OSPF-NSSA

RouterOS Forwarding Protocols
1

Does anyone like to send me back message.
My question is about the OSPF AREA-NSSA. In which situation can i play with

-------Translator role--------------

  • translate neve,
  • translate always
  • translate candidate.

Thanks

2

https://ccieblog.co.uk/ospf/ospf-nssa-translator-election-forwarding-address

In a nutshell - this helps decide which ABR will re-originate the Type7 LSA into the backbone as Type5 LSA.
This can affect how traffic is routed towards the destination.

3

Redistribution into an NSSA area creates a special type of link-state advertisement (LSA) known as type 7, which can only exist in an NSSA area. An NSSA autonomous system boundary router (ASBR) generates this LSA and an NSSA area border router (ABR) translates it into a type 5 LSA, which gets propagated into the OSPF domain. The network diagram demonstrates this principle.

4

I spent a little time googling to find out what the exact difference is between type 5 and type 7 LSAs.

I already know that 5’s are external routes, and that 7’s are external routes for NSSA - but WHY this difference? Why two types that basically do exactly the same thing? Was the format of type 7 different somehow? I never quite found a definitive answer as far as what I wanted to know about the difference. All I really found was discussions about what I already knew: 7 is for NSSA and 5 is for standard areas and the backbone area.

So I’ve had to make my own conclusion from the various tidbits I’ve picked up.
Essentially, they’re the same, but this makes the filtering of externals easier.
stub = no external routes
nssa = nobody else’s external routes
I guess it was just easier to distinguish the area’s own externals from the domain’s externals by using a different LSA type inside the NSSA.

5

Thanks

6

A stub area blocks Type 5 LSAs.
Type 7 makes it possible to bring in external routes in your routing system (i.e. backbone area) THROUGH a stub area WITHOUT breaking the stub area (i.e.: no Type 5). After passing the ABR of the NSSA, the Type 7 will be advertised as a Type 5 in the rest of the system.

7

They could just as easily have used type 5 in NSSA and made the distinction be that ABRs will not repeat Type5 from other areas into an NSSA, but will repeat Type5 from NSSA into other areas.
I’m not making an argument - my post is simply musings on why it was chosen to be done by defining a new construct which is for all intents and purposes the same thing.

Also of note is that in addition to blocking Type5 LSAs, stub/NSSA areas also do not get the Type4 LSAs which point to the ASBRs.