This post represents the solution and explanation for quiz-5.
Have a look at the quiz to understand the problem.

Quiz Explained

At first look, the quiz seemed tricky because of the OSPFv3/IPv6, but in reality the same problem may affect OSPFv2 in the same manner. I used IPv6 intentionally to give the impression that the problem is related to OSPFv3 only.

As most of you already indicated in the Comments section, the junior engineer did make a mistake: he put/left the Vlan102 interface in area 0.0.0.0 (clearly shown in the output of command show ipv6 ospf int br). As a result Dist-2 router becomes an ABR (Area Border Router).

You may think: "so, what?... why can't an ABR accept that default route received from another ABR (the COREs in our quiz)?" ... let's see why:

We all know that the motivation of having an NSSA is to allow external routes (learned from different routing domains) into that area and they will be carried as Type-7 LSA.

As specified in RFC 1587, there are some bits in the Options field that are important in our context:
  • N-bit = used in the Hello packets to indicated that the router has NSSA capability on that interface (two routers will not form an ajancency unless they agree on the N-bit, meaning they are both configured as NSSA for that interface)
  • P-bit = (P - propagate) only used in type-7 LSAs to tell the ABRs to translate that type-7 LSA into a type-5 LSA. This P-bit represents a routing loop prevention mechanism.

Have a look at the picture below:

NSSA_and_P-bit_in_type-7_lsa

On the left-hand side, the ABR-1 and ABR-2 (COREs in our quiz) originate type-7 default route and they MUST NOT set the P-bit. When these type-7 LSAs reach other ABRs, since they don't have the P-bit, they will not be considered for SFP calculations and will not get to the routing table.

On the right-hand side, the ASBR (connected to other routing domains) will send the external prefixes as type-7 LSAs with the P-bit set. These LSAs will be translated into type-5 LSAs on both ABRs.

Let's have a look at OSPF's database on our Dist-2: 

Dist-2#sh ipv ospf database
...
                Type-7 AS External Link States (Area 192.168.1.0)

ADV Router      Age         Seq#        Prefix
192.168.255.1   231         0x80000001  ::/0
192.168.255.2   220         0x80000001  ::/0
...
Dist-2#sh ipv ospf database nssa-external

            OSPFv3 Router with ID (192.168.255.4) (Process ID 1)

                Type-7 AS External Link States (Area 192.168.1.0)

  LS age: 263
  LS Type: AS External Link
  Link State ID: 1
  Advertising Router: 192.168.255.1
  LS Seq Number: 80000001
  Checksum: 0x6565
  Length: 28
  Prefix Address: ::
  Prefix Length: 0, Options: None
  Metric Type: 2 (Larger than any link state path)
  Metric: 1

  LS age: 252
  LS Type: AS External Link
  Link State ID: 1
  Advertising Router: 192.168.255.2
  LS Seq Number: 80000001
  Checksum: 0x5F6A
  Length: 28
  Prefix Address: ::
  Prefix Length: 0, Options: None
  Metric Type: 2 (Larger than any link state path)
  Metric: 1

Dist-2#sh ipv route ::/0
% Route not found

As you can see, Dist-2 knows the two default injected by cores in the OSPF database, but does not put them into the routing table - as an ABR, it does not accept type-7 LSA without P-bit, in order to avoid routing loops.

Solution

Now, let's fix the mistake ( configure area 192.168.1.0 on Vlan102 interface) and check again: 

Dist-2(config)#int vlan 102
Dist-2(config-if)#ipv6 ospf 1 area 192.168.1.0
Dist-2(config-if)#end
Dist-2#
Dist-2#sh ipv route ::/0
...
ON2  ::/0 [110/1]
     via FE80::C001:1CFF:FE3C:10, FastEthernet0/1
     via FE80::C000:1CFF:FE3C:10, FastEthernet0/0
Dist-2#

Dist-2#sh ipv osp int br
Interface    PID   Area            Intf ID    Cost  State Nbrs F/C
Vl102        1     192.168.1.0     30         1     DR    0/0
Vl201        1     192.168.1.0     35         1     DR    0/0
Vl200        1     192.168.1.0     34         1     DR    0/0
Vl105        1     192.168.1.0     33         1     DR    0/0
Vl104        1     192.168.1.0     32         1     DR    0/0
Vl103        1     192.168.1.0     31         1     DR    0/0
Vl101        1     192.168.1.0     29         1     DR    0/0
Vl100        1     192.168.1.0     28         1     DR    0/0
Fa0/1        1     192.168.1.0     5          1     DR    1/1
Fa0/0        1     192.168.1.0     4          1     DR    1/1
Lo0          1     192.168.1.0     27         1     LOOP  0/0

Voila!... DIST-2 is not an ABR anymore - it's an internal OSPF router with all interfaces in NSSA area 192.168.1.0, so it accepts the default routes from ABRs (the core).

Interesting enough, if you have a look again at the OSPF database: 

Dist-2#sh ipv ospf database nssa-external

            OSPFv3 Router with ID (192.168.255.4) (Process ID 1)

                Type-7 AS External Link States (Area 192.168.1.0)

  Routing Bit Set on this LSA
  LS age: 543
  LS Type: AS External Link
  Link State ID: 1
  Advertising Router: 192.168.255.1
  LS Seq Number: 80000001
  Checksum: 0x6565
  Length: 28
  Prefix Address: ::
  Prefix Length: 0, Options: None
  Metric Type: 2 (Larger than any link state path)
  Metric: 1

  Routing Bit Set on this LSA
  LS age: 532
  LS Type: AS External Link
  Link State ID: 1
  Advertising Router: 192.168.255.2
  LS Seq Number: 80000001
  Checksum: 0x5F6A
  Length: 28
  Prefix Address: ::
  Prefix Length: 0, Options: None
  Metric Type: 2 (Larger than any link state path)
  Metric: 1

you'll notice the additional information Routing Bit Set on this LSA (that did not exist before !!). This is a Cisco implementation to indicate that the LSA is valid (passed all sanity checks) for SFP calculation.

ATTENTION

This "routing-bit" is not an actual bit in the LSA Option field, it is stored only locally, not propagated to other OSPF routers!

In the end, let's have a look how a type-7 LSA with P-bit SET looks like: suppose ASBR (Dist-1 in our case) redistributes the external prefix 2003:1:1:1::/64 

Dist-2#sh ipv route 2003:1:1:1::/64
...
ON2  2003:1:1:1::/64 [110/20]
     via FE80::C001:1CFF:FE3C:10, FastEthernet0/1
     via FE80::C000:1CFF:FE3C:10, FastEthernet0/0
Dist-2#
Dist-2#sh ipv ospf database nssa-external
...
  Routing Bit Set on this LSA
  LS age: 42
  LS Type: AS External Link
  Link State ID: 2
  Advertising Router: 192.168.255.3
  LS Seq Number: 80000001
  Checksum: 0x89B4
  Length: 36
  Prefix Address: 2003:1:1:1::
  Prefix Length: 64, Options: P
  Metric Type: 2 (Larger than any link state path)
  Metric: 20

As you can see, this type-7 LSA will be flooded into entire NSSA area and it has the P-bit set (also notice the "Routing Bit Set on this LSA").

Thanks everyone for your comments in the quiz !