Jul
31

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:

blogospf

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!


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14 Responses to “OSPF Areas, Part 4, the Not-So-Stubby Area”

 
  1. anna says:

    Superb !!!

    Thanks a lot

  2. George Coulter says:

    Hi

    Link to part 1 no longer available

  3. Raheel says:

    Great Explaination….. amazing skills to turn confusing stuff into a peace of cake.

  4. anna says:

    IT would be great if you could explain
    Network Types in OSPF especially NBMA ones.

    Thanks a lot

    Bye
    anna

  5. pradeep says:

    Very good explaination.Thank You.

  6. Nadeem Rafi says:

    good explanation…

  7. George Coulter:

    I fixed that – thanks so much!

  8. George Coulter says:

    luvverly

  9. Márcio A Costa says:

    The “translation” happens on ABR from LSA 7 to LSA 5 and vice versa LSA 5 to LSA 7, Am I right ?

  10. Link to Part 3 is broken.

  11. [...] 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 [...]

  12. RichardB says:

    As type 5 LSA’s from another area do not make it into a NSSA, is there any way to have those routes translated into anything that would influence routing in the desired direction instead of relying on the default route (which doesn’t take the desire path toward the destinations)? This would be akin to a reverse type5-to-type7 translation happening from outside the NSSA to inside it.

 

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