OSPF stub area’s do not allow Type 5
LSAs. This is also why we are not able to see the 9.9.9.9/32 route originating from R9.
In order to enable the redistribution we need to turn off only stub and instead use not-sostub-area (nssa). We turn on nssa on all routers in that specific area.
R4_3(config-router)#do sh run | sec router ospf
router ospf 1
router-id 43.43.43.43
log-adjacency-changes
area 1 nssa
redistribute eigrp 1043 subnets
network 43.43.43.43 0.0.0.0 area 1
network 172.16.4.0 0.0.0.255 area 1
R4(config)#do sh run | sec router ospf
router ospf 1
log-adjacency-changes
area 1 nssa
network 4.4.4.4 0.0.0.0 area 1
network 172.16.4.0 0.0.0.255 area 1
network 192.168.45.0 0.0.0.255 area 0
What a difference as you can see in both the routing table as well as the OSPF database.
R4(config)#do sh ip route ospf | inc 10.10
O N2 10.10.10.10/32 [110/20] via 172.16.4.3, 00:03:42, FastEthernet1/0
O N2 10.10.43.0/24 [110/20] via 172.16.4.3, 00:03:42, FastEthernet1/0
Just remember that Type 7 LSAs exist within the NSSA area but outside they are
translated to Type 5.
Router R4 is a great demonstration of the LSAs we use within the NSSA area and outside
of it. We use Type 7 LSAs within this NSSA area and outside. If you note the Type 5
LSAs are coming from two places. The first NSSA is coming from 172.16.5.1 (Router
5_1) and the second from us since we will be converting the Type 7 LSA into a Type 5
when we are outside of this area.
Router Link States (Area 1)
Link ID
count
41.41.41.41
43.43.43.43
172.16.4.2
192.168.45.4
ADV Router
Age
Seq#
Checksum Link
41.41.41.41
43.43.43.43
172.16.4.2
192.168.45.4
577
596
557
595
0x80000044
0x8000001F
0x80000041
0x80000009
0x00529A
0x008273
0x004C62
0x0060F2
Net Link States (Area 1)
Link ID
172.16.4.123
ADV Router
192.168.45.4
Age
557
Seq#
Checksum
0x8000005C 0x00C08A
Summary Net Link States (Area 1)
Link ID
51.51.51.51
52.52.52.52
53.53.53.53
172.16.5.0
192.168.45.0
ADV Router
192.168.45.4
192.168.45.4
192.168.45.4
192.168.45.4
192.168.45.4
Age
600
601
601
601
601
Seq#
0x80000009
0x80000009
0x80000009
0x80000009
0x80000009
Checksum
0x00A91B
0x007B45
0x004D6F
0x0016BA
0x0026D6
Type-7 AS External Link States (Area 1)
2
2
2
2
Link ID
10.10.10.10
10.10.43.0
ADV Router
43.43.43.43
43.43.43.43
Age
360
360
Seq#
Checksum Tag
0x80000001 0x00E430 0
0x80000001 0x00DC21 0
Type-5 AS External Link States
Link ID
9.9.9.9
10.10.10.10
10.10.43.0
ADV Router
172.16.5.1
192.168.45.4
192.168.45.4
Age
297
360
360
Seq#
0x8000000C
0x80000001
0x80000001
Checksum
0x007638
0x002809
0x0020F9
Tag
0
0
0
The Wireshark capture also highlights that this is a LS Type NSSA AS-External_LSA.
Totally Stub Area
It would make further sense in this situation, when we have the network topology
configured as a hub (R4_1, R4_2, and R4_3) and spoke model (R4) that we only need the
default route for all routes outside of our area. This is where Totally Stub Area comes in.
This is what it looks like from R4_1.
R4_1(config-router)#do sh ip route
Codes: 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
Gateway of last resort is 172.16.4.123 to network 0.0.0.0
4.0.0.0/32 is subnetted, 1 subnets
4.4.4.4 [110/2] via 172.16.4.123, 00:00:11, FastEthernet1/0
172.16.0.0/24 is subnetted, 1 subnets
C
172.16.4.0 is directly connected, FastEthernet1/0
42.0.0.0/32 is subnetted, 1 subnets
O
42.42.42.42 [110/2] via 172.16.4.2, 00:00:11, FastEthernet1/0
43.0.0.0/32 is subnetted, 1 subnets
O
43.43.43.43 [110/2] via 172.16.4.3, 00:00:11, FastEthernet1/0
41.0.0.0/32 is subnetted, 1 subnets
C
41.41.41.41 is directly connected, Loopback0
O*IA 0.0.0.0/0 [110/2] via 172.16.4.123, 00:00:11, FastEthernet1/0
O
The simple keyword needs to be added on the ABR (R4).
router ospf 1
log-adjacency-changes
area 1 stub no-summary
network 4.4.4.4 0.0.0.0 area 1
network 172.16.4.0 0.0.0.255 area 1
network 192.168.45.0 0.0.0.255 area 0
We can see that the Type 3 LSA’s have been reduced to just having the default route. No
Type 4 or Type 5 (routes outside of the OSPF domain) routes are permitted which is fine
R4_1(config-router)#do sh ip ospf data
OSPF Router with ID (41.41.41.41) (Process ID 1)
Router Link States (Area 1)
Link ID
count
ADV Router
Age
Seq#
Checksum Link
41.41.41.41
43.43.43.43
172.16.4.2
192.168.45.4
41.41.41.41
43.43.43.43
172.16.4.2
192.168.45.4
176
175
171
176
0x80000034
0x80000008
0x80000031
0x80000033
0x00EA1A
0x0023F3
0x00E4E1
0x007EB4
2
2
2
2
Net Link States (Area 1)
Link ID
172.16.4.123
ADV Router
192.168.45.4
Age
172
Seq#
Checksum
0x80000042 0x006DFF
Summary Net Link States (Area 1)
Link ID
0.0.0.0
ADV Router
192.168.45.4
Age
779
Seq#
Checksum
0x80000001 0x005152
Before adding the no-summary option at a packet level the Summary-LSA (Type 3) can
be seen as multiple routes.
Once the area 1 stub no-summary command is executed on R4 those LSAs are replaced
with just the Link State ID of 0.0.0.0 and netmask of 0.0.0.0.
What happens when we want to add a route to this area but it is not a OSPF router,
instead it is an EIGRP router.
However, one thing to note, when configuring any stub, nssa, or totally nssa, the packet
will change. Take a look.
You can see that the External Routing Capability is set from R4_1 (172.16.4.1). Let us
go ahead and add the area 1 stub to the router configuration.
R4_1(config-router)#area 1 stub
*May 25 16:30:45.734: %OSPF-5-ADJCHG: Process 1, Nbr 43.43.43.43 on
FastEthernet1/0 from 2WAY to DOWN, Neighbor Down: Adjacency forced to
reset
*May 25 16:30:45.734: %OSPF-5-ADJCHG: Process 1, Nbr 172.16.4.2 on
FastEthernet1/0 from 2WAY to DOWN, Neighbor Down: Adjacency forced to
reset
*May 25 16:30:45.738: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.45.4 on
FastEthernet1/0 from FULL to DOWN, Neighbor Down: Adjacency forced to
reset
R4_1(config-router)#do sh run | sec router ospf
router ospf 1
router-id 41.41.41.41
log-adjacency-changes
area 1 stub
network 41.41.41.41 0.0.0.0 area 1
network 172.16.4.0 0.0.0.255 area 1
As you can see once that happens all neighbors adjacency is reset. They also do not
come back. Why?
R4_1(config-router)#do sh ip ospf nei
Neighbor ID
Pri
Interface
43.43.43.43
0
FastEthernet1/0
172.16.4.2
0
FastEthernet1/0
192.168.45.4
1
FastEthernet1/0
State
Dead Time
Address
DOWN/DROTHER
-
172.16.4.3
DOWN/DROTHER
-
172.16.4.2
DOWN/DROTHER
-
172.16.4.123
Once those adjacencies reset I would expect R4_1 to reset those adjacencies. The reason
that this is not happening is because the hello packet has changed.
The one thing you do see is that the External Routing Capability is not set.
The remaining routers R4_2, R4_3, and R4 all have this bit set.
In order to solve this problem we need to set all of the routers, especially R4 to also be
aware that this area is a stub area.
Once we add the area 1 stub command under the router’s R4_2 and R4_3 ospf
configuration we see the following.
R4_2(config-router)#do sh ip ospf nei
Neighbor ID
Pri
Interface
41.41.41.41
0
FastEthernet1/0
43.43.43.43
0
FastEthernet1/0
192.168.45.4
1
FastEthernet1/0
State
Dead Time
Address
2WAY/DROTHER
00:00:32
172.16.4.1
2WAY/DROTHER
00:00:35
172.16.4.3
DOWN/DROTHER
-
172.16.4.123
Remember that R4 (192.168.45.4) has it’s priority at the default of 1 and the other routers
have a priority of 0 (Pri) which means that they will not participate in the Designated
router / Backup Designated router elections.
Totally Not-So-Stub-Area (Totally NSSA)
The Totally NSSA is much like the Totally Stub in that we configure it in basically the
same way and we get similar results.
On the ABR we have area 1 configured to be a nssa area. Which means that we can have
an ASBR (namely R4_3, 172.16.4.3) redistribute the EIGRP route (10.10.10.10) into the
area. Below is the configuration for the R4 router as well as the routing table of one of
the inter-area (within the same area) routers.
R4#sh run | sec router ospf
router ospf 1
log-adjacency-changes
area 1 nssa
network 4.4.4.4 0.0.0.0 area 1
network 172.16.4.0 0.0.0.255 area 1
network 192.168.45.0 0.0.0.255 area 0
R4_2(config-router)#do sh ip route ospf
51.0.0.0/32 is subnetted, 1 subnets
O IA
51.51.51.51 [110/4] via 172.16.4.123, 00:05:54, FastEthernet1/0
O IA 192.168.45.0/24 [110/2] via 172.16.4.123, 00:05:54,
FastEthernet1/0
4.0.0.0/32 is subnetted, 1 subnets
O
4.4.4.4 [110/2] via 172.16.4.123, 00:12:19, FastEthernet1/0
53.0.0.0/32 is subnetted, 1 subnets
O IA
53.53.53.53 [110/4] via 172.16.4.123, 00:05:54, FastEthernet1/0
172.16.0.0/24 is subnetted, 2 subnets
O IA
172.16.5.0 [110/3] via 172.16.4.123, 00:05:54, FastEthernet1/0
52.0.0.0/32 is subnetted, 1 subnets
O IA
52.52.52.52 [110/4] via 172.16.4.123, 00:05:54, FastEthernet1/0
43.0.0.0/32 is subnetted, 1 subnets
O
43.43.43.43 [110/2] via 172.16.4.3, 00:12:19, FastEthernet1/0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O N2
10.10.10.10/32 [110/20] via 172.16.4.3, 00:05:49,
FastEthernet1/0
O N2
10.10.43.0/24 [110/20] via 172.16.4.3, 00:05:49,
FastEthernet1/0
41.0.0.0/32 is subnetted, 1 subnets
O
41.41.41.41 [110/2] via 172.16.4.1, 00:12:19, FastEthernet1/0
Those O Intra-Area routes of 52.52.52.52 , 53.53.53.53 and 172.16.5.0/24 appear but
much like the Totally Stub we can collapse these into one default route by simply placing
the no-summary keyword on router 4.
R4_2(config-router)#do sh ip route ospf
4.0.0.0/32 is subnetted, 1 subnets
O
4.4.4.4 [110/2] via 172.16.4.123, 00:22:07, FastEthernet1/0
43.0.0.0/32 is subnetted, 1 subnets
O
43.43.43.43 [110/2] via 172.16.4.3, 00:22:07, FastEthernet1/0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O N2
10.10.10.10/32 [110/20] via 172.16.4.3, 00:04:15,
FastEthernet1/0
O N2
10.10.43.0/24 [110/20] via 172.16.4.3, 00:04:15,
FastEthernet1/0
41.0.0.0/32 is subnetted, 1 subnets
O
41.41.41.41 [110/2] via 172.16.4.1, 00:22:07, FastEthernet1/0
O*IA 0.0.0.0/0 [110/2] via 172.16.4.123, 00:04:20, FastEthernet1/0
What is more interesting is the LSUpdate packets that are sent out. We not only send out
the default LS Update of 0.0.0.0/0.0.0.0 but we also send out the LS Updates of the more
specific routes as well.