What is the exact definition of ABR in ospf?

[Ravi Pande]

Is it the router that connects 2 different areas or router which has one interface in area 0(backbone area) and another interface in any other area.

Thank You

ravi

[Richard Burts]

Ravi

This is an interesting question and the answer depends on your frame of reference. A strict reading of the RFCs about OSPF basically says that ABR connects two areas and does not specifically require that one be area 0. So in the frame of reference of the RFC area 0 is not required.

But the Cisco implementation of OSPF does require that one area be area 0. So from the frame of reference of Cisco router behavior then area 0 is required for ABR.

HTH

Rick 

[Rolf Fischer]

Rick,

A strict reading of the RFCs about OSPF basically says that ABR connects two areas and does not specifically require that one be area 0.

I agree that the "Area border router" paragraph of RFC 2328 doesn't state the backbone area requrirement but the very next ("Backbone routers") does:

3.3.  Classification of routers

Area border routers
A router that attaches to multiple areas.
Area border routers run multiple copies of the basic algorithm, one copy for each attached area.
Area border routers condense the topological information of their attached areas for distribution to the backbone.
The backbone in turn distributes the information to the other areas.

Backbone routers
A router that has an interface to the backbone area.
This includes all routers that interface to more than one area (i.e., area border routers).
However, backbone routers do not have to be area border routers.
Routers with all interfaces connecting to the backbone area are supported.

As you know, that's sometimes the trouble especially with the larger RFCs: The information is often divided over several paragraphs and even chapters.

Best regards
Rolf

[Joseph W. Doherty]

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Posting

Hmm, I would say Rick's answer, from the RFC, is correct, i.e. an ABR is just an OSPF router connected to more than one area.  An area zero connection isn't required.

An OSPF router configured as an ABR, without a connection to area zero, will route traffic within and even between its connected areas.  However, it won't share one area's routing information with another area's OSPF neighbors w/o area zero.

Having an ABR w/o an area zero connection would be unusual.  I.e. ABRs are usually also, by RFC definition, "backbone" routers too.

(BTW, without reading the RFC information posted by Rolf, off-the-cuff, I would have answered OP question that an ABR requires an area zero connection in addition to another area or areas connections.  By the RFC, though, that appears to be incorrect, although again, that's normal usage.)

[Rolf Fischer]

It wasn't my intention to say Rick's answer is incorrect, and I understand Rick's and your point: The RFC indeed doesn't say explicitly that an OSPF router is not an ABR when it doesn't have a backbone connection.

Instead you can find statements like this:

3.1 (...) "The OSPF backbone always contains all area border routers."

Hard to say what John Moy had in mind when he choosed such formulations. However, the main functionaltity I associate with an ABR is inter-area routing, and this doesn't happen (at least not for their neighbor-routers) without a backbone connection. Just my two cents ;)

EDITED:

Found this in the abstract of RFC 3509 (Alternative Implementations of OSPF ABRs):

"Though the definition of the Area Border Router (ABR) in the OSPF specification does not require a router with multiple attached areas to have a backbone connection, it is actually necessary to provide successful routing to the inter-area and external destinations.  If this requirement is not met, all traffic destined for the areas not connected to such an ABR or out of the OSPF domain, is dropped."

[Richard Burts]

Rolf

I certainly did not interpret your response as saying that my answer is incorrect and I hope that Joseph did not either. As I said before I believe that what we see as an answer depends on our perspective looking at the question. I believe that a person doing a very strict reading of the RFC could look at individual points and individual paragraphs and come away saying that area zero was not required for ABR. You make a very excellent point that when we take a broader look at the RFC and the relationship between paragraphs that it is quite clear that the intent was to have area zero connection for an ABR. And there is no doubt at all that the Cisco implementation of OSPF does require area zero connection for ABR.

HTH

Rick

[Joseph W. Doherty]

Rick, you note Cisco's implementation won't allow a router to run connected to more than one OSPF area w/o one being area zero?  I think (?) I've done it in the past; some funky DMZ setup.  Could you further explain how Cisco's implementation blocks this?

BTW, if you're thinking about such a set up doesn't share one area's routes with any other area, w/o an area zero, that's true.  But that router's route table should contain all the areas routes.

[Richard Burts]

Joseph

You misunderstand what I said. I did not say that Cisco implementation wont allow a router to run connected to more than one OSPF area without one being area zero. Quite obviously Cisco implementation does allow a single router to be connected to multiple areas with none of the areas being area zero. What I said was that in this situation the router would not function as ABR.

The critical thing about an ABR is that it takes the intra area routes (based on LSA type 1 and 2 within an area) and generates inter area routes (LSA type 3) and advertises inter area routes between areas. With a connection to area zero the router does this and without a connection to area zero the router does not do this.

HTH

Rick

[Joseph W. Doherty]

Rick, you're correct, I did misunderstand.

Assuming we're all in agreement that an ABR interconnects area zero and other areas, I guess what threw me was your mention "And there is no doubt at all that the Cisco implementation of OSPF does require area zero connection for ABR."

I was wondering whether you knew of any other (non-Cisco) vendor that operated differently.  If not, then all vendors would appear to likewise agree with the foregoing definition of an ABR.  (The latter would help address Ravi's 2nd posting's questions.)

[Richard Burts]

Joseph

I believe that we are in agreement  from a practical standpoint about the requirement for area zero and mostly in agreement from the theoretical standpoint. While I know some about what some other vendors do I am certainly not authoritative on other vendor implementations of OSPF and wanted to qualify my answer in terms of what I do know to be the case. I do not know of any other vendor who implements an ABR without connection to area zero. But I stop short of saying that there are none.

HTH

Rick

[Joseph W. Doherty]

"HTH" - it does - thanks Rick.

[Joseph W. Doherty]

Ah, with that statement, I would now say the RFC does expect/require an ABR to have a connection to area zero.  If so, my initial off-the-cuff answer would have been correct, and Rick's answer, less than correct.  ;)

[Ravi Pande]

Thank  you for your comment.

If i am asked in an interview so what should i say?

Because when it comes to practical world, apart from cisco how other vendors define?

[Richard Burts]

I would suggest that your answer would depend on whether you prefer to keep things simple or whether you are comfortable discussing things when there is some complexity and perhaps some ambiguity about the topic.

The safe and simple answer is that ABR needs a connection to the backbone and to at least one other area.

HTH

Rick