A Networker Blog

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
External Route Tag: 0

Now If you do not know about 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 0×80000142 0×00BB4E 4
4.4.4.4 4.4.4.4 20 0×80000005 0×00EA1B 1
6.6.6.6 6.6.6.6 897 0×8000012B 0×005A47 3
9.9.9.9 9.9.9.9 25 0×80000143 0×006822 4
10.10.10.10 10.10.10.10 87 0×8000002B 0×00766C 2

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
192.168.14.10 9.9.9.9 26 0×80000001 0×00DB84
192.168.100.1 1.1.1.1 70 0×80000002 0×00F71A

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
2.2.2.0 4.4.4.4 16 0×80000001 0×00AC5A 0

Looking at the OSPF External 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
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#
RTR-TS1#4
[Resuming connection 4 to R4 ... ]

*Mar 2 01:09:09.225: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on
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)#
RTR-TS1#2
[Resuming connection 2 to sw1 ... ]

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: 0×2564
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#

So in short, if you do not know about the forwarding
address (192.168.24.2, above), possibly because of lsa type 3 filtering, you
can suppress the FA, this results in the FA being 0.0.0.0 which forces the
use of the ASBR to reach the destination in question.

Have Fun!

Victor Cappuccio.-