OSPF NSSA area: Does the ASBR needs to be directly connected to the ABR

[Steph1963]

Hi to All,

I am presently preparing for my Troute exam and I would like to understand where an ASBR fits in an OSPF Not-So-Stubby Area.

I would like to know if the ASBR must be directly connected to the ABR or of it can be connected via other routers that are part of the NSSA. Is there any advantages of connecting the ASBR to the ABR in that type of area. I am also curious to know what the other rotuers in the NSSA area are doing with LSA type 7, can they used these LSA in their routing table or will they route packets to unknown destination via the default network generated by the ABR following the ASBR presence in the NSSA?

Can we also expect an ASBR to redistribute LSA type 7 generated by an ABR into a NSSA area into a different protocol.

Thanks for your help

Stéphane

[Eehab Suliman]

It can be connected via other routes. ASBR does not have to be directly connected to ABR. There is not advantate of connecting the ASBR directly to the ABR.

Internal routers in the NSSA will use the type 7 to build a route to the external destination advertised by the type 7 LSA.

It will send send type 7 to all the routers in the NSSA including the ABR.

When the type 7 reaches the ABR it will inspect the P bit on the type 7 packet which is normally set by the ASBR when it first generate the type 7 packet.

The ABR will then translate the type 7 to a type 5 and send it to the rest of the OSPF domain (other areas) or block the type 7 and not forward it to other areas. This decision is made based on the P bit.

Type 7 will be create an ospf route after the ospf algorithm runs again the LS database. All OSPF routes can certaily be redistributed into other protocols.

Please rate answers you find helpful.

[Giuseppe Larosa]

Hello Stéphane,

an ASBR within an NSSA doesn't need to be directly connected to the NSSA area ABR(s)

But only one ABR node, the one with the highest OSPF router-id will convert to LSA type 5 those LSAs with appropriate P settings (propagate).

NSSA internal routers use LSA type 7 for routing purposes because they receive ASBR router LSA with appropriate bits set to signal it is an ABR, they don't need to see an LSA type 4, the same happens in a standard LSA for LSA type 5 generated by an ASBR that is within that area.

>> Can we also expect an ASBR to redistribute LSA type 7 generated by an ABR into a NSSA area into a different protocol.

actually an ASBR is a router that injects routes into the OSPF domain.

OSPF routes injected in another routing protocol are regulated by the usual rules:

-only routes used and installed in the routing table can be redistributed

- the receving protocol may need a seed metric to work properly ( i.e. for EIGRP is mandatory)

Hope to help

Giuseppe

[Steph1963]

Hi Giuseppe,

Thanks for your answer,

Just for my understanding, are you saying that a LSA type 4 is require for a router to recognize and acknowledge an LSA type 5 in a backbone or nomal area and router in a NSSA area does not need somehing as a LSA type 4 to install a LSA type 7  in it`s routing table.

I was never able to find out the utility of a LSA type 4.

Thanks for your help

Stephane

[Giuseppe Larosa]

Hello Stephane,

LSA type 4 are used to decide if an OSPF LSA type 5 can be used or not, when the ASBR that originated that LSA type 5 is outside the local OSPF area.

if the ASBR is within the local area, the local node can directly check some bits in the router LSA of the ASBR, and it doesn't need to consult an LSA type 4, and indeed an LSA type 4 for an ASBR located in area X exists only in OTHER areas ( 0 and not totally stub areas) created by ABRs, as a way to relay ASBR health check.

See it another way: OSPF router-id can be advertised or not , so LSA type 4 provides the check that says:

"that remote ASBR is really alive you can use that LSA type 5 originated by him."

Hope to help

Giuseppe

[Steph1963]

Hi Giuseppe,

Just need a last clarification, this LSA type 4 mechanism does not exists in a NSSA area. Does the router install LSA type 7 without checking anything since the ASBR is in the same aera ?

Many thanks for your very useful help

Stéphane

[Giuseppe Larosa]

Hello Stephane,

other routers that are inside the same NSSA can install the LSA type 7 by checking ASBR router LSA that is available to them (E bit meaning the node is injecting routes into the OSPF domain)

clearly LSA type 4 for ASBR nodes outside the NSSA area are not needed as the corresponding LSA type 5 are blocked by ABR nodes.

so yes at the end you shouldn't find any LSA type4 inside the database of an NSSA area

Hope to help

Giuseppe