Stub router

Stub router

• The purpose is to scale.
• Topology summarization
• Hide the details of what the graph looks like in other areas.

• Summarization can be done only in two places.
• ABR - uses area x range
• ASBR - uses the summary-address

• Per LSA summarization.
• Remove all inter-area routes and replace them with shortest match routing (default route)
• STUB area or NSSA
• Remove all external routes to the area and replace them with shortest match routing (default route)

• Totally STUBBY on TNSSA
• Stub routing basic requirements 
• Area 0 cannot be a special area type
• We cannot create virtual link across a special area type.

• There are four types of special areas.
• STUBBY
• Knows how to get to the ABR (SPT with Type 1 LSA)
• Allows only Type 3 LSA to enter the Area.
• A default router will be injected automatically by the ABR to represent the NLRI to the suppressed Type 5 (no E1 or E2).
• Because Type 5 are not allowed, Type 4 are not necessary.
• My ABR knows how to get to the ASBR in another area (Type 4 and Type 5)
• If I can default to my ABR, I do not need the details for the external routes.

Example: Area 1 has been set to STUBBY.

R4, R7 and R12 have been configured as stub

router ospf 1

area 1 stub

• OSPF Database of R12

R12#sh ip ospf database

            OSPF Router with ID (12.12.12.12) (Process ID 1)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count

4.4.4.4         4.4.4.4         234         0x80000008 0x0010A2 2

7.7.7.7         7.7.7.7         233         0x8000000B 0x0042A0 5

12.12.12.12     12.12.12.12     259         0x80000008 0x000218 3

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum

0.0.0.0         4.4.4.4         234         0x80000001 0x0039F4

1.1.1.1         4.4.4.4         234         0x80000003 0x001116

5.5.5.5         4.4.4.4         234         0x80000003 0x0062B3

10.1.2.0        4.4.4.4         234         0x80000003 0x009A83

10.1.3.0        4.4.4.4         234         0x80000003 0x008F8D

10.2.4.0        4.4.4.4         234         0x80000003 0x006EAD

10.2.5.0        4.4.4.4         234         0x80000003 0x006DAC

10.3.4.0        4.4.4.4         234         0x80000003 0x0062B8

10.3.10.0       4.4.4.4         234         0x80000003 0x002AE9

10.4.10.0       4.4.4.4         234         0x80000007 0x000C04

10.10.10.10     4.4.4.4         234         0x80000003 0x007191

192.168.0.0     4.4.4.4         234         0x80000003 0x00833F

• R12 RIB - Only Default, O and O IA.

R12#sho ip route ospf

Gateway of last resort is 10.7.12.7 to network 0.0.0.0

O*IA  0.0.0.0/0 [110/3] via 10.7.12.7, 00:14:40, GigabitEthernet1

      1.0.0.0/32 is subnetted, 1 subnets

O IA     1.1.1.1 [110/4] via 10.7.12.7, 00:14:40, GigabitEthernet1

      5.0.0.0/32 is subnetted, 1 subnets

O IA     5.5.5.5 [110/5] via 10.7.12.7, 00:14:40, GigabitEthernet1

      7.0.0.0/32 is subnetted, 1 subnets

O        7.7.7.7 [110/2] via 10.7.12.7, 00:15:07, GigabitEthernet1

      10.0.0.0/8 is variably subnetted, 11 subnets, 2 masks

O IA     10.1.2.0/24 [110/4] via 10.7.12.7, 00:14:40, GigabitEthernet1

O IA     10.1.3.0/24 [110/4] via 10.7.12.7, 00:14:40, GigabitEthernet1

O IA     10.2.4.0/24 [110/3] via 10.7.12.7, 00:14:40, GigabitEthernet1

O IA     10.2.5.0/24 [110/4] via 10.7.12.7, 00:14:40, GigabitEthernet1

O IA     10.3.4.0/24 [110/3] via 10.7.12.7, 00:14:40, GigabitEthernet1

O IA     10.3.10.0/24 [110/4] via 10.7.12.7, 00:14:40, GigabitEthernet1

O        10.4.7.0/24 [110/2] via 10.7.12.7, 00:15:07, GigabitEthernet1

O IA     10.4.10.0/24 [110/3] via 10.7.12.7, 00:14:40, GigabitEthernet1

O IA     10.10.10.10/32 [110/4] via 10.7.12.7, 00:14:40, GigabitEthernet1

O IA  192.168.0.0/24 [110/3] via 10.7.12.7, 00:14:40, GigabitEthernet1

• TOTALLY STUBBY
• Knows how to get to the ABR (SPT with Type 1 LSA)
• Allows only default route into the area.
• A default router will be injected automatically by the ABR to represent the NLRI to the suppressed Type 3 (no IA, E1 or E2).
• Because Type 3 and Type 5 are not allowed, Type 4 are not necessary.
• My ABR knows how to get to the inter-area networks (Type 3) and the ASBR in another area (Type 4 and Type 5)
• If I can default to my ABR, I do not need the details for any routes outside of my area.

Example: Area 1 has been set to TOTALLY STUBBY.

R4 has been configured as TOTALLY STUBBY, R7 and R12 still configured as stub.

R7 and R12

router ospf 1

area 1 stub

R4 

router ospf 1

area 1 stub no-summary

• OSPF Database of R12

R12#sho ip ospf database

            OSPF Router with ID (12.12.12.12) (Process ID 1)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count

4.4.4.4         4.4.4.4         1271        0x80000008 0x0010A2 2

7.7.7.7         7.7.7.7         829         0x8000000C 0x0040A1 5

12.12.12.12     12.12.12.12     1296        0x80000008 0x000218 3

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum

0.0.0.0         4.4.4.4         29          0x80000002 0x0037F5

• R12  RIB - Only Default and O routes.

R12#sho ip route ospf

O*IA  0.0.0.0/0 [110/3] via 10.7.12.7, 00:21:13, GigabitEthernet1

      7.0.0.0/32 is subnetted, 1 subnets

O        7.7.7.7 [110/2] via 10.7.12.7, 00:21:40, GigabitEthernet1

      10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks

O        10.4.7.0/24 [110/2] via 10.7.12.7, 00:21:40, GigabitEthernet1

If there are two ABRs in the Stub area, you use the command area x default cost xxx to influence the metric of the default route being injected inside the stub area by one the ABRs. It will not affect O nor O IA paths.

• NSSA
• Filter exactly like a Stubby Area with the exception that we can have an ASBR in the AREA.
• ASBR int the area will generate type 7 LSA (NSSA external)
• ABR in that area will convert those Type 7 into Type 5 as they leave the area into area 0.
• All inbound Type 5 and Type 4 LSAs are filtered into the area.
• The ARB in this case does not introduce a default route into the special area by default.

Example: Area 3 has been set to NSSA.

R3, R4 and R10 have been configured as NSSA

router ospf 1

 area 3 nssa

• OSPF Database of R10 - Only Type 1, Type 3 and Type 7. Type 4 and 5 are filtered, example. 5.5.5.5 is a Type 5 which has been filtered into the area.

R10#sho ip ospf DATabase

            OSPF Router with ID (10.10.10.10) (Process ID 1)

                Router Link States (Area 3)

Link ID         ADV Router      Age         Seq#       Checksum Link count

3.3.3.3         3.3.3.3         1390        0x8000000C 0x0056BD 3

4.4.4.4         4.4.4.4         75          0x8000000F 0x005C33 2

10.10.10.10     10.10.10.10     73          0x80000016 0x009A96 8

                Summary Net Link States (Area 3)

Link ID         ADV Router      Age         Seq#       Checksum

1.1.1.1         3.3.3.3         956         0x80000009 0x00AA72

1.1.1.1         4.4.4.4         75          0x8000000B 0x00888E

2.2.2.2         3.3.3.3         195         0x80000004 0x008697

2.2.2.2         4.4.4.4         75          0x80000004 0x0068B1

3.3.3.3         3.3.3.3         1472        0x80000002 0x0052CA

3.3.3.3         4.4.4.4         75          0x80000004 0x003ADB

4.4.4.4         3.3.3.3         1947        0x80000001 0x0030E8

4.4.4.4         4.4.4.4         75          0x80000002 0x00060F

7.7.7.7         3.3.3.3         1947        0x80000001 0x00AF5C

7.7.7.7         4.4.4.4         75          0x8000000B 0x00738B

10.1.2.0        3.3.3.3         956         0x80000009 0x0034DF

10.1.2.0        4.4.4.4         75          0x8000000B 0x0012FB

10.1.3.0        3.3.3.3         956         0x80000009 0x001FF4

10.1.3.0        4.4.4.4         75          0x8000000B 0x000706

10.2.4.0        3.3.3.3         195         0x8000000B 0x000E01

10.2.4.0        4.4.4.4         75          0x8000000A 0x00E725

10.2.5.0        3.3.3.3         195         0x8000000C 0x00010C

10.2.5.0        4.4.4.4         75          0x8000000B 0x00E425

10.3.4.0        3.3.3.3         956         0x80000009 0x00FB15

10.3.4.0        4.4.4.4         75          0x8000000A 0x00DB30

10.3.8.0        3.3.3.3         956         0x80000009 0x00CF3D

10.3.8.0        4.4.4.4         75          0x8000000B 0x00B74E

10.4.7.0        3.3.3.3         1947        0x80000001 0x00E82B

10.4.7.0        4.4.4.4         75          0x8000000A 0x00AE59

10.7.12.0       3.3.3.3         1947        0x80000001 0x009773

10.7.12.0       4.4.4.4         75          0x8000000B 0x005BA2

12.12.12.12     3.3.3.3         1947        0x80000001 0x00D224

12.12.12.12     4.4.4.4         75          0x8000000B 0x009653

192.1.1.1       3.3.3.3         1727        0x80000002 0x00FB68

192.1.1.1       4.4.4.4         75          0x80000004 0x00D984

192.1.2.1       3.3.3.3         1727        0x80000002 0x00F072

192.1.2.1       4.4.4.4         75          0x80000004 0x00CE8E

192.1.3.1       3.3.3.3         1727        0x80000002 0x00E57C

192.1.3.1       4.4.4.4         75          0x80000004 0x00C398

192.2.1.1       3.3.3.3         1978        0x80000004 0x00EB75

192.2.1.1       4.4.4.4         75          0x80000004 0x00CD8F

192.2.2.1       3.3.3.3         1978        0x80000004 0x00E07F

192.2.2.1       4.4.4.4         75          0x80000004 0x00C299

192.2.3.1       3.3.3.3         1978        0x80000004 0x00D589

192.2.3.1       4.4.4.4         75          0x80000004 0x00B7A3

192.3.1.1       3.3.3.3         1472        0x80000002 0x00D989

192.3.1.1       4.4.4.4         75          0x80000004 0x00C19A

192.3.2.1       3.3.3.3         1472        0x80000002 0x00CE93

192.3.2.1       4.4.4.4         75          0x80000004 0x00B6A4

192.3.3.1       3.3.3.3         1472        0x80000002 0x00C39D

192.3.3.1       4.4.4.4         75          0x80000004 0x00ABAE

192.4.1.1       3.3.3.3         1947        0x80000001 0x00D988

192.4.1.1       4.4.4.4         75          0x80000002 0x00AFAE

192.4.2.1       3.3.3.3         1947        0x80000001 0x00CE92

192.4.2.1       4.4.4.4         75          0x80000002 0x00A4B8

192.4.3.1       3.3.3.3         1947        0x80000001 0x00C39C

192.4.3.1       4.4.4.4         75          0x80000002 0x0099C2

192.168.0.0     3.3.3.3         956         0x80000009 0x001D9B

192.168.0.0     4.4.4.4         75          0x8000000B 0x00FAB7

                Type-7 AS External Link States (Area 3)

Link ID         ADV Router      Age         Seq#       Checksum Tag

10.10.11.0      10.10.10.10     467         0x8000000B 0x009389 0

11.11.11.11     10.10.10.10     467         0x80000005 0x0018FD

• R10 OSPF RIB - Only O and O IA are permitted to enter. 

R10#sho ip route ospf

Gateway of last resort is not set

      1.0.0.0/32 is subnetted, 1 subnets

O IA     1.1.1.1 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

                 [110/3] via 10.3.10.3, 01:07:28, GigabitEthernet1

      2.0.0.0/32 is subnetted, 1 subnets

O IA     2.2.2.2 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

                 [110/3] via 10.3.10.3, 00:37:01, GigabitEthernet1

      3.0.0.0/32 is subnetted, 1 subnets

O IA     3.3.3.3 [110/2] via 10.3.10.3, 00:57:50, GigabitEthernet1

      4.0.0.0/32 is subnetted, 1 subnets

O IA     4.4.4.4 [110/2] via 10.4.10.4, 00:34:25, GigabitEthernet3

      7.0.0.0/32 is subnetted, 1 subnets

O IA     7.7.7.7 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

      10.0.0.0/8 is variably subnetted, 15 subnets, 2 masks

O IA     10.1.2.0/24 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

                     [110/3] via 10.3.10.3, 01:07:28, GigabitEthernet1

O IA     10.1.3.0/24 [110/2] via 10.3.10.3, 01:07:28, GigabitEthernet1

O IA     10.2.4.0/24 [110/2] via 10.4.10.4, 00:34:25, GigabitEthernet3

O IA     10.2.5.0/24 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

                     [110/3] via 10.3.10.3, 00:37:01, GigabitEthernet1

O IA     10.3.4.0/24 [110/2] via 10.4.10.4, 00:34:25, GigabitEthernet3

                     [110/2] via 10.3.10.3, 01:07:28, GigabitEthernet1

O IA     10.3.8.0/24 [110/2] via 10.3.10.3, 01:07:28, GigabitEthernet1

O IA     10.4.7.0/24 [110/2] via 10.4.10.4, 00:34:25, GigabitEthernet3

O IA     10.7.12.0/24 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

      12.0.0.0/32 is subnetted, 1 subnets

O IA     12.12.12.12 [110/4] via 10.4.10.4, 00:34:24, GigabitEthernet3

      30.0.0.0/32 is subnetted, 1 subnets

O        30.30.30.30 [110/2] via 10.3.10.3, 00:25:07, GigabitEthernet1

      192.1.1.0/32 is subnetted, 1 subnets

O IA     192.1.1.1 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

                   [110/3] via 10.3.10.3, 01:02:21, GigabitEthernet1

      192.1.2.0/32 is subnetted, 1 subnets

O IA     192.1.2.1 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

                   [110/3] via 10.3.10.3, 01:02:21, GigabitEthernet1

      192.1.3.0/32 is subnetted, 1 subnets

O IA     192.1.3.1 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

                   [110/3] via 10.3.10.3, 01:02:21, GigabitEthernet1

      192.2.1.0/32 is subnetted, 1 subnets

O IA     192.2.1.1 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

                   [110/3] via 10.3.10.3, 00:37:01, GigabitEthernet1

      192.2.2.0/32 is subnetted, 1 subnets

O IA     192.2.2.1 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

                   [110/3] via 10.3.10.3, 00:37:01, GigabitEthernet1

      192.2.3.0/32 is subnetted, 1 subnets

O IA     192.2.3.1 [110/3] via 10.4.10.4, 00:34:24, GigabitEthernet3

                   [110/3] via 10.3.10.3, 00:37:01, GigabitEthernet1

      192.3.1.0/32 is subnetted, 1 subnets

O IA     192.3.1.1 [110/2] via 10.3.10.3, 00:57:57, GigabitEthernet1

      192.3.2.0/32 is subnetted, 1 subnets

O IA     192.3.2.1 [110/2] via 10.3.10.3, 00:57:57, GigabitEthernet1

      192.3.3.0/32 is subnetted, 1 subnets

O IA     192.3.3.1 [110/2] via 10.3.10.3, 00:57:57, GigabitEthernet1

      192.4.1.0/32 is subnetted, 1 subnets

O IA     192.4.1.1 [110/2] via 10.4.10.4, 00:34:25, GigabitEthernet3

      192.4.2.0/32 is subnetted, 1 subnets

O IA     192.4.2.1 [110/2] via 10.4.10.4, 00:34:25, GigabitEthernet3

      192.4.3.0/32 is subnetted, 1 subnets

O IA     192.4.3.1 [110/2] via 10.4.10.4, 00:34:25, GigabitEthernet3

O IA  192.168.0.0/24 [110/2] via 10.4.10.4, 00:34:24, GigabitEthernet3

                     [110/2] via 10.3.10.3, 01:07:28, GigabitEthernet1

From R3 and R4 we are learning the routes redistributed by R10 (ASBR) from EIGRP as N2. 

• R3

R3#sho ip route ospf | in 10.4.10|10.3.10

O        10.4.10.0/24 [110/2] via 10.3.10.10, 00:21:15, GigabitEthernet5

O        10.10.10.10/32 [110/2] via 10.3.10.10, 00:05:59, GigabitEthernet5

O N2     10.10.11.0/24 [110/20] via 10.3.10.10, 00:05:54, GigabitEthernet5

O N2     11.11.11.11 [110/20] via 10.3.10.10, 00:05:54, GigabitEthernet5

O        192.10.1.1 [110/2] via 10.3.10.10, 00:05:49, GigabitEthernet5

O        192.10.2.1 [110/2] via 10.3.10.10, 00:05:40, GigabitEthernet5

O        192.10.3.1 [110/2] via 10.3.10.10, 00:04:32, GigabitEthernet5

• R4

R4#sho ip route ospf | in 10.4.10

O        10.3.10.0/24 [110/2] via 10.4.10.10, 00:31:16, GigabitEthernet5

O        10.10.10.10/32 [110/2] via 10.4.10.10, 00:06:41, GigabitEthernet5

O N2     10.10.11.0/24 [110/20] via 10.4.10.10, 00:06:37, GigabitEthernet5

O N2     11.11.11.11 [110/20] via 10.4.10.10, 00:06:37, GigabitEthernet5

O        30.30.30.30 [110/3] via 10.4.10.10, 00:21:58, GigabitEthernet5

O        192.10.1.1 [110/2] via 10.4.10.10, 00:06:32, GigabitEthernet5

O        192.10.2.1 [110/2] via 10.4.10.10, 00:06:23, GigabitEthernet5

O        192.10.3.1 [110/2] via 10.4.10.10, 00:05:15, GigabitEthernet5

From another routers in area 0, example R1, we can see the redistributed routes from NSSA are reaching as E2, meaning that Type 7 were translated to Type 5 when advertised outside the area by the ABRs R3 and R4. Other routes advertised by R10 are seen as normal O IA.

R1#sho ip route ospf | in 192.10|E2

       E1 - OSPF external type 1, E2 - OSPF external type 2

O E2     5.5.5.5 [110/20] via 192.168.0.2, 00:29:07, GigabitEthernet4

O E2     10.10.11.0/24 [110/20] via 192.168.0.4, 00:01:42, GigabitEthernet4

O E2     11.11.11.11 [110/20] via 192.168.0.4, 00:01:42, GigabitEthernet4

      192.10.1.0/32 is subnetted, 1 subnets

O IA     192.10.1.1 [110/3] via 192.168.0.4, 00:01:43, GigabitEthernet4

      192.10.2.0/32 is subnetted, 1 subnets

O IA     192.10.2.1 [110/3] via 192.168.0.4, 00:01:38, GigabitEthernet4

      192.10.3.0/32 is subnetted, 1 subnets

O IA     192.10.3.1 [110/3] via 192.168.0.4, 00:00:30, GigabitEthernet4

• TOTALLY NOT SO STUBBY AREA (NOT SO TOTALLY STUBBY AREA)
• Filter exactly like a TOTALLY STUBY AREA with the exception that we can have an ASBR in the area.
• A default router will be injected automatically by the ABR to represent the NLRI to the suppressed Type 3 (no IA, E1 or E2).
• ASBR int the area will generate type 7 LSA (NSSA external)
• ABR in that area will convert those Type 7 into Type 5 as they leave the
• All inbound Type 3, Type 5 and Type 4 LSAs are filtered into the area.

Example: Area 3 has been set to TNSSA.

R3, R4 and have been configured has TNSSA, R10 still configured as NSSA.

router ospf 1

 area 3 nssa no-summary

• OSPF Database of R10 - Only Type 1, Type 3 (This a default route from the ABR, that's the reason why it is showing as type 3, but no IA will be introduced into the area) and Type 7. Type 4 and 5 are filtered, example. 5.5.5.5 is a Type 5 which has been filtered into the area.

R10#sho ip ospf database

            OSPF Router with ID (192.10.3.1) (Process ID 1)

                Router Link States (Area 3)

Link ID         ADV Router      Age         Seq#       Checksum Link count

3.3.3.3         3.3.3.3         465         0x80000007 0x0095DC 3

4.4.4.4         4.4.4.4         433         0x80000006 0x00D31E 2

192.10.3.1      192.10.3.1      1572        0x80000006 0x00A64D 8

                Summary Net Link States (Area 3)

Link ID         ADV Router      Age         Seq#       Checksum

0.0.0.0         3.3.3.3         465         0x80000001 0x00DE4B

0.0.0.0         4.4.4.4         433         0x80000001 0x00C065

                Type-7 AS External Link States (Area 3)

Link ID         ADV Router      Age         Seq#       Checksum Tag

10.10.11.0      192.10.3.1      1572        0x80000002 0x00A0DE 0

11.11.11.11     192.10.3.1      1572        0x80000002 0x001959 0

• R10 OSPF RIB - Only O and a default route showing as O IA are permitted to enter. 

R10#sho ip route ospf

Gateway of last resort is 10.4.10.4 to network 0.0.0.0

O*IA  0.0.0.0/0 [110/2] via 10.4.10.4, 00:06:11, GigabitEthernet3

                [110/2] via 10.3.10.3, 00:06:43, GigabitEthernet1

      30.0.0.0/32 is subnetted, 1 subnets

O        30.30.30.30 [110/2] via 10.3.10.3, 00:58:53, GigabitEthernet1

Similar to NSSA scenario, from R3 and R4 we are learning the routes redistributed by R10 (ASBR) from EIGRP as N2. 

• R3

R3#sho ip route ospf | in 10.4.10|10.3.10

O        10.4.10.0/24 [110/2] via 10.3.10.10, 00:12:03, GigabitEthernet5

O        10.10.10.10/32 [110/2] via 10.3.10.10, 00:12:03, GigabitEthernet5

O N2     10.10.11.0/24 [110/20] via 10.3.10.10, 00:12:03, GigabitEthernet5

O N2     11.11.11.11 [110/20] via 10.3.10.10, 00:12:03, GigabitEthernet5

O        192.10.1.1 [110/2] via 10.3.10.10, 00:12:03, GigabitEthernet5

O        192.10.2.1 [110/2] via 10.3.10.10, 00:12:03, GigabitEthernet5

O        192.10.3.1 [110/2] via 10.3.10.10, 00:12:03, GigabitEthernet5

• R4

R4#sho ip route ospf | in 10.4.10|10.3.10

O        10.3.10.0/24 [110/2] via 10.4.10.10, 00:11:41, GigabitEthernet5

O        10.10.10.10/32 [110/2] via 10.4.10.10, 00:11:41, GigabitEthernet5

O N2     10.10.11.0/24 [110/20] via 10.4.10.10, 00:11:41, GigabitEthernet5

O N2     11.11.11.11 [110/20] via 10.4.10.10, 00:11:41, GigabitEthernet5

O        30.30.30.30 [110/3] via 10.4.10.10, 00:11:41, GigabitEthernet5

O        192.10.1.1 [110/2] via 10.4.10.10, 00:11:41, GigabitEthernet5

O        192.10.2.1 [110/2] via 10.4.10.10, 00:11:41, GigabitEthernet5

O        192.10.3.1 [110/2] via 10.4.10.10, 00:11:41, GigabitEthernet5

Similar to NSSA, from another routers in area 0, example R1, we can see the redistributed routes from NSSA are reaching as E2, meaning that Type 7 were translated to Type 5 when advertised outside the area by the ABRs R3 and R4. Other routes advertised by R10 are seen as normal O IA.

R1#sho ip route ospf | in 192.10|E2

       E1 - OSPF external type 1, E2 - OSPF external type 2

O E2     5.5.5.5 [110/20] via 192.168.0.2, 01:05:37, GigabitEthernet4

O E2     10.10.11.0/24 [110/20] via 192.168.0.4, 01:06:11, GigabitEthernet4

O E2     11.11.11.11 [110/20] via 192.168.0.4, 01:06:02, GigabitEthernet4

      192.10.1.0/32 is subnetted, 1 subnets

O IA     192.10.1.1 [110/3] via 192.168.0.4, 01:06:11, GigabitEthernet4

      192.10.2.0/32 is subnetted, 1 subnets

O IA     192.10.2.1 [110/3] via 192.168.0.4, 01:06:11, GigabitEthernet4

      192.10.3.0/32 is subnetted, 1 subnets

O IA     192.10.3.1 [110/3] via 192.168.0.4, 01:06:11, GigabitEthernet4

• Type 7 LSA (External NSSA)
• Flooded within the area.
• Changed to Type 5 at the ABR when they leave the area.
• Describes the external route being advertised.
• Seed metric (Default 20)
• Metric Type of either O N1 or O N2
• Forwarding address - who I route towards to reach that destination.
• We can set Route Tag.

• Stubby area filter traffic entering an area at transit point (ABR). We are not limiting what leaves the area, but what enters.
• OSPF State machine Route Selection Process
• O > O IA > O E1 > O N1 > O E2 > O N2

• Summarization
• It is done in the ABR and ASBR only.
• Unless is the ASBR converting Type7 to type 5, only internal routes (O) are summarized. Routes coming outside the area are not summarized to other areas. Make sure the right ABR or ASB does the summary.
• The metric for the summary route is based on the best metric of all the routes included in the summary.

• Example 1 - ABR summarization.
• Always summarizes from the Area and not to the area. 
• In the example the summary if from Area 0 to any other Areas R3 is connected.

R3 will summarize routes from Area0 towards R8. 

router ospf 1

 router-id 3.3.3.3

 area 0 range 192.1.0.0 255.255.0.0

 area 0 range 192.2.0.0 255.255.0.0

 area 0 range 192.3.0.0 255.255.0.0

 area 0 range 192.4.0.0 255.255.0.0

area 0 range 192.5.0.0 255.255.0.0

R8 verification. Route to 192.5.0.0/16 is not in the RIB because this route is not present on R3's RIB.

R8#sho ip route ospf | in 192.

O IA  192.1.0.0/16 [110/3] via 10.3.8.3, 00:00:05, GigabitEthernet1

O IA  192.2.0.0/16 [110/3] via 10.3.8.3, 00:00:32, GigabitEthernet1

O IA  192.3.0.0/16 [110/2] via 10.3.8.3, 00:00:36, GigabitEthernet1

O IA  192.4.0.0/16 [110/3] via 10.3.8.3, 00:00:55, GigabitEthernet1

• Example 2 - ABR summarizing routes coming from ASBR in the same NSSA area.
• This can be done only in case the ABR is the one doing the translation from Type7 to Type 5.
• Int the example topology, Area 3 is NSSA. R3 and R4 are connected to this area and summarizing the Prefixes the ASPR R10 is redistributing via EIGRP.

• We will go step by step to better understand it.

R11

R11 has 4 subnets that are being advertised to EIGRP.

Interfaces

• Loopback1              192.11.1.1      
• Loopback2              192.11.2.1      
• Loopback3              192.11.3.1      
• Loopback4              192.11.4.1      

router eigrp 1

 network 192.11.0.0 0.0.255.255

R10

R10 is learning these routes and redistributing them from EIGRP to OSPF.

R10#sh ip route  | in 192.11

D     192.11.1.0/24 [90/130816] via 10.10.11.11, 00:10:26, GigabitEthernet2

D     192.11.2.0/24 [90/130816] via 10.10.11.11, 00:10:26, GigabitEthernet2

D     192.11.3.0/24 [90/130816] via 10.10.11.11, 00:10:26, GigabitEthernet2

D     192.11.4.0/24 [90/130816] via 10.10.11.11, 00:10:26, GigabitEthernet2

router ospf 1

 redistribute eigrp 1 subnets

R3 and R4 are learning the routes from R10 and are summarizing it. As we can see, only R4 is installing the discard router, because is has been selected as the translator for Type 7 int Type5.

router ospf 1

 summary-address 192.11.0.0 255.255.0.0

R3#sh ip route | in 192.11

O E2  192.11.0.0/16 [110/20] via 192.168.0.4, 00:10:51, GigabitEthernet4

O N2  192.11.1.0/24 [110/20] via 10.3.10.10, 00:12:36, GigabitEthernet5

O N2  192.11.2.0/24 [110/20] via 10.3.10.10, 00:12:36, GigabitEthernet5

O N2  192.11.3.0/24 [110/20] via 10.3.10.10, 00:12:36, GigabitEthernet5

O N2  192.11.4.0/24 [110/20] via 10.3.10.10, 00:12:36, GigabitEthernet5

We can see above that R3 did not install the Discard router, instead it received the summarized route from R4 (O E2  192.11.0.0/16 [110/20] via 192.168.0.4, 00:10:51, GigabitEthernet4)

router ospf 1

 summary-address 192.11.0.0 255.255.0.0

R4#sh ip route | in 192.11

O     192.11.0.0/16 is a summary, 00:10:59, Null0

O N2  192.11.1.0/24 [110/20] via 10.4.10.10, 00:12:43, GigabitEthernet5

O N2  192.11.2.0/24 [110/20] via 10.4.10.10, 00:12:43, GigabitEthernet5

O N2  192.11.3.0/24 [110/20] via 10.4.10.10, 00:12:43, GigabitEthernet5

O N2  192.11.4.0/24 [110/20] via 10.4.10.10, 00:12:43, GigabitEthernet5

If we check from Area 0, we can see that the summarized route E2 is received via R4, because it was selected as the translator. R3 can be selected as the translator in case R4 fails.

R1#sho ip route | in 192.11

O E2  192.11.0.0/16 [110/20] via 192.168.0.4, 00:15:34, GigabitEthernet4

• Example 3 - ASBR summarization.
• R3 and R4 are no longer summarizing the routes from the ABR in Area 3. Now R10 is doing the summarization, as it is the ASBR.

R10

router ospf 1

 summary-address 192.11.0.0 255.255.0.0

• Verificarion

R3#sho ip route | in 192.11

O N2  192.11.0.0/16 [110/20] via 10.3.10.10, 00:02:14, GigabitEthernet5

R4#sho ip route | in 192.11

O N2  192.11.0.0/16 [110/20] via 10.4.10.10, 00:02:23, GigabitEthernet5

R1#sh ip route | in 192.11

O E2  192.11.0.0/16 [110/20] via 192.168.0.4, 00:02:49, GigabitEthernet4

ASBR can filter the advertisement of the summary route to make it only available in the NSSA area only.

R10 has been set to send the summarized route to NSSA only, as seen below.

router ospf 1

 summary-address 192.11.0.0 255.255.0.0 nssa-only

• Verification
• Routers inside Area 0 except R3 and R4 which are connected to Area 3 NSSA, and routers in other Areas will no longer see the routes coming from Area 3.

R1#sho ip route ospf | in 192.11

R1#ping 192.11.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.11.1.1, timeout is 2 seconds:

.....

Success rate is 0 percent (0/5)

• Discard route
• When OSPF advertises a summary route, discard route is added in the RIB. This discard route addition can be disabled using the command no discard-route under OSPF process.
• Example.

R10 is inserting the summary into the Area 3.

router ospf 1

 summary-address 192.11.0.0 255.255.0.0 nssa-only

• Verification

R10#sho ip route ospf | in 192.11

O     192.11.0.0/16 is a summary, 03:07:33, Null

Now we will add the command to not install the discard route.

router ospf 1

 no discard-route external

R10#sho ip route ospf | in 192.11 - No route to Null0 is in the RIB.