STUB Areas

STUBBY AREAS

ABRs in stub areas advertise a default route into the stub area. At the same time, the ABR chooses to not advertise external routes (5 LSAs) into the area, or even instead to no longer advertise interarea routes (in Type 3 LSAs) into the area. As a result, all routers in the stub area can still route to the destinations (based on the default route), but the routers require less memory and processing.

The following list summarizes these features of stub areas for easier study and review:

·         ABRs create a default route, using a Type 3 LSA, listing subnet 0.0.0.0 and mask 0.0.0.0, and flood that into the stub area.

·         ABRs do not flood Type 5 LSAs into the stub area.

·         ABRs may not flood other Type 3 LSAs into the area.

·         The default route has a metric of 1 unless otherwise configured using the OSPF sub-command area (area-num) default-cost (cost).

·         Routers inside stub areas cannot redistribute external routes into the stubby area, because that would require a Type 5 LSA in the area.

·         All routers in the area must be configured to be stubby; if not, neighbor relationships cannot form between potential neighbors based on this mismatched configuration.

OSPF has several types of areas, which can be defined according to the needs of a network:

     Normal area

     Stub area

     Totally stubby area

     Not-so-stubby area (NSSA)

     Totally not-so-stubby area

Normal Areas

When the area is defined by default, it is considered a normal or regular area. Normal areas have the following characteristics:

·         Summary LSAs from other areas are injected. LSA-3

·         External LSAs are injected. LSA-5

·         External default LSAs can be injected.

Stub Areas

In stub areas, no external LSAs are allowed. Recall the Options field in OSPF Hello packet. One of the bits in that option field is the E bit. In cases of stub areas, the E bit is clear, indicating that the area is incapable of importing any external LSAs.

Stub areas have the following characteristics:

·         Summary LSAs from other areas are injected.

·         The default route is injected as a summary route.

·         External LSAs are not injected.

router ospf 1

 area 1 stub

Totally Stubby Areas

Totally stubby areas are the most restricted form of area. Routers in this type of area rely on only the injection of a default summary route from the ABR. No other external or summary information is included in the routing table. This is an extension to the stub area, so all the characteristics are still true for this area.

This area has the following characteristics:

·         No summary LSAs are allowed.

·         No external LSAs are allowed.

·         The default route is injected as a summary route.

router ospf 1

 area 1 stub no-summary

Not-So-Stubby Areas

This is also an extension of the stub area. Suppose an Area XYZ is defined as a stub area and there is a requirement of redistribution of routes into that area. If Area XYZ were defined as stub, this would not be possible. To redistribute routes into Area XYZ, Area XYZ must be changed into an NSSA. When Area XYZ is changed into an NSSA, it will allow redistribution and then routes can be redistributed into the NSSA area as Type 7 LSAs.

NSSAs were created to inject external routes from stub areas into the OSPF domain. In the NSSA, when the ASBR injects a route into the AS, it generates a Type 7 LSA. The ABR translates this LSA to a Type 5 LSA, which is propagated to the rest of the autonomous system. The Type 7 LSA flooding  scope is within the NSSA area.

NSSA is supported starting in Cisco IOS Software Release 11.2. NSSAs have the following characteristics:

·         Type 7 LSAs carry external information within an NSSA.

·         Type 7 LSAs are converted into Type 5 LSAs at the NSSA ABR.

·         No external LSA are allowed.

·         Summary LSAs are injected.

Totally Not-So-Stubby Areas

Totally NSSAs have the following characteristics:

·         No summary LSAs are allowed.

·         No external LSAs are allowed.

·         The default route is injected as a summary route.

·         Type 7 LSAs are converted into Type 5 LSAs at the NSSA ABR.

In some situations, there is no need to inject external routes into the NSSA as Type 7 routes. This situation usually occurs when an ASBR is also an NSSA ABR.

Default Routes in NSSA

There are two ways to have a default route in an NSSA:

·         When you configure an area as an NSSA, the NSSA ABR doesn't generate a default summary

route, by default.

·         In the case of a stub area or an NSSA, totally stubby area, the NSSA ABR generates a default

summary route.

Default Summary Route

By defining an area as an NSSA, totally stubby area, the NSSA ABR generates a default summary route. As mentioned earlier, if the NSSA area were not defined as a totally stubby area, a default summary route would not be generated by the NSSA ABR. This is done by applying the no-summary option on the NSSA ABR.

1.       Make Router 2,3 and 4 as STUB routers

After making router as a stub, all networks redistributed at Router5 blocked on Router2,3 &4

2.       To make Area 234 as totally stubby, configure ABR Routers with no summary command

As for area 234, there are not ABRS R3 and R4, the networks coming from R4 are blocked for 234. Now there are LSA3 only from the router3

Check the Show ip route COMMAND

Router2

5.0.0.0/32 is subnetted, 8 subnets

O IA    5.5.137.129 [110/3] via 23.23.23.2, 00:00:16, FastEthernet1/0

O IA    5.5.137.161 [110/3] via 23.23.23.2, 00:00:16, FastEthernet1/0

O IA    5.5.137.193 [110/3] via 23.23.23.2, 00:00:16, FastEthernet1/0

O IA    5.5.137.225 [110/3] via 23.23.23.2, 00:00:16, FastEthernet1/0

O IA    5.5.137.1 [110/3] via 23.23.23.2, 00:00:18, FastEthernet1/0

O IA    5.5.137.33 [110/3] via 23.23.23.2, 00:00:18, FastEthernet1/0

O IA    5.5.137.65 [110/3] via 23.23.23.2, 00:00:18, FastEthernet1/0

O IA    5.5.137.97 [110/3] via 23.23.23.2, 00:00:18, FastEthernet1/0

     23.0.0.0/30 is subnetted, 1 subnets

C       23.23.23.0 is directly connected, FastEthernet1/0

     24.0.0.0/30 is subnetted, 1 subnets

C       24.24.24.0 is directly connected, FastEthernet2/0

     56.0.0.0/30 is subnetted, 1 subnets

O IA    56.56.56.0 [110/3] via 23.23.23.2, 00:00:19, FastEthernet1/0

     26.0.0.0/30 is subnetted, 3 subnets

O IA    26.26.26.8 [110/6] via 23.23.23.2, 00:00:19, FastEthernet1/0

O IA    26.26.26.0 [110/4] via 23.23.23.2, 00:00:19, FastEthernet1/0

O IA    26.26.26.4 [110/5] via 23.23.23.2, 00:00:19, FastEthernet1/0

     12.0.0.0/30 is subnetted, 1 subnets

C       12.12.12.0 is directly connected, FastEthernet0/0

     45.0.0.0/30 is subnetted, 1 subnets

O IA    45.45.45.0 [110/3] via 23.23.23.2, 00:00:19, FastEthernet1/0

O*IA 0.0.0.0/0 [110/2] via 24.24.24.2, 00:00:14, FastEthernet2/0

               [110/2] via 23.23.23.2, 00:00:14, FastEthernet1/0

The destinations are only from Router3 because this ABR is still advertising LSA3 into area 234.

It automatically add the default route out R3 and R4 because of these routes now all the networks can be still reachable to router2. Any packets for unknown networks ( or networks of area 0 and area 69) will forwarded to Router3 and Router4.

Check the OSPF database on Router2

OSPF automatically show Default route from Router3 as it is not made totally stubby yet. All other networks can be seen through Router3

R2#sh ip ospf data

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Router Link States (Area 234)

Link ID         ADV Router      Age         Seq#       Checksum Link count

2.2.2.2         2.2.2.2         1460        0x8000000A 0x008FE2 10

3.3.3.3         3.3.3.3         1272        0x80000006 0x00827D 9

4.4.4.4         4.4.4.4         1545        0x80000004 0x00D310 9

                Net Link States (Area 234)

Link ID         ADV Router      Age         Seq#       Checksum

23.23.23.2      3.3.3.3         1698        0x80000001 0x00C80F

24.24.24.2      4.4.4.4         1689        0x80000001 0x00A824

                Summary Net Link States (Area 234)

Link ID         ADV Router      Age         Seq#       Checksum

0.0.0.0         3.3.3.3         1706        0x80000001 0x0057DA

0.0.0.0         4.4.4.4         990         0x80000002 0x0037F5

5.5.137.1       3.3.3.3         1199        0x80000001 0x00F0AB

5.5.137.33      3.3.3.3         1199        0x80000001 0x00AFCC

5.5.137.65      3.3.3.3         1199        0x80000001 0x006EED

5.5.137.97      3.3.3.3         1204        0x80000001 0x002D0F

5.5.137.129     3.3.3.3         1204        0x80000001 0x00EB30

5.5.137.161     3.3.3.3         1204        0x80000001 0x00AA51

5.5.137.193     3.3.3.3         1204        0x80000001 0x006972

5.5.137.225     3.3.3.3         1204        0x80000001 0x002893

26.26.26.0      3.3.3.3         1712        0x80000002 0x00AB38

35.35.35.0      3.3.3.3         1278        0x80000003 0x005079

45.45.45.0      3.3.3.3         1278        0x80000003 0x00F0B9

56.56.56.0      3.3.3.3         1712        0x80000002 0x006525

NSSA Practice

I have redistributed networks at Router3 to make it NSSA, but it doesn’t work as it is. Because to make it work, first we have to change configuration of STUB to NSSA on all routers in that area.

Remove area 234 stub command on all routers and configure it with area 234 nssa.

I have now configured all areas with NSSA commands.

Not So Stubby Area will have all the netwoks from the full ospf domain because LSA 5 is converted into LSA 7. It is because LSA5 cannot be advertised into any STUB area. Therefore LSA  works instead of LSA5.

Check the Route table of Router 2

It has all the networks whom are advertides in OSPF Domain. You can see networks 3.3.136.0/24 redistribute at router3. And also others netwoks not only from router3 also from Router4

3.0.0.0/27 is subnetted, 8 subnets

O N2    3.3.136.128 [110/20] via 23.23.23.2, 00:00:04, FastEthernet1/0

O N2    3.3.136.160 [110/20] via 23.23.23.2, 00:00:05, FastEthernet1/0

O N2    3.3.136.192 [110/20] via 23.23.23.2, 00:00:05, FastEthernet1/0

O N2    3.3.136.224 [110/20] via 23.23.23.2, 00:00:05, FastEthernet1/0

O N2    3.3.136.0 [110/20] via 23.23.23.2, 00:00:05, FastEthernet1/0

O N2    3.3.136.32 [110/20] via 23.23.23.2, 00:00:05, FastEthernet1/0

O N2    3.3.136.64 [110/20] via 23.23.23.2, 00:00:05, FastEthernet1/0

O N2    3.3.136.96 [110/20] via 23.23.23.2, 00:00:06, FastEthernet1/0

     4.0.0.0/32 is subnetted, 8 subnets

O       4.4.136.129 [110/2] via 24.24.24.2, 00:14:05, FastEthernet2/0

O       4.4.136.161 [110/2] via 24.24.24.2, 00:14:05, FastEthernet2/0

O       4.4.136.193 [110/2] via 24.24.24.2, 00:14:05, FastEthernet2/0

O       4.4.136.225 [110/2] via 24.24.24.2, 00:14:05, FastEthernet2/0

O       4.4.136.1 [110/2] via 24.24.24.2, 00:14:05, FastEthernet2/0

O       4.4.136.33 [110/2] via 24.24.24.2, 00:14:05, FastEthernet2/0

O       4.4.136.65 [110/2] via 24.24.24.2, 00:14:05, FastEthernet2/0

O       4.4.136.97 [110/2] via 24.24.24.2, 00:14:05, FastEthernet2/0

     5.0.0.0/32 is subnetted, 8 subnets

O IA    5.5.137.129 [110/3] via 24.24.24.2, 00:14:05, FastEthernet2/0

                                  [110/3] via 23.23.23.2, 00:14:05, FastEthernet1/0

O IA    5.5.137.161 [110/3] via 24.24.24.2, 00:14:05, FastEthernet2/0

                                  [110/3] via 23.23.23.2, 00:14:05, FastEthernet1/0

O IA    5.5.137.193 [110/3] via 24.24.24.2, 00:14:07, FastEthernet2/0

                                  [110/3] via 23.23.23.2, 00:14:07, FastEthernet1/0

O IA    5.5.137.225 [110/3] via 24.24.24.2, 00:14:07, FastEthernet2/0

                                  [110/3] via 23.23.23.2, 00:14:07, FastEthernet1/0

O IA    5.5.137.1 [110/3] via 24.24.24.2, 00:14:07, FastEthernet2/0

                                 [110/3] via 23.23.23.2, 00:14:07, FastEthernet1/0

O IA    5.5.137.33 [110/3] via 24.24.24.2, 00:14:07, FastEthernet2/0

                                [110/3] via 23.23.23.2, 00:14:07, FastEthernet1/0

O IA    5.5.137.65 [110/3] via 24.24.24.2, 00:14:10, FastEthernet2/0

                        [110/3] via 23.23.23.2, 00:14:10, FastEthernet1/0

O IA    5.5.137.97 [110/3] via 24.24.24.2, 00:14:10, FastEthernet2/0

                       [110/3] via 23.23.23.2, 00:14:10, FastEthernet1/0

     23.0.0.0/30 is subnetted, 1 subnets

C       23.23.23.0 is directly connected, FastEthernet1/0

     24.0.0.0/30 is subnetted, 1 subnets

C       24.24.24.0 is directly connected, FastEthernet2/0

     56.0.0.0/30 is subnetted, 1 subnets

O IA    56.56.56.0 [110/3] via 24.24.24.2, 00:14:10, FastEthernet2/0

                   [110/3] via 23.23.23.2, 00:14:10, FastEthernet1/0

     12.0.0.0/30 is subnetted, 1 subnets

C       12.12.12.0 is directly connected, FastEthernet0/0

     45.0.0.0/30 is subnetted, 1 subnets

Totally NSSA

T NSSA is very useful area if we have only one exit point and in which we also want to redistribute networks. Then Router ABR will send default summary route  and there will not be any LSA 7 for area T.NSSA.

If there is an ABR and other router is ASBR, then ABR will block LSA 7 so that networks from other area don’t get entry and will send a default summary route. And other router as ASBR will advertise external routes.

But T NSSA is best for practice when there is ABR is working as ASBR in the OSPF domain.