OSPF - Type 5 and Type 7 LSA Comparison

R2 --- R5 is in standard area 25.
R5 is and ABR connected to area 0 as well.
R2 is redistributing connected interfaces.
These show up as E2 routes on R5.

Here is how he type LSA for R2's loopback 2.2.2.2:

R5# show ip ospf database external 2.2.2.2

OSPF Router with ID (5.5.5.5) (Process ID 1)

Type-5 AS External Link States

Routing Bit Set on this LSA
LS age: 121
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 2.2.2.2 (External Network Number )
Advertising Router: 2.2.2.5
LS Seq Number: 80000001
Checksum: 0x3D50
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0

Now Let's convert area 25 to an nssa area:

R5(config)#router ospf 1
R5(config-router)#area 25 nssa

R2(config)#router ospf 1
R2(config-router)#area 25 nssa

Here is how the type 7 LSA looks, can you see the difference?

R2#show ip ospf database nssa-external 2.2.2.2

OSPF Router with ID (2.2.2.5) (Process ID 1)

Type-7 AS External Link States (Area 25)

LS age: 85
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 2.2.2.2 (External Network Number )
Advertising Router: 2.2.2.5
LS Seq Number: 80000001
Checksum: 0x7A8
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 172.12.25.2
External Route Tag: 0

R5 converts this to a type 5 LSA before flooding to area 0. Here's how it looks now:

R5# show ip ospf database external 2.2.2.2

OSPF Router with ID (5.5.5.5) (Process ID 1)

Type-5 AS External Link States

LS age: 100
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 2.2.2.2 (External Network Number )
Advertising Router: 5.5.5.5
LS Seq Number: 80000001
Checksum: 0x535D
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 172.12.25.2
External Route Tag: 0

Notice that the forwarding address is listed as 0.0.0.0 in the first example. After area 25 was changed to a nssa, then the forwarding address appeared in the type 7 lsa and in the translated type 5 lsa.

Also notice the type 7 lsa has an advertising router of 2.2.2.5 (R2's router-id) but the type is advertised by R5 for flooding into area 0 and beyond. In the original type 5 lsa the advertising router was 2.2.2.5 throughout the ospf domain. Take a look below at R3 in a distant area 345.

When area 25 is standard area:

R3#show ip osp database external 2.2.2.2 | inc Advertising
Advertising Router: 2.2.2.5

When area 25 is nssa:

R3#show ip osp database external 2.2.2.2 | inc Advertising
Advertising Router: 5.5.5.5

We can also supress the advertisement of the forwarding address with this command on R5, the ABR:

R5(config-router)#are 25 nssa translate type7 suppress-fa

R3#show ip osp database external 2.2.2.2 | inc Forward
Forward Address: 0.0.0.0

Why would we do this? not exactl sure right now, but I have read the when the forward address is 0.0.0.0 then the router sends the packet to the advertising router...it seems this would have happened anyway...right?

I remember running into problems with type7/5 translation when I was doing labs studying for the CCNP routing exam. There were some interesting issues and I'll do some more research. This was just a quick lab to see what happens to external lsa depending on the type of area.

Here's a link that explains more:

When to Suppress OSPF Forwarding Address in Translated Type-5 LSAs