OSPF Stub Area routes are Internal Routes to OSPF domain.

[mahesh18]

Hi all,

Need to confirm scene below.

Say R1 is running EIGRP R2 is in area 0 and it is redistributing OSPF routes to EIGRP domain.

R3 is ABR connects to Area 0 and Area 1 .

Area 1 is stub area.

So when OSPF routes come to OSPF Domain area 0 and EIGRP router R1  they will come as internal routes as stub does not allow LSA type 4 and 5 Right?

Thanks

[Giuseppe Larosa · Accepted solution]

Hello Mahesh,

LSA type 5 and LSA type 4 are filtered at stub area boundary by ABR R3. They are simply not sent out of the interfaces associated with the stub area. That's all.

A stub area is an area with a  minimized database size. ABR router injects an O IA default route into the stub area in order to give an exit point to routers internal to the stub area.

LSA type 5 or 4 are not converted into internal routes

No ASBR can be internal to  a stub area and only internal routes are there.

if a router is ASBR in area 0 and it is also connected to a stub area, it does not inject LSA type 5 into the stub and does not provide any form of conversion / translation.

note: NSSA provides a way to have an ASBR in a NSSA area with control of propagation of LSA type 7 at area border. (conversion to LSA type 5 to area 0 that is the opposite direction of what we are discussing here)

In the case of a totally stub area the only O IA route is the default route injected by ABR and also all LSA type 3 are filtered/ blocked at area boundary.  This provides the minimum database size for routers that are internal to the  totally stub area

Hope to help

Giuseppe

[Richard Burts]

Mahesh

I am somewhat not understanding your question. You seem to be asking about OSPF routes coming to R1. But if I understand your question then R1 is running only EIGRP and so will not receive any OSPF advertisement. R1 will learn the routes that originated in OSPF from R2 and will learn them as EIGRP external routes.

But your point is quite correct that a stub area will have only OSPF internal routes inside the area and all OSPF routes that originate in a stub area are advertised to area 0 as OSPF internal routes (O IA based on type 3 LSA).

HTH

Rick

[mahesh18]

Hi Rick,

Yes i was asking about any OSPF routes coming to R1 as R1 is running EIGRP only and connected to R2 which is in Area 0.

When you say this --

R2 will learn the routes that originated in OSPF from R2 and will learn them as EIGRP external routes.

Does it mean that R1 will learn the routes that originates in OSPF from R2 and will learn them as EIGRP external routes only if R2 is redistributing OSPF routes in the EIGRP doamin right ??

Thanks

Mahesh

[smehrnia · Accepted solution]

Hi Mahesh,

Look... from R1 point of view, it doesn't see no OSPF network / topology at all. it just Sync its EIGRP database with R2. since R2 does run both OSPF and EIGRP routing protocols and is redistributing between them, whatever route that R2 redistributes from OSPF into EIGRP is considered eigrp External and passes it to R1 as it is (external). although you should keep that in mind that both R1 and R2 must operate under a same eigrp Autonomous System to sync databases.

Also OSPF Stub area only knows about its own internal networks and to get to the outside networks, it just needs a default route (all to shrink the datebase and process load on the stub routers)

plz Rate if it helped.

Sorush

[Richard Burts · Accepted solution]

Mahesh

I see that I made a mistake in the names in my post. I said:

R2 will learn the routes that originated in OSPF from R2 and will learn them as EIGRP external routes.

and what I meant to say is

R1 will learn the routes that originated in OSPF from R2 and will learn them as EIGRP external routes.

And it is correct that R1 will learn OSPF routes from R2 as EIGRP external only if R2 has learned the OSPF routes and has redistributed them into EIGRP.

HTH

Rick

[mahesh18]

Hi All,

Many thanks again for answering my question.

Regards

MAhesh