Re: single area ospf with or without are 0

Hi Tomasz,

Just to expand on Scott's reply:-

Summary (Type-3) LSAs can only be generated by an ABR:

1) From a Non-Backbone area to Area 0 for:
- Connected routes
- Intra-Area routes

2) From Area 0 to a Non-Backbone area for:
- Connected routes
- Intra-Area 0 routes

No summary routes are generated unless area 0 is one of the two areas
involved. So inter area connectivity can only occur if there is an area 0.
If Summary LSAs are learned via area 0 on an ABR, they may of course be sent
into connected non-backbone areas. But a summary received from a non
backbonea area will not be forwarded into any other areas. While an ABR
must have an interface in area 0, it may or may not have a neighbor in area
0; which can change its behaviour slightly.

If a Type-3 advertisemet is received by an Area 0 ABR on a non backbone
interface, and the router did not also learn about the prefix from area 0
(i.e. multiple ABRs are connected via area 0), then it indicates that Area 0
is discontiguous. How such received type-3 advertisemets are handled by an
ABR depends on whether the router has an active neighbor in Area 0. If it
has an active neighbor in area 0 it will ignore the route advertisement. If
it does not have an active neighbor in area 0 they it will use the route
itself, but will not advertise it on into other non backbone areas.

The conditional handling of inter-area LSAs allows for an interesting (but
useless) topology to be created. Connect three routers in a triangle using
three different non-backbone areas, then create a loopback on each and
assign them to area 0.
- Although you have three discontiguous area 0s, you still have full
connectivity. Each of the routers will generate type 3 LSAs for its
connected interfaces, and the immediately adjacent routers will accept those
advertisements (they have no active neighbors in area 0).
- If one link fails, you will no longer have full connectivity, since the
two routers on either side of the failure will not be able to contact each
others loopbacks (the transit router like any other ABR will not forwarded
type-3's received from a non backbone area into any other area).
- WIth the failure fixed and your triangle of routers intact once more, if
you connect any of the three routers to a fourth router using area 0 you
will loose full connectivity (that router will no longer use type-3s
received on non-backbone interfaces).

The topology is interesting, but does not scale and does not comply with the
rfc. OSPF uses multiple areas and summary routes for scalability.
Summarsation looses some topology information, so it uses a single area 0 to
interconnect those areas in order to prevent loops in the topology. That is
why the rfc specifies that you can have only one area 0.

As has already been said, if you only need one area, pick any number you
like.

Hope this helps,

Paul.

On Wed, Nov 11, 2009 at 1:27 AM, Scott Morris <smorris_at_ine.com> wrote:

> Intra area communication is always good no matter what the area number.
> If there is to be INTER-area communications, the area 0 must exist. So
> only only one area, you could care less what the number is.
>
>
>
>
> *Scott Morris*, CCIE/x4/ (R&S/ISP-Dial/Security/Service Provider) #4713,
>
> JNCIE-M #153, JNCIS-ER, CISSP, et al.
>
> JNCI-M, JNCI-ER
>
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> Tomasz Zajac wrote:
> > Hello
> >
> > I have a simple question about ospf. Does single area (non zero)
> > implementation need connection to area 0.
>
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Received on Wed Nov 11 2009 - 19:37:29 ART