OSPF Areas, Part 4, the Not-So-Stubby Area

Click here for Part 1 of the series or click here for Part 3 of the series.

Now it is time for us to examine yet another OSPF special area type - the Not-So-Stubby Area. I am sure you recall out topology from the previous parts, but here it is again:

When we left Area 11 in the last post, it was a Totally Stubby Area. This prevented LSA Types 3, 4, and 5 from entering the area, with the exception of the special default route (Type 3) generated by the Area Border Router.

The Not-So-Stubby Area (NSSA) allows us to bring in some redistributed routes into the stub area! Wow. I guess the area really isn't that stubby at all. These redistributed routes propagate through the NSSA as Type 7, and are then converted on the ABR to Type 5. At least that is what we read in the RFCs. Let us see it in action at the command line!

I have removed the previous Totally Stubby configuration and I have placed the following command on all routers in Area 11 - area 11 nssa. After doing this, let us take a look at the OSPF database and 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 29 0x80000005 0x00A78B 1
3.3.3.3 3.3.3.3 28 0x80000004 0x0062CB 1

Net Link States (Area 11)

Link ID ADV Router Age Seq# Checksum
192.168.1.3 3.3.3.3 28 0x80000001 0x00DACB

Summary Net Link States (Area 11)

Link ID ADV Router Age Seq# Checksum
10.10.10.0 2.2.2.2 58 0x80000003 0x00E91B
172.16.10.0 2.2.2.2 58 0x80000003 0x0069F1

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, 00:00:33, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
O IA 10.10.10.0 [110/20] via 192.168.1.2, 00:00:33, FastEthernet0/0
C 192.168.1.0/24 is directly connected, FastEthernet0/0
R3#

Notice how at this point,it looks just like a stub area. Type 4 and 5 LSAs have been filtered. Now, I will configure some loopbacks on R3, run RIP on them, and redistribute them into OSPF. Let us examine the OSPF database on R3 now:

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 2.2.2.2 765 0x80000005 0x00A78B 1
3.3.3.3 3.3.3.3 10 0x80000005 0x0066C4 1

Net Link States (Area 11)

Link ID ADV Router Age Seq# Checksum
192.168.1.3 3.3.3.3 764 0x80000001 0x00DACB

Summary Net Link States (Area 11)

Link ID ADV Router Age Seq# Checksum
10.10.10.0 2.2.2.2 794 0x80000003 0x00E91B
172.16.10.0 2.2.2.2 794 0x80000003 0x0069F1

Type-7 AS External Link States (Area 11)

Link ID ADV Router Age Seq# Checksum Tag
33.33.33.0 3.3.3.3 9 0x80000001 0x00F06E 0
44.44.44.0 3.3.3.3 11 0x80000001 0x0063DA 0
R3#

I guess the RFC did not lie to us! There they are - the Type 7 LSAs! Let us examine the OSPF database and IP routing table on R2 (the ABR):

R2#show ip ospf database

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

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1120 0x80000002 0x0025E3 2
2.2.2.2 2.2.2.2 1023 0x80000003 0x007B5A 1

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
10.10.10.1 1.1.1.1 1120 0x80000001 0x009476

Summary Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
192.168.1.0 2.2.2.2 1119 0x80000001 0x00F4CB

Router Link States (Area 11)

Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 994 0x80000005 0x00A78B 1
3.3.3.3 3.3.3.3 241 0x80000005 0x0066C4 1

Net Link States (Area 11)

Link ID ADV Router Age Seq# Checksum
192.168.1.3 3.3.3.3 996 0x80000001 0x00DACB

Summary Net Link States (Area 11)

Link ID ADV Router Age Seq# Checksum
10.10.10.0 2.2.2.2 1025 0x80000003 0x00E91B
172.16.10.0 2.2.2.2 1025 0x80000003 0x0069F1

Type-7 AS External Link States (Area 11)

Link ID ADV Router Age Seq# Checksum Tag
33.33.33.0 3.3.3.3 242 0x80000001 0x00F06E 0
44.44.44.0 3.3.3.3 242 0x80000001 0x0063DA 0

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
11.11.11.0 1.1.1.1 1171 0x80000001 0x0075AB 0
33.33.33.0 2.2.2.2 235 0x80000001 0x00A3C9 0
44.44.44.0 2.2.2.2 239 0x80000001 0x001636 0

R2#show ip route

Gateway of last resort is not set

33.0.0.0/24 is subnetted, 1 subnets
O N2 33.33.33.0 [110/20000] via 192.168.1.3, 00:04:12, FastEthernet0/1
172.16.0.0/24 is subnetted, 1 subnets
O 172.16.10.0 [110/11] via 10.10.10.1, 00:17:16, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, FastEthernet0/0
11.0.0.0/24 is subnetted, 1 subnets
O E2 11.11.11.0 [110/20000] via 10.10.10.1, 00:04:12, FastEthernet0/0
C 192.168.1.0/24 is directly connected, FastEthernet0/1
44.0.0.0/24 is subnetted, 1 subnets
O N2 44.44.44.0 [110/20000] via 192.168.1.3, 00:04:12, FastEthernet0/1
R2#

Holy LSAs Batman! This router has indeed converted the Type 7s to Type 5s and installed them in the routing table. Notice the N2 designations for NSSA area prefixes. Now let us examine R1 - an internal backbone router:

R1#show ip ospf database

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

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1704 0x80000002 0x0025E3 2
2.2.2.2 2.2.2.2 1609 0x80000003 0x007B5A 1

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
10.10.10.1 1.1.1.1 1704 0x80000001 0x009476

Summary Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
192.168.1.0 2.2.2.2 1705 0x80000001 0x00F4CB

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
11.11.11.0 1.1.1.1 1755 0x80000001 0x0075AB 0
33.33.33.0 2.2.2.2 819 0x80000001 0x00A3C9 0
44.44.44.0 2.2.2.2 820 0x80000001 0x001636 0

R1#show ip route

Gateway of last resort is not set

33.0.0.0/24 is subnetted, 1 subnets
O E2 33.33.33.0 [110/20000] via 10.10.10.2, 00:13:45, FastEthernet0/0
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.10.0 is directly connected, Loopback1
10.0.0.0/24 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, FastEthernet0/0
11.0.0.0/24 is subnetted, 1 subnets
C 11.11.11.0 is directly connected, Loopback11
O IA 192.168.1.0/24 [110/20] via 10.10.10.2, 00:26:50, FastEthernet0/0
44.0.0.0/24 is subnetted, 1 subnets
O E2 44.44.44.0 [110/20000] via 10.10.10.2, 00:13:45, FastEthernet0/0
R1#

This is what we would expect as well! This device just sees these prefixes as good, old-fashioned, Type 5 LSAs.

But wait, I think we have an issue now in this OSPF domain. Let us examine the routing table on R3:

R3#show ip route

Gateway of last resort is not set

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, 00:18:43, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
O IA 10.10.10.0 [110/20] via 192.168.1.2, 00:18:43, 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
R3#

We are not getting the Type 5 being generated on R1 as expected. But we are also not getting a default route from the ABR anymore. The default route is not automatic with the NSSA. All we need to do is modify the area 11 nssa command on R2 to area 11 nssa default-information-originate. Now everyone is happy, happy. Check out the routing table now 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, 00:21:51, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
O IA 10.10.10.0 [110/20] via 192.168.1.2, 00:21:51, 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, 00:00:12, FastEthernet0/0
R3#

I hope you will join me for the next part of this blog series where we examine the Jumbo Shrimp of OSPF areas, the Totally Not-So-Stubby area!