OSPF Areas, Part 2, Normal and Stub Areas

Welcome back to our series on OSPF areas. Click here for Part 1 of the series. It is time to focus on normal areas and stub areas in this post. Recall our topology:


We have gone to R1 and created a prefix (11.11.11.0/24) using a loopback interface. We run RIP version 2 on this interface and redistribute this into OSPF Area 0. What should this create on R3 in Area 11 (a normal OSPF area)? That’s right  – a Type 5 LSA for an External prefix. Let us examine the OSPF database on R3 now and the accompanying IP routing table:

R3#show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 1)

  Router Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         1216        0x80000002 0x00023C 1
3.3.3.3         3.3.3.3         1215        0x80000002 0x00C075 1

  Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.1.3     3.3.3.3         1215        0x80000001 0x003577

  Summary Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.10.0      2.2.2.2         1281        0x80000001 0x0048C4
172.16.10.0     2.2.2.2         1241        0x80000001 0x00C79B

  Summary ASB Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         2.2.2.2         449         0x80000001 0x0075B0

  Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
11.11.11.0      1.1.1.1         456         0x80000001 0x0075AB 0
R3#

R3#show ip route

Gateway of last resort is not set

     172.16.0.0/24 is subnetted, 1 subnets
O IA    172.16.10.0 [110/21] via 192.168.1.2, 00:24:41, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
O IA    10.10.10.0 [110/20] via 192.168.1.2, 00:24:41, FastEthernet0/0
     11.0.0.0/24 is subnetted, 1 subnets
O E2    11.11.11.0 [110/20000] via 192.168.1.2, 00:11:53, FastEthernet0/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
R3#

Sure enough, there is the Type 5 prefix in the normal area. And we cannot forget about the LSA Type 4 (Summary ASB Link State). This informs the OSPF domain of the location of the Autonomous System Boundary Router (ASBR). I am sure you have been noticing how some of the LSAs in the database do not translate directly into routing table entries. For example, the LSA Type 4. This is reminiscent of the EIGRP topology table. That protocol sure tries to act link state as well!
OK, well let us see what happens when we convert Area 11 into a STUB AREA. Remember, this is a simple configuration. All we need to do is go to ALL of the routers in the stub area (there can be many), and issue the router configuration command area 11 stub. Now that we have done that, let us examine the databases on R3.

R3#show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 1)

  Router Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         7           0x80000005 0x001A23 1
3.3.3.3         3.3.3.3         6           0x80000005 0x00D85C 1

  Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.1.3     3.3.3.3         6           0x80000004 0x004D5E

  Summary Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         2.2.2.2         33          0x80000001 0x0075C0
10.10.10.0      2.2.2.2         33          0x80000003 0x0062AA
172.16.10.0     2.2.2.2         33          0x80000003 0x00E181
R3#

R3#show ip route

Gateway of last resort is 192.168.1.2 to network 0.0.0.0

     172.16.0.0/24 is subnetted, 1 subnets
O IA    172.16.10.0 [110/21] via 192.168.1.2, 00:01:23, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
O IA    10.10.10.0 [110/20] via 192.168.1.2, 00:01:23, FastEthernet0/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
O*IA 0.0.0.0/0 [110/11] via 192.168.1.2, 00:01:23, FastEthernet0/0
R3#

Wow, things really changed here. Notice the Stub Area effect worked just as advertised in our Core Knowledge studies. The Type 4 and 5 LSAs were removed from the OSPF database! They were replaced with a “special” LSA Type 3. It is special because it is an automatically generated default route by the Area Border Router (ABR).
Join us in the next part of this blog series where we examine the next OSPF area type, the Totally Stubby Area.