OSPF Area Types

ccnp-routing
Compare standard, stub, totally stubby, NSSA, and totally NSSA areas and observe how each area type changes the LSAs in the OSPF database.
Published

Dec 11, 2017

Standard Area

image
Back to the earlier exercise 
B1#show ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

        Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         168         0x8000000E 0x003369 1
2.2.2.2         2.2.2.2         18          0x8000000B 0x00F79F 1
3.3.3.3         3.3.3.3         502         0x80000006 0x00E1B3 1

        Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
172.16.1.3      3.3.3.3         200         0x8000000C 0x00A5A0

        Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.0.0       2.2.2.2         214         0x80000007 0x0050C9
10.10.0.0       3.3.3.3         206         0x80000005 0x0036E1

        Summary ASB Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
99.99.99.98     2.2.2.2         214         0x80000001 0x008124
99.99.99.98     3.3.3.3         206         0x80000001 0x00633E
99.99.99.99     2.2.2.2         214         0x80000001 0x00772D
99.99.99.99     3.3.3.3         206         0x80000001 0x005947

        Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
0.0.0.0         99.99.99.98     1145        0x80000002 0x00998B 1
0.0.0.0         99.99.99.99     1385        0x80000002 0x009390 1
1.1.1.1         1.1.1.1         161         0x80000002 0x0099FD 0
Remember 0.0.0.0s in Type-5 LSA are generated by default-information originate 
WAN1#show ip route 1.1.1.1
Routing entry for 1.1.1.1/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 1
  Last update from 172.16.1.1 on GigabitEthernet0/0, 00:04:37 ago
  Routing Descriptor Blocks:
  * 172.16.1.1, from 1.1.1.1, 00:04:37 ago, via GigabitEthernet0/0
      Route metric is 20, traffic share count is 1

Stub Area

stub
B1(config-router)#area 1 stub
*Dec 12 02:03:13.067: %OSPF-4-ASBR_WITHOUT_VALID_AREA: Router is currently an ASBR while having only one area which is a stub area
*Dec 12 02:03:13.100: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on GigabitEthernet0/0 from FULL to DOWN, Neighbor Down: Adjacency forced to reset
*Dec 12 02:03:13.101: OSPF-1 LSGEN: Scheduling rtr LSA for area 1, build flag 0x41 (from 0x2D7BB7E)
*Dec 12 02:03:13.103: OSPF-1 LSGEN: Scheduling rtr LSA for area 1, build flag 0x41 (from 0x2D51EF5)
*Dec 12 02:03:13.104: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on GigabitEthernet0/0 from FULL to DOWN, Neighbor Down: Adjacency forced to reset
*Dec 12 02:03:13.105: OSPF-1 LSGEN: Scheduling rtr LSA for area 1, build flag 0x41 (from 0x2D7BB7E)
*Dec 12 02:03:13.106: OSPF-1 LSGEN: Scheduling network LSA on GigabitEthernet0/0, build flag 0x40 (from 0x2DD43BC)
*Dec 12 02:03:13.106: OSPF-1 LSGEN: Scheduling rtr LSA for area 1, build flag 0x41 (from 0x2D51EF5)
*Dec 12 02:03:14.050: OSPF-1 LSGEN: Reset old DR on intf GigabitEthernet0/0
*Dec 12 02:03:14.056: OSPF-1 LSGEN: Build router LSA for area 1, router ID 1.1.1.1, seq 0x80000016
*Dec 12 02:03:14.057: OSPF-1 LSGEN: No full nbrs on intf GigabitEthernet0/0 to build Net LSA
We have to configure all routers in an area as stub to become OSPF neighbors 
WAN2(config-router)#area 1 stub
*Dec 12 01:56:56.734: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on GigabitEthernet0/0 from FULL to DOWN, Neighbor Down: Adjacency forced to reset
*Dec 12 01:57:01.554: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on GigabitEthernet0/0 from LOADING to FULL, Loading Done
See the result of debug ip ospf lsa-generation on B1 
B1(config-router)#
*Dec 12 02:04:15.543: OSPF-1 LSGEN: Scheduling rtr LSA for area 1, build flag 0x41 (from 0x2D51EF5)
*Dec 12 02:04:15.544: OSPF-1 LSGEN: Scheduling rtr LSA for area 1, build flag 0x41 (from 0x2D51EF5)
*Dec 12 02:04:15.632: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on GigabitEthernet0/0 from LOADING to FULL, Loading Done
*Dec 12 02:04:15.633: OSPF-1 LSGEN: Scheduling rtr LSA for area 1, build flag 0x51 (from 0x2D7BB7E)
*Dec 12 02:04:16.099: OSPF-1 LSGEN: Reset old DR on intf GigabitEthernet0/0
*Dec 12 02:04:16.101: OSPF-1 LSGEN: Build router LSA for area 1, router ID 1.1.1.1, seq 0x80000017
*Dec 12 02:04:28.791: OSPF-1 LSGEN: Scheduling rtr LSA for area 1, build flag 0x41 (from 0x2D51EF5)
*Dec 12 02:04:29.294: OSPF-1 LSGEN: No change in router LSA, area 1
*Dec 12 02:04:58.630: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on GigabitEthernet0/0 from DOWN to DOWN, Neighbor Down: Dead timer expired
*Dec 12 02:04:58.634: OSPF-1 LSGEN: Scheduling rtr LSA for area 1, build flag 0x41 (from 0x2D51EF5)
*Dec 12 02:04:59.136: OSPF-1 LSGEN: No change in router LSA, area 1
And one more router within area 1 needed to be configured stub 
WAN1(config)#router ospf 1
WAN1(config-router)#area 1 stub
Now, let us see the result of show ip ospf database on B1 
B1#show ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

        Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         245         0x80000017 0x00395E 1
2.2.2.2         2.2.2.2         105         0x80000012 0x00088A 1
3.3.3.3         3.3.3.3         246         0x8000000C 0x00D5B9 1

        Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
172.16.1.3      3.3.3.3         104         0x80000015 0x00B18D

        Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         2.2.2.2         107         0x80000001 0x0075C0
0.0.0.0         3.3.3.3         250         0x80000001 0x0057DA
10.10.0.0       2.2.2.2         107         0x8000000A 0x0068B0
10.10.0.0       3.3.3.3         250         0x80000008 0x004EC8

Voila, there are no type 3 and 5 LSAs

Totally Stub Area

totally_stub
  • To configure, we issue command area X stub on all routers inside area X except ABRs.
  • On ABRs we issue command area X stub no-summary
WAN2(config-router)#area 1 stub no-summary
B1#show ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

        Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         580         0x80000017 0x00395E 1
2.2.2.2         2.2.2.2         440         0x80000012 0x00088A 1
3.3.3.3         3.3.3.3         581         0x8000000C 0x00D5B9 1

        Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
172.16.1.3      3.3.3.3         440         0x80000015 0x00B18D

        Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         2.2.2.2         443         0x80000001 0x0075C0
0.0.0.0         3.3.3.3         11          0x80000003 0x0053DC
10.10.0.0       2.2.2.2         443         0x8000000A 0x0068B0

WAN1(config-router)#area 1 stub no-summary
B1#show ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

        Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         650         0x80000017 0x00395E 1
2.2.2.2         2.2.2.2         510         0x80000012 0x00088A 1
3.3.3.3         3.3.3.3         651         0x8000000C 0x00D5B9 1

        Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
172.16.1.3      3.3.3.3         509         0x80000015 0x00B18D

        Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         3.3.3.3         81          0x80000003 0x0053DC

Not-So-Stubby Area (NSSA)

nssa
  • Issue area X nssa on all routers in area X
Here is an example configuration on R1: 
WAN1#show running-config | section ospf
router ospf 1
 auto-cost reference-bandwidth 1000
 area 0 range 10.10.0.0 255.255.0.0
 area 1 nssa
 network 10.10.0.0 0.0.255.255 area 0
 network 172.16.1.0 0.0.0.255 area 1

We have to configure area 1 nssa on B1 and WAN2 as well. Let us see the result on B1:

B1#show ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

        Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         521         0x8000001D 0x00BACC 1
2.2.2.2         2.2.2.2         603         0x80000017 0x008BF7 1
3.3.3.3         3.3.3.3         786         0x8000000F 0x005D25 1

        Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
172.16.1.3      3.3.3.3         790         0x80000018 0x003301

        Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.0.0       2.2.2.2         814         0x80000003 0x00FD1A
10.10.0.0       3.3.3.3         792         0x80000003 0x00DF34

        Type-7 AS External Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Tag
1.1.1.1         1.1.1.1         521         0x80000001 0x004E81 0
As we can see, there is no default route injected into NSSA although Type-3 LSAs can pass. We have to explicitly configure it on ABRs. 
WAN1(config)#router ospf 1
WAN1(config-router)#area 1 nssa default-information-originate
WAN2(config)#router ospf 1
WAN2(config-router)#area 1 nssa default-information-originate
B1#show ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

        Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         703         0x8000001D 0x00BACC 1
2.2.2.2         2.2.2.2         784         0x80000017 0x008BF7 1
3.3.3.3         3.3.3.3         967         0x8000000F 0x005D25 1

        Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
172.16.1.3      3.3.3.3         971         0x80000018 0x003301

        Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.0.0       2.2.2.2         995         0x80000003 0x00FD1A
10.10.0.0       3.3.3.3         973         0x80000003 0x00DF34

        Type-7 AS External Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Tag
0.0.0.0         2.2.2.2         5           0x80000001 0x00D0D8 0
0.0.0.0         3.3.3.3         21          0x80000001 0x00B2F2 0
1.1.1.1         1.1.1.1         702         0x80000001 0x004E81 0

Redistribution in NSSA

  • We cannot redistribute routes into Stub and Totally Stub areas
  • But we can redistribute into NSSA

Totally NSSA

  • Like NSSA but 2 differences:
    1. Type-3 LSA cannot pass
    2. Default route is automatically injected. There is no need to area 1 nssa default-information-originate
  • To configure, on ABRs we add no-summary keyword at the end of area 1 nssa OSPF router subcommand
WAN1#show run | s ospf
router ospf 1
 auto-cost reference-bandwidth 1000
 area 0 range 10.10.0.0 255.255.0.0
 area 1 nssa no-summary
 network 10.10.0.0 0.0.255.255 area 0
 network 172.16.1.0 0.0.0.255 area 1

WAN2#show run | s ospf
router ospf 1
 auto-cost reference-bandwidth 1000
 area 0 range 10.10.0.0 255.255.0.0
 area 1 nssa no-summary
 network 10.10.0.0 0.0.255.255 area 0
 network 172.16.1.0 0.0.0.255 area 1

B1#show run | s ospf
router ospf 1
 auto-cost reference-bandwidth 1000
 area 1 nssa
 redistribute connected subnets
 network 172.16.1.0 0.0.0.255 area 1

B1#show ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

        Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         1159        0x8000001D 0x00BACC 1
2.2.2.2         2.2.2.2         1240        0x80000017 0x008BF7 1
3.3.3.3         3.3.3.3         1423        0x8000000F 0x005D25 1

        Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
172.16.1.3      3.3.3.3         1427        0x80000018 0x003301

        Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         2.2.2.2         163         0x80000001 0x00FC31
0.0.0.0         3.3.3.3         155         0x80000001 0x00DE4B

        Type-7 AS External Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Tag
1.1.1.1         1.1.1.1         1158        0x80000001 0x004E81 0
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