EIGRP A.S AND OSPF AREA

[mainodin87]

Dear All,

What is the concept behind introducing A.S Autonomous System in EIGRP and Area in OSPF.

i have gone through cisco website and know how it works just want to clear some concepts, like what is actual difference between A.S and AREA is any of it related to limitation of using routers per AREA or per A.S.

What is difference when "A.S" is used with OSPF and when its is used with EIGRP.

if any one can explain with simple basic terms ?

Regards,

Mainodin

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Mainodin, warning, what I'm going to write may also cause you more confusion - it's really to (hopefully) make Peter's head hurt (mine does - laugh).

In my prior post, I wrote "BTW, for R2 to be an ASBR, it might need to redistribute between the two ASs.  If it only routes between them, I'm unsure it's truly an ASBR.  (Peter, your thoughts?)".  Peter's response, addresses my statement, and basically what he is saying is a router that redistributes into an OSPF process is an ASBR, and he's 100% (technically) correct.  He further clarifies that an ASBR can be asymetric, i.e. it doesn't do mutual distribution.  Again he's 100% (technically) correct, although this means R2 can be both an ASBR and not an ASBR at the same time, its role depends on what OSPF AS you're referencing.

However, the reason why an ASBR needing to redistribute is fuzzy (to me), is because you can configure routing for R2 to function as an ASBR without it actually being (technically) an ASBR.  I.e, if a router routes between ASs but doesn't redistribute, what kind of router is it?  Technically it's not an OSPF ASBR.

For example of functioning as an ASBR when not an OSPF ASBR, if both R1 and R3 also had a static default route to R2, you would route between the two OSPF ASs, so then R2 (technically) isn't an ASBR (or ABR).

Or, you make R2/R3 area 1 also an area 0 (continuing to assume R1/R2 and R2/R3 don't share an OSPF process) and then you always orginated a default into both ASs, again R2 would route between the two ASs without being (assuming this doesn't count as bringing in routes from the outside) an ASBR.

Or, more complex, on R2 you define two new loopbacks and define them as area 0 interfaces into the two different OSPF processes on R2, and then also change R1/R2 to area 1, and make both R1/R2 and R2/R3 area 1s a stub, again R2 would route between the two ASs without R2 being (technically) an ASBR (although now it would be an ABR - for both ASs).

[mainodin87]

Joseph, dont know about Peter but my head is spining right now.

I am doing some OSPF practical on GNS.

I've many questions in my mind but for now will go step by step let me complete some practical labs on OSPF, will also implement your example as well and will see the result it throughs out.

till then if Peter has anything on your inputs He may continue, I'll be reading (silent).

Thank you,

Mainodin

[Peter Paluch]

Hi Joe,

We're embarking on a somewhat philosophical debate here.

When you say that you can force R2 to act as an ASBR even without performing redistribution on R2 (for example, by having R1 and R3 configured with static default routes pointing toward R2), you are of course right. However, in terms of OSPF operation, this router is not an ASBR because it does not perform any functions in OSPF mandated by an ASBR.

If an OSPF router acts as an ASBR then there is a set of extra operations in OSPF this router needs to do (just what I can recall from top of my mind):

1. Advertise its ASBR status in its own LSA-1 by setting the E-bit.
2. Originate an LSA-5 or LSA-7 for every redistributed network and flood it appropriately.
3. If also acting as an ABR between an NSSA area and other regular areas (one of them must be area 0) and being the ABR with the highest OSPF RID, perform the LSA Type 7-to-5 translation
4. If operating in a VRF environment and redistributing networks from BGP, set the DN bit on all resulting Type 3, 5, and 7 LSAs and the Domain Tag on all resulting Type 5 and 7 LSAs, and during SPF computation, ignore all LSAs having the DN bit set or the Domain Tag equal to the Domain ID value.

So to OSPF, an ASBR has specific tasks to do that distinguish this router in its operation from other routers that are not OSPF ASBRs.

In a broader, non-OSPF-related sense, any router that interconnects two distinct routing domains can be considered an ASBR - but not necessarily an OSPF ASBR.

The question now is - are we talking about a generic ASBR, or about an OSPF ASBR?

Best regards,
Peter

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Peter, you've left me with nothing to debate (laugh), because you've succinctly stated what I was (poorly) trying to say, i.e. "In a broader, non-OSPF-related sense, any router that interconnects two distinct routing domains can be considered an ASBR - but not necessarily an OSPF ASBR."

In answer to your question " - are we talking about a generic ASBR, or about an OSPF ASBR?", I was initially talking generically, expecially when we were addressing both EIGRP and OSPF, but when we got more into OSPF, specifically, I didn't well clarify the distinction.  Again, your other statement does that very well; thank you.

PS:

BTW, Mainodin, in Peter's post he mentions VRF.  In that enviorment, R2 could be configured to connect to both ASs but not be an ASBR (even generically).

[Peter Paluch]

Joseph,

Thank you - I am honored!

Best regards,
Peter

[mainodin87]

Hope you got my question ?

[mainodin87]

Hey Peter,

Thanks a lot for your detail explanation, that was really helpful, am going through configuration of OSPF will get back to you if anything else is required.

Thank you,

Mainodin