Company ABC has multiple buildings (A, B, C and D) and two internet connections to ISP-1 (in Building-B) and ISP-2 (in Building-C). Building-A has a CORE router connected to the Border Router in Building-B (BR-B).
Both BR-B and BR-C receive a default route via eBGP from the ISPs and are configured identically to inject it into the OSPF Area 0 that covers all internal routers as shown in the diagram below:

quiz-24 "OSPF Default-Information Originate Always"


As you can see, Area 0 contains two external LSAs for the default-information (0.0.0.0) injected by each Border Routers, BR-B (192.168.15.1) and BR-C (192.168.12.2).
At this time, connectivity to the Internet (eg. 34.34.34.4) is working fine: 

CORE-A#traceroute 34.34.34.4

Type escape sequence to abort.
Tracing the route to 34.34.34.4

  1 192.168.15.1 24 msec 48 msec 8 msec
  2 1.1.1.1 32 msec 124 msec 40 msec
  3 34.34.34.4 88 msec *  68 msec
CORE-A#

Note that each BR performs NAT Overload on outside interface s0/0 (but this does not affect the quiz)!

At some moment, the link to ISP-1 (1.1.1.1) is brought down as the ISP requires some maintenance on the circuit. You assume that everything will work fine since traffic will reach internet via BR-C -> ISP-2 (2.2.2.2). Unfortunatelly, you soon find out that this is not the case: entire Building-A looses the internet access for the time that link to ISP-1 is down: 

CORE-A#traceroute 34.34.34.4

Type escape sequence to abort.
Tracing the route to 34.34.34.4

  1 192.168.15.1 52 msec 40 msec 36 msec
  2 192.168.15.1 !H  *  !H
CORE-A#

Routing and OSPF database for Area 0 look fine: 

CORE-A#sh ip ospf data
...
                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
0.0.0.0         192.168.12.2    664         0x80000003 0x00CA6E 1
0.0.0.0         192.168.15.1    1114        0x80000002 0x00BD7A 1
CORE-A#
CORE-A#sh ip route ospf
...
O*E2 0.0.0.0/0 [110/1] via 192.168.15.1, 00:51:22, FastEthernet0/0
CORE-A#

What is the problem ?


DEVICES' CONFIGURATIONS:


hostname CORE-A
!
!
!
interface FastEthernet0/0
 ip address 192.168.15.5 255.255.255.0
 speed 100
 full-duplex
!
!
router ospf 1
 log-adjacency-changes
 network 192.168.0.0 0.0.255.255 area 0
!
!
!
            
hostname BR-B
!
!
!
interface FastEthernet0/0
 ip address 192.168.12.1 255.255.255.0
 ip nat inside
 ip virtual-reassembly
 speed 100
 full-duplex
!
interface Serial0/0
 ip address 1.1.1.2 255.255.255.252
 ip nat outside
 ip virtual-reassembly
 clock rate 2000000
!
interface FastEthernet0/1
 ip address 192.168.15.1 255.255.255.0
 ip nat inside
 ip virtual-reassembly
 speed 100
 full-duplex
!
router ospf 1
 log-adjacency-changes
 network 192.168.0.0 0.0.255.255 area 0
 default-information originate always
!
router bgp 65001
 no synchronization
 bgp log-neighbor-changes
 neighbor 1.1.1.1 remote-as 100
 no auto-summary
!
ip nat inside source list ACL_NAT interface Serial0/0 overload
!
ip access-list standard ACL_NAT
 permit 192.168.0.0 0.0.255.255
!
!
            
hostname BR-C
!
!
interface FastEthernet0/0
 ip address 192.168.12.2 255.255.255.0
 ip nat inside
 ip virtual-reassembly
 speed 100
 full-duplex
!
interface Serial0/0
 ip address 2.2.2.1 255.255.255.252
 ip nat outside
 ip virtual-reassembly
 clock rate 2000000
!
!
router ospf 1
 log-adjacency-changes
 network 192.168.0.0 0.0.255.255 area 0
 default-information originate always
!
router bgp 65001
 no synchronization
 bgp log-neighbor-changes
 neighbor 2.2.2.2 remote-as 200
 no auto-summary
!
ip nat inside source list ACL_NAT interface Serial0/0 overload
!
ip access-list standard ACL_NAT
 permit 192.168.0.0 0.0.255.255
!
!
!
            
hostname ISP-1
!
!
interface FastEthernet0/0
 ip address 34.34.34.3 255.255.255.0
 speed 100
 full-duplex
!
interface Serial0/0
 ip address 1.1.1.1 255.255.255.252
 clock rate 512000
!
!
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 neighbor 1.1.1.2 remote-as 65001
 neighbor 1.1.1.2 default-originate
 neighbor 34.34.34.4 remote-as 200
 no auto-summary
!
ip route 2.2.2.0 255.255.255.252 34.34.34.4
!
!
            
hostname ISP-2
!
!
interface FastEthernet0/0
 ip address 34.34.34.4 255.255.255.0
 speed 100
 full-duplex
!
interface Serial0/0
 ip address 2.2.2.2 255.255.255.252
 clock rate 512000
!
router bgp 200
 no synchronization
 bgp log-neighbor-changes
 neighbor 2.2.2.1 remote-as 65001
 neighbor 2.2.2.1 default-originate
 neighbor 34.34.34.3 remote-as 100
 no auto-summary
!
ip route 1.1.1.0 255.255.255.252 34.34.34.3
!
!
!

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