OSPF Area-id

[vpalania]

Hi ,

Is OSPF Area-id significant locally?Then, why is it also inculded in OSPF packet header( for type 3, type5 packets which are transmitted to other areas)?

Consider there are two 'area2' , when the backbone receives update from these 2 areas, then how it differentiates them?

Could someone explain it with an example?

Thanks,

Vijay

[joyride_us]

H,

I am not sure of the answer but here is mine!

You are right that 'OSPF area#' is included in the Hello messages : this is actually how ABR's are automatically found!

If you have 2 'area2' : it depends how they are connected : if they are connected then they are actually 1 area! If not, which I would think is your case, I think it should be fine because routing is not based on areaID but subnet addresses. The AreaID just helps define the raltionship of routers with each other either whether they are in the same area or not.

If you area2's are totally stubby, still no problem as the area0 between them still see their respective subnets...

Let's see what others think!

Hope this helps a bit!

[kamlesh.sharma]

Hi,

It may be because of area authentication. The authentication type is configurable on per-area basis.and for the same area configuration there should be diffrent ABR for the same area.like

area2 ----ABR1----area0-----ABR2-----area2

And in this topology you may have different authentication and possibly for putting ASBR in same areas.

[pkhatri]

Hi Vijay,

The OSPF area-ID is NOT locally significant. All routers in an area must agree that they belong to the same area for them to actually belong to the same area. However, the only enforcement of this understanding is done on a link-link basis. Therefore, including the Area ID in the OSPF packet header allows routers to determine what area the neighboring router belongs to and whether or not to establish a neighbor relationship with this neighbor. So that should answer your first question.

I presume that when you say "type 3, type 5 packets" you are meaning type-3 and type-5 LSAs and not OSPF packet types 3 and 5, which are two different things. You need to understand that all OSPF packets only travel a single hop only (TTL=1). LSAs are carried in OSPF packets and during the process of flooding, they are carried within OSPF packets that change on every hop.

If you have two areas with the same number, the following possibilities may occur:

1. If the two areas are logically discontiguous, the backbone treats them as two separate areas. This is poor design but does work.

2. If the two(sic) areas are not logically discontigous, they are in effect the same area.

Apart from ABRs, the backbone does not have an understanding of area numbers. Every OSPF packet travelling within the backbone is associated with Area 0. The fact that LSAs carried within these packets originate from non-backbone areas is immaterial.

Hope that helps :-)

Paresh.

[vpalania]

Hi,

Thanks for your explanation.I think you have

misunderstood the words 'Area-id significant locally', here locally means within an Area and from your explanation I understood 'Area-id significant locally'.

For your second explanation,I didn't understand 'logically discontigous'.

Thanks,

Vijay

[pkhatri]

Hi Vijay,

What I meant by that was: the two areas should not have any links between them which is configured with the same area-id as those two routers.. because this just makes it one big area instead of two separate areas.

Paresh.