OSPF Areas, Part 5, the Totally Not-So-Stubby Area

As a former English Major at the University of Massachusetts, Amherst, I really loved the oxymoron. You remember those…”sharply dull” or “cruel kindness”. Well, the OSPF protocol has one whopper of an oxymoron in its special areas – The Totally, Not-So-Stubby area!
When we last left our Area 11 in Part 4 of this blog series, it was a Not-So-Stubby Area, with the default-information-originate command used on the Area Border Router (ABR) in order to ensure a default route existed in the area. Here is the topology, and a look at the existing routing table on R3:

R3#show ip route

Gateway of last resort is 192.168.1.2 to network 0.0.0.0

 33.0.0.0/24 is subnetted, 1 subnets
C       33.33.33.0 is directly connected, Loopback33
 172.16.0.0/24 is subnetted, 1 subnets
O IA    172.16.10.0 [110/21] via 192.168.1.2, 00:00:14, FastEthernet0/0
 10.0.0.0/24 is subnetted, 1 subnets
O IA    10.10.10.0 [110/20] via 192.168.1.2, 00:00:14, FastEthernet0/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
 44.0.0.0/24 is subnetted, 1 subnets
C       44.44.44.0 is directly connected, Loopback44
O*N2 0.0.0.0/0 [110/1] via 192.168.1.2, 00:00:09, FastEthernet0/0
R3#

Notice I have highlighted some Type 3 LSAs that exist on this router. These, of course, are Inter-Area routes that are permitted into the Not-So-Stubby area. If we want to eliminate them, we need to make the area Totally Not-So-Stubby.
Let me go to the Area Border Router (R2) and remove the command area 11 nssa default-information-originate and replace it with the command area 11 nssa no-summary. Now, with that accomplished, let us examine the OSPF database and the routing table 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         6           0x80000003 0x00A194 1
3.3.3.3         3.3.3.3         181         0x80000003 0x0060CD 1

 Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.1.2     2.2.2.2         6           0x80000002 0x001199

 Summary Net Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         2.2.2.2         12          0x80000001 0x00FC31

 Type-7 AS External Link States (Area 11)

Link ID         ADV Router      Age         Seq#       Checksum Tag
33.33.33.0      3.3.3.3         180         0x80000001 0x00F06E 0
44.44.44.0      3.3.3.3         180         0x80000001 0x0063DA 0

R3#show ip route

Gateway of last resort is 192.168.1.2 to network 0.0.0.0

 33.0.0.0/24 is subnetted, 1 subnets
C       33.33.33.0 is directly connected, Loopback33
C    192.168.1.0/24 is directly connected, FastEthernet0/0
 44.0.0.0/24 is subnetted, 1 subnets
C       44.44.44.0 is directly connected, Loopback44
O*IA 0.0.0.0/0 [110/11] via 192.168.1.2, 00:00:17, FastEthernet0/0
R3#

Notice that the Type 3 LSAs do indeed disappear from the area. Also, notice that we do not need to instruct the Area Border Router to send the default route any more. It happens “automagically” once again like in a Totally Stubby area. Of course the Type 7 LSAs still exist in the area as a method to transport the redistributed routes in to the Area Border Router for the Type 5 LSA conversion process.
I sincerely hope you enjoyed this blog series on OSPF areas. As always, thanks for choosing INE to assist you in your Cisco certification needs.
Keep following the blog of course…I want to do a post soon for one of our awesome CCIE 2.0 customers, Terry Vinson. Terry wants me to take on the powerful VLAN Access Control Lists (VACLs) feature of Catalayst switches.