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ospf and distance vector

How dos ospf prevent ospf summary's generated from backbone area from entering the backbone area ?

7 REPLIES
Cisco Employee

Re: ospf and distance vector

Parwal,

ABR will never advertise inter-area routes into the backbone.

From RFC2328, section 12.4.3:

Note that only intra-area routes are advertised into the backbone, while both intra-area and inter-area routes are advertised into the other areas.

Regards,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
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Re: ospf and distance vector

Harold,

Actually i've read this RFC many times, and i found many confusing statements in it, one of them was the statement that you've quoted, simply speaking, backbone area must have inter-area routes in order to be able to reach routes in other areas, and further on to be able to relay these routes to other areas, now how can you explain this statement in this context.

BR,

Mohammed Mahmoud.

Hall of Fame Super Silver

Re: ospf and distance vector

Mohammed

I am not sure that I fully understand your question. But let me try to answer it in this way. As a starting point let us consider an ABR. The ABR has an interface(s) in some non-zero area and an interface(s) in the area zero backbone. For simplicity let us assume that the ABR has an interface in area 1 and in area 0.

So the ABR will have learned intra-area routes from its neighbors in area 1 which it inserts into its area 1 data base. (technically we should be talking about LSAs not routes, but for this discussion we can simplify and talk about routes) And based on the intra-area routes (LSAs type 1 and type 2) it will generate inter-area routes which it inserts into its area 0 data base (LSA type 3) and which it then advertises to its neighbors in area 0. So intra-area routes are advertised from area 1 into area 0 (but an inter-area route which got into the area 1 data base from area 0 will not be advertised back into area 0).

And the ABR will learn routes from its neighbors in area 0. Some of these routes will be intra-area routes for subnets in area 0 and some will be inter-area routes which were subnets learned from other areas. The ABR has these routes in its area 0 data base. It will convert the area 0 intra-area routes (LSAs type 1 and type 2) into inter-area routes (LSA type 3) and insert these routes into its area 1 data base and will advertise them to its neighbors in area 1.

So one way to think about it is that type 1 and type 2 are always translated into type 3 and inserted into the other area data base. But type 3s are advertised from area 0 into other areas but not from other areas back into area 0.

HTH

Rick

Re: ospf and distance vector

Rick,

Thank you very much, what you've described is what i exactly understand, but i find it conflicting with the RFC statement, the statement states that "Note that only intra-area routes are advertised into the backbone, while both intra-area and inter-area routes are advertised into the other areas.", if we assume that there is a backbone router (a router having all its interfaces in Area0), this router will learn all the inter-area routes, how does this contrast with the RFC statement.

BR,

Mohammed Mahmoud.

Re: ospf and distance vector

Rick,

To compose a comprehensive statement according to my understanding, despite of the RFC statement which i find misleading, i think it is more appropriate to say "On an ABR, intra-area routes (Type1 and 2) are inserted from all the areas to area 0 as summary route (Type3), while inter-area routes in the other areas database are never inserted into area 0 database, on the other hand area 0 intra and inter area routes in area 0 database are inserted into non area 0 database as summary routes".

[edit] Sorry, i know that it might seem that i am just playing with the words, but this statement in its words has always annoyed me.

BR,

Mohammed Mahmoud.

Super Bronze

Re: ospf and distance vector

Is the statement conflicting?

Consider a router that has both area 1 and area 2 on it. None of the intra-area routes, within both areas 1 and 2 are advertised between areas 1 and 2.

However, if the router has areas 0, 1 and 2 on it, areas 1 and 2 will advertise their intra-area routes to area 0, the backbone, and area 0 will advertise to both areas 1 and 2, not only its intra-area routes, but the inter-area routes, i.e. areas 0 and 1 routes to area 2 and areas 0 and 2 routes to area 1.

Isn't "Note that only intra-area routes are advertised into the backbone, while both intra-area and inter-area routes are advertised into the other areas." just saying inter-area routes are advertised via the backbone?

Re: ospf and distance vector

Hi Joseph,

I like your analysis for the statement :), this is my point, Rick has explained it in a way (which complied to my understanding of the OSPF operation while studying OSPF operation in the past), and you analyzed it in another context "saying inter-area routes are advertised via the backbone", while i was misleaded by the statement when i read it, i was just sharing one of my historical problems, and i believe that it is clear for everybody now.

BR,

Mohammed Mahmoud.

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