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ospf autonomous system number

Answered Question
Apr 2nd, 2006
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Hi,

I have one doubt from many days. In case IGRP, EIGRP, BGP we will specify autonomous system number.

But coming to RIP and OSPF we don't specify autonomous system number.

If this is the case, either RIP or OSPF belongs to which autonomous system how to know?


Regards

skrao

Correct Answer by pkhatri about 11 years 4 months ago

Hi,


There is absolutely no concept of AS number with RIP or OSPF. The only way to enfore AS boundaries in OSPF and RIP is by the careful use of the 'network' statement so that you only enable the protocols on links which belong to your 'autonomous system'. If you make a mistake and enable RIP on an external-facing interface, and the router at the end is also configured for RIP, RIP will happily exchange routes with this neighbor. As you know, that will not happen with EIGRP or BGP since the AS numbers have to match up.


So, in a nutshell, the answer is that there is no way to enforce the boundary within the protocol itself - you have to set up the boundaries manually.


Pls do remember to rate posts.


Paresh

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Correct Answer
pkhatri Sun, 04/02/2006 - 04:07
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Hi,


There is absolutely no concept of AS number with RIP or OSPF. The only way to enfore AS boundaries in OSPF and RIP is by the careful use of the 'network' statement so that you only enable the protocols on links which belong to your 'autonomous system'. If you make a mistake and enable RIP on an external-facing interface, and the router at the end is also configured for RIP, RIP will happily exchange routes with this neighbor. As you know, that will not happen with EIGRP or BGP since the AS numbers have to match up.


So, in a nutshell, the answer is that there is no way to enforce the boundary within the protocol itself - you have to set up the boundaries manually.


Pls do remember to rate posts.


Paresh

sivakondalarao Sun, 04/02/2006 - 04:16
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Thanks paresh,

suppose I am running OSPF and EIGRP in my network. On the boarder router( which is connecting OSPF network and EIGRP network) I configured redistribution (i.e, from OSPF->EIGRP, EIGRP->OSPF).

Now is these two protocols domains treated as two autonomous systems?

Otherwise I am running only OSPF protocol, but with two process numbers. Now these two process domains are treated as two autonomous systems?

Like these confusions many to me...............


Regards

skrao

pkhatri Sun, 04/02/2006 - 04:23
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The OSPF and EIGRP networks will be treated as two separate routing domains (you can use the term autonomous system but that term is probably more of an indication of an administrative boundary,not a technical one).


Two separate OSPF processes are considered to support two separate OSPF domains. You can consider these two domains to be different ASs but this once again depends on who has administrative control of these domains. If you are using the technical definition of an AS, then yes, these two processes would be analogous to two separate EIGRP ASs.


Pls do remember to rate posts.


Paresh

mheusinger Sun, 04/02/2006 - 17:09
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Hello,


maybe this is more a question about the definition of an AS. Ans autonomous systems is a collection of networks under a single administration. This is how BGP defines an AS. Now within a BGP AS there can be many different IGPs (like EIGRP and OSPF). And as Paresh already mentioned: there is no concept of an AS within OSPF, just OSPF domains. So an OSPF area could be span across several BGP AS (though a very unlikely case unless in BGP confederations).


And within one BGP AS you could setup a portion of your network with EIGRP and another portion with OSPF and use mutual redistribution.


One thing to remember is, that each IP routing protocol will only "see and understand" it´s own world. Any information in another routing protocol is not visible (ships in the night concept) unless you insert the info through redistribution.


Thus also AS numbers in EIGRP and in BGP are not checked against each other and OSPF is not aware of any AS number at all.


If you look at one IP routing protocol then forget about all terms and wording you learned within the context of another routing protocol. They might or might not mean the same and could cause confusion. In fact the did, as I assume :-)


Hope this helps! Please rate all posts.


Regards, Martin

Odys (CSC) Wed, 03/07/2012 - 15:45
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no "Autonomous System" concept at OSPF ? what does then ASBR stand for ?

Peter Paluch Wed, 03/07/2012 - 23:27
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Hi Hydir,


Martin has described the issue with "autonomous systems" and IGP protocols very nicely. The concept of autonomous system is used in two slightly different approaches.


The first approach is how BGP uses the autonomous systems: to associate a set of networks as being reachable in or through a particular autonomous system. In other words, in this approach, the entire network topology is split into individual standalone entities - the autonomous systems, and the routing is concerned with knowing which ASes must be traversed in order to reach the particular destination. Routing protocols here must be capable of carrying routing information between ASes and to tag the routing information with the AS numbers it has traversed so that both routing loops are prevented and shortest paths (in terms of number of ASes to traverse) are used. In this approach, the routing protocol runs both within and between ASes, or above ASes, so to say.


The second approach is how IGP protocols use the concept of autonomous systems: to define the boundaries of a particular network topology beyond which the IGP should not run. Note that this is quite different from the first approach: while the first approach was concerned about carrying routing information from one AS to another, here, we are trying to contain the routing information within the AS without leaking it into another. That is precisely what EIGRP "autonomous system" numbers do - two routers with different EIGRP AS setting are not going to establish an adjacency (note the difference to BGP - in BGP, two routers in different ASes happily establish a peering after a proper configuration). So once again, in IGP protocols, we do not talk about autonomous systems as standalone entities with the intention of knowing which network is in which AS and what ASes must be traversed to reach that network (in essence, looking on the entire network "from above" and knowing all ASes and their contents), rather, the autonomous system in IGP approach is simply the boundary where the particular IGP stops and does not go further.


The ASBR in OSPF should be understood in this very sense. It is simply a router where the OSPF domain ends and some other routing domain starts, but that is all. The ASBR exists in order to import information from outside the OSPF domain into it in a controlled manner, but it is not supposed to identify other ASes, their numbers etc. - rather, it is only a router at our network boundary that allows us to see what is beyond that boundary in a limited sense. Also note that the OSPF does not take care of exporting our information from our AS to another AS - again diverging from the first approach to understanding the concept of AS.


Does this help a little?


Best regards,

Peter

Odys (CSC) Sat, 03/10/2012 - 10:33
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If I understand it correctly, let me say these points:


i--  Concerning EGP protocols like BGP

The standard concept of AS belongs to it. So BGP works as inter-AS protocol.




ii-- Concerning IGP protocols like EIGRP & OSPF


An AS in EIGRP is similar to an Area in OSPF (because all the routers within that AS share the same ASnr and routing info, just like the concept of Area with OSPF)


An AS in OSPF represents a network domain concept, not a real AS.


ASBR would help to pick up routing info from a remote router in another AS, but it won't fully connect to that AS.


something like that ?


by the way..... Martin ? do you still know the link to his post ?


Best Regards

Peter Paluch Sat, 03/10/2012 - 15:38
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Hello Hydir,


i--  Concerning EGP protocols like BGP

The standard concept of AS belongs to it. So BGP works as inter-AS protocol.


Yes, I agree.


An AS in EIGRP is similar to an Area in OSPF (because  all the routers within that AS share the same ASnr and routing info,  just like the concept of Area with OSPF)


I do not agree with this description. In OSPF there is, by default, unlimited flow of routing information (not topological information) between areas. In EIGRP, two routers in different ASes do not even establish an adjacency.


An AS in EIGRP is a set of EIGRP routers mutually exchanging routing information. The boundary between two ASes is also the boundary where two EIGRP routers won't create adjacencies and won't exchange any routing information.


An AS in OSPF represents a network domain concept, not a real AS.


Yes, I agree, but this would be a perfect explanation for the AS in EIGRP as well.


ASBR would help to pick up routing info from a remote router in another AS, but it won't fully connect to that AS.


In principle, true. A particular IGP routing protocol is concerned about carrying its own routing information and about importing foreign routing information into it. No IGP cares about exporting its information to other routing protocols (it is not necessary - other routing protocols care about importing routes which is the same problem, just from a different end). So it is true that an ASBR picks up routes from different ASes and imports it into OSPF. If that also happens in the opposite direction - it may, but it is no longer the responsibility of OSPF.


Best regards,

Peter

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