area . nssa translate type7 suppress-fa and the FA Address

The OSPF Forwarding Address Suppression in Translated Type-5 LSAs feature causes
a not-so-stubby area (NSSA) area border router (ABR) to translate Type-7 link state
advertisements (LSAs) to Type-5 LSAs, but use the address 0.0.0.0 for the forwarding
address instead of that specified in the Type-7 LSA. This feature causes routers that
are configured not to advertise forwarding addresses into the backbone to direct
forwarded traffic to the translating NSSA ABRs

For Example:
SW1#show ip ospf database external 2.2.2.0

OSPF Router with ID (9.9.9.9) (Process ID 1)

Type-5 AS External Link States

Routing Bit Set on this LSA
LS age: 58
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 2.2.2.0 (External Network Number )
Advertising Router: 4.4.4.4
LS Seq Number: 80000001
Checksum: 0xAC5A
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 192.168.24.2 --------- Forwarding address
External Route Tag: 0
This could happen if you do not know how to reach the Forwarding address (192.168.24.2)
SW1#show ip route 192.168.24.2
% Network not in table

SW1#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address
Interface
4.4.4.4 1 FULL/BDR 00:00:32 192.168.14.4 Vlan14
1.1.1.1 255 FULL/DR 00:00:37 192.168.100.1
Vlan100
10.10.10.10 1 FULL/BDR 00:00:36 192.168.100.10
Vlan100

SW1#show ip ospf database

OSPF Router with ID (150.150.0.1) (Process ID 100)

OSPF Router with ID (9.9.9.9) (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 86 0x80000142 0x00BB4E 4
4.4.4.4 4.4.4.4 20 0x80000005 0x00EA1B 1
6.6.6.6 6.6.6.6 897 0x8000012B 0x005A47 3
9.9.9.9 9.9.9.9 25 0x80000143 0x006822 4
10.10.10.10 10.10.10.10 87 0x8000002B 0x00766C 2

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
192.168.14.10 9.9.9.9 26 0x80000001 0x00DB84
192.168.100.1 1.1.1.1 70 0x80000002 0x00F71A

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
2.2.2.0 4.4.4.4 16 0x80000001 0x00AC5A 0
The 192.168.24.2 was filtered,  lets look at the External OSPF database

SW1#show ip ospf database external 2.2.2.0

OSPF Router with ID (150.150.0.1) (Process ID 100)

OSPF Router with ID (9.9.9.9) (Process ID 1)

Type-5 AS External Link States

LS age: 23
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 2.2.2.0 (External Network Number )
Advertising Router: 4.4.4.4
LS Seq Number: 80000001
Checksum: 0xAC5A
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 192.168.24.2  --------- Forwarding address
External Route Tag: 0

SW1#show ip route 192.168.24.0
% Network not in table
SW1#ping 2.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
SW1#
We see that we still have the Forwarding Address there, even
if the route is not present in the Routing Table.

Not lets use the area . nssa translate type7 suppress-fa command

Serial0.42 from LOADING to FULL, Lo
R4(config-router)#router ospf 100
R4(config-router)#
R4(config-router)#area 42 nssa translate type7 suppress-fa
R4(config-router)#
SW1#show ip ospf database external 2.2.2.0

OSPF Router with ID (150.150.0.1) (Process ID 100)

OSPF Router with ID (9.9.9.9) (Process ID 1)

Type-5 AS External Link States

Routing Bit Set on this LSA
LS age: 8
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 2.2.2.0 (External Network Number )
Advertising Router: 4.4.4.4
LS Seq Number: 80000002
Checksum: 0x2564
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 0.0.0.0 --------- Forwarding address modified
External Route Tag: 0

SW1#ping 2.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 59/62/67 ms
SW1#

SW1#show ip route 2.2.2.2
Routing entry for 2.2.2.0/24
Known via "ospf 1", distance 110, metric 20, type extern 2, forward
metric 1
Last update from 192.168.14.4 on Vlan14, 00:02:09 ago
Routing Descriptor Blocks:
* 192.168.14.4, from 4.4.4.4, 00:02:09 ago, via Vlan14
Route metric is 20, traffic share count is 1

SW1#show ip route 0.0.0.0
% Network not in table
SW1#
In short, if routers do not have the knowledge on how to reach the forwarding address,
due to some kind of lsa filtering, you can suppress the FA route advertisement and suppress 
this value, this would results that the FA be equal to 0.0.0.0 which forces the use of 
the ASBR to reach the destination.

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