OSPF Question

[kevin.hu]

Hi all,

Is it true that a router can only learn inter-area routes thru an ABR even though one of its intra area OSPF neighbor has that inter-area route? Is there any documentation on that? Thank you!

[Richard Burts]

Kevin

If I understand your question correctly then the answer is yes routers only learn inter-area routes from an ABR.

To clarify let us assume a network with two routers (routerA and routerB). Let us assume that routerA has interfaces in area 1 and also in area 2. (this means that it has routes for two areas, but since it does not have interfaces in area 0 it is not an ABR.) Let us assume that routerA is connected to routerB via an interface in area 1 and that routerB has its interfaces all in area 1. routerA will not advertise routes from area 2 over the area 1 interface and routerB will not learn the area 2 routes from routerA.

Right now I am not sure that I can point to documentation to support this, but I have tested this in the lab and I am very sure that this is how the Cisco implementation of OSPF works.

HTH

Rick

[marikakis]

When routing a packet between two non-backbone areas the backbone is used.

The OSPF backbone area is responsible for the dissemination of inter-area routing information.

Inter-area routing can be pictured as forcing a star configuration on the Autonomous System,

with the backbone as hub and each of the non-backbone areas as spokes.

In order to be able to route to destinations outside of the area, the area border routers inject

additional routing information into the area. This additional information is

a distillation of the rest of the Autonomous System's topology.

This distillation is accomplished as follows:

Each area border router is by definition connected to the backbone.

Each area border router summarizes the topology of its attached non-backbone areas

for transmission on the backbone, and hence to all other area border routers.

An area border router then has complete topological information concerning the backbone,

and the area summaries from each of the other area border routers.

From this information, the router calculates paths to all inter-area destinations.

The router then advertises these paths into its attached areas. This enables the area's internal

routers to pick the best exit router when forwarding traffic inter-area destinations.

3,4 Summary-LSAs

Originated by area border routers, and flooded through-out the LSA's associated area.

Flooding procedure may decide not to flood an LSA out a particular interface, if there

is a high probability that the attached neighbors have already received the LSA.

The above is a selection I consider relevant from the RFC 2328 (OSPF v2, especially Sections 3.2,4.1,13).

You can find this in e.g. ftp://ftp.rfc-editor.org/in-notes/rfc2328.txt

If you go to the above URL, you can do a find (on This Page) with keywords : inter-area, flooding.

In summary:

You do need backbone connection somewhere/somehow as already pointed out by Rick.

The ABR will inject inter-area information concerning other areas into each of its attached areas,

and then flooding inside the area will follow.

Flooding might not be done in some cases (e.g. back the broadcast interface information was received from).

Do you have a specific problem with a particular topology, or is this question purely academic ?

Regards,

Maria