Solved: EIGRP A.S AND OSPF AREA

mainodin87 · ‎11-30-2015

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

Peter Paluch · ‎12-02-2015

Hi Mainodin and Joseph,

R1 with OSPF process 10 interface connects to R2 with a process 10 interface (Area 0)
R2 with OSPF process 30 interface connects to R3 with a process 40 interface (Area 1)

Under these assumptions, if there is no redistribution configured on R2 between OSPF processes 10 and 30 then R2 is neither ABR nor ASBR.

R2 would be an ABR only if both its f0 and s0 interfaces were added to the same single OSPF process (either 10 or 30). The setup quoted at the beginning of the post states clearly, however, that f0 is in OSPF process 10 while s0 is in OSPF process 30. Because of that, R2 cannot act as an ABR between areas 0 and 1 in this exhibit. To R2, these areas live in entirely distinct and independent OSPF processes. Basically, OSPF process 10 on R2 knows nothing about OSPF process 30, and vice versa.

With two different OSPF processes, R2 can at most act as an ASBR. Which OSPF process would consider R2 to be an ASBR depends on the direction of redistribution:

If R2 was configured to redistribute routes from process 10 into process 30, it would act as an ASBR in process 30. Process 10 would not consider R2 to be an ASBR.
If R2 was configured to redistribute routes from process 30 into process 10, it would act as an ASBR in process 10. Process 30 would not consider R2 to be an ASBR.
If R2 was configured to mutually redistribute routes between processes 10 and 30, it would act as an ASBR to both of these processes.

Essentially, if a router is to be considered an ASBR, it must bring in some routes "from outside".

Please feel welcome to ask further!

Best regards,
Peter

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Peter Paluch · ‎11-30-2015

Hi Mainodin,

EIGRP autonomous systems and OSPF areas are not directly comparable - they do not serve the same purpose.

EIGRP (and its predecessor, IGRP) is designed from the beginning as an Interior Gateway Protocol (IGP). It is intended to be run inside an autonomous system, and specifically, it is not designed to provide the exchange of routing information between two or more autonomous systems. Designers of IGRP decided to make sure that two autonomous systems, both running IGRP, will not exchange routing information even if they are interconnected by a link that has been added to the IGRP processes in both autonomous systems. IGRP does this by having the autonomous system number specified as a part of its configuration, transmitting this autonomous system number in each IGRP packet, and comparing this number in received packets to its own configuration. If it differs, the packet is silently dropped. EIGRP being designed as a drop-in replacement for IGRP has retake this policy.

It should be mentioned that the autonomous system number in EIGRP does not really need to match the number of the autonomous system where the EIGRP is running - it can be an arbitrary number as long as it is the same on all EIGRP routers within that autonomous system. EIGRP uses the AS number to prevent routers from talking to routers in a different autonomous system. Beyond that, the exact AS value in EIGRP is not important.

In OSPF, we do not configure autonomous system numbers at all. The number in the router ospf or ipv6 router ospf or router ospfv3 command is only a process number to distinguish between multiple OSPF processes running on the same router, but it is entirely independent of the autonomous system number in which the OSPF is running. Moreover, as opposed to EIGRP, OSPF does not send out this process number in packets, so different routers can use different OSPF process IDs and still be able to talk to each other. Having different OSPF process IDs is not a best practice but from a pure technical standpoint, it does not prevent the routers from establishing an OSPF adjacency.

However, OSPF allows you to partition the entire autonomous system in which it is running into several non-overlapping parts called areas. Routers in a single area will have a detailed knowledge about the topology and the networks inside it, but regarding other areas, routers will only have an information about networks, not the topology anymore. The use of areas is traditionally explained as helping OSPF routers keeping their link-state databases reasonably small - without areas, in a network with 1000 routers and 10000 links, each single router, no matter where, would be forced to know the detailed map of the entire network including all those 1000 routers and 10000 links down to the smallest detail. Introducing areas can help significantly decrease the size of the link-state database. Today, however, the primary reasons for using multiple areas in a single OSPF domain are route summarization and route filtering. As opposed to EIGRP (and any distance-vector routing protocol), in OSPF, the only place where you can summarize multiple networks into one, or entirely filter out some networks, is the border router between two areas. OSPF does not allow route summarization or route filtering inside an area.

To sum it up, to EIGRP, the AS number is just a mechanism to make sure that EIGRP does not talk to a router that is in some other autonomous system. OSPF does not have AS numbers; instead, it has process IDs (that are only local and are never advertised anywhere), and it has areas that allow you to partition an autonomous system into a set of components which allow you to simplify the complexity of information OSPF routers in an area have to process, and allow you to perform route summarization/filtering on the area borders.

Please feel welcome to ask further!

Best regards,
Peter

mainodin87 · ‎11-30-2015

Hey Peter,

Thanks for your valuable time, most of the concept are clear now but,

as you said OSPF doesnot have A.S number and again you mentioned it has area which allows you to partition an A.S now what is this A.S which is getting partitioned into set of components, and where is it mentioned ?

Peter Paluch · ‎11-30-2015

Hello Mainodin,

as you said OSPF doesnot have A.S number and again you mentioned it has area which allows you to partition an A.S now what is this A.S which is getting partitioned into set of components, and where is it mentioned ?

In this context, an autonomous system is simply the set of all routers and their interconnections where OSPF is running. In fact, the RFC 2328 where OSPF is standardized says it very nicely:

        Autonomous System
            A group of routers exchanging routing information via a
            common routing protocol.  Abbreviated as AS.

It is assumed here that if you are an administrator of an autonomous system, you would run OSPF on all its routers, and if you had a link to another autonomous system, you would make sure in your configuration that your autonomous system boundary router and the boundary router in the different AS would not establish an OSPF adjacency to keep these two autonomous systems apart. This would isolate these two ASes and keep their OSPF instances separate, thus meeting the definition above.

Please feel welcome to ask further!

Best regards,
Peter

Joseph W. Doherty · ‎11-30-2015

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

This may confuse, but a wrinkle of OSPF, you can run more than one OSPF process on the same router, and OSPF will only naturally share its topology with other devices that form an adjacency with a particular process.

For example:

R1 with OSPF process 10 interface connects to R2 with a process 20 interface

(NB: as Peter noted, process numbers don't need to match, but they usually do)

R2 with OSPF process 30 interface connects to R3 with a process 40 interface

This creates two ASs

R1 will not know networks in (R2's process 30 or R3's process 40)

and

R3 will not know networks in (R1's process 10 or R2's process 20)

unless R2 redistributes routes between its processes 20 and 30.

R2 will know all networks in both ASs.

R2 is an ASBR.

Athough, R2 doesn't, by default, share routes between the two ASs, also by default, it will route between them.

mainodin87 · ‎11-30-2015

Damn ! why it is so confusing or i'm the only one who is confused.

as per your example, what i am getting is interface with different process are different A.Ses and what should be the area or it is nothing to do with Area ?

one more thing if R1 R2 and R3 use same process with same Area then what such network is called ?

Thank you

Joseph W. Doherty · ‎12-01-2015

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

As Peter as noted, between boxes, OSPF routers don't see each others process number. So the number itself doesn't indicate an AS. However if you see two (or more) OSPF processes on the same router, you're then probably looking at different ASs.

What makes an AS is a routing topology being shared between routers.

For example, in my prior example, if R1 adds a new subnet and inserts it into its process 10, R2 will know of it, but R3 will not (by default). So R1 and R2 are in one AS and R2 and R3 are in another AS.

If R2 redistributes routes from the R1/R2 AS to the R2/R3 AS, then R3 may also know of the newly added subnet added to R1, but this takes administrative action, it's not automatic. So R2 is an AS boundary, which in OSFP makes R2 an ASBR.

OSPF areas on the other hand don't, by default, stop sharing routes within an OSPF AS, but they share routes differently within an area vs. between areas. Within an OSPF area, link information is shared. Each OSPF area has a database of the area's topology. However, between areas, only network-prefixes/routes are shared.

In my prior example, if R1 and R2 are in one OSPF area (and there's only one process, say 25), and R2 and R3 are in another area, again if I added a network on R1, R2 and R3 would know of the network prefix, but R2 would know that the network is connected off R1, while R3 would only know the network is somewhere on the other side of R2 (R2 now being an ABR).

mainodin87 · ‎12-02-2015

Hey Joseph,

Very nice explanation provided with example, just let me know if i got it right ?

If,

R1 with OSPF process 10 interface connects to R2 with a process 10 interface (Area 0)

R2 with OSPF process 30 interface connects to R3 with a process 40 interface (Area 1)

Will this still creates two ASs ?

and

Now R2 works as an ABR for R1 and ASBR for R3 ?

Joseph W. Doherty · ‎12-02-2015

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

R1 with OSPF process 10 interface connects to R2 with a process 10 interface (Area 0)

R2 with OSPF process 30 interface connects to R3 with a process 40 interface (Area 1)

Will this still creates two ASs ?

Yes (because there are two ASs on R2)

and

Now R2 works as an ABR for R1 and ASBR for R3 ?

No and yes (because R2 has two different ASs, each AS is a single area, athough you have an area zero and one, they are not in the same AS, so you don't have an ABR)

mainodin87 · ‎12-02-2015

on the basis of above mentioned example and as per my attachment, what is R2 ABR or ASBR ?

Joseph W. Doherty · ‎12-02-2015

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

Based on your example posting, R2 would be an ASBR. However, normally the diagram would presume R2 is an ABR. What makes the difference is what OSPF process all the interfaces are in. That's not shown on the diagram, but you do note that in your "example" posting.

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?)

If R2 only had a single OSPF process, then it would be an ABR.

mainodin87 · ‎12-02-2015

Hmm .. That is where i'm getting confused, hope Peter is reading our comments :)

Peter please provide your inputs on same.

Peter Paluch · ‎12-02-2015

Hi Mainodin and Joseph,

R1 with OSPF process 10 interface connects to R2 with a process 10 interface (Area 0)
R2 with OSPF process 30 interface connects to R3 with a process 40 interface (Area 1)

Under these assumptions, if there is no redistribution configured on R2 between OSPF processes 10 and 30 then R2 is neither ABR nor ASBR.

R2 would be an ABR only if both its f0 and s0 interfaces were added to the same single OSPF process (either 10 or 30). The setup quoted at the beginning of the post states clearly, however, that f0 is in OSPF process 10 while s0 is in OSPF process 30. Because of that, R2 cannot act as an ABR between areas 0 and 1 in this exhibit. To R2, these areas live in entirely distinct and independent OSPF processes. Basically, OSPF process 10 on R2 knows nothing about OSPF process 30, and vice versa.

With two different OSPF processes, R2 can at most act as an ASBR. Which OSPF process would consider R2 to be an ASBR depends on the direction of redistribution:

If R2 was configured to redistribute routes from process 10 into process 30, it would act as an ASBR in process 30. Process 10 would not consider R2 to be an ASBR.
If R2 was configured to redistribute routes from process 30 into process 10, it would act as an ASBR in process 10. Process 30 would not consider R2 to be an ASBR.
If R2 was configured to mutually redistribute routes between processes 10 and 30, it would act as an ASBR to both of these processes.

Essentially, if a router is to be considered an ASBR, it must bring in some routes "from outside".

Please feel welcome to ask further!

Best regards,
Peter

Masoud Pourshabanian · ‎12-02-2015

Hello Peter,

Thanks for your timely and accurate answers. I encounterd an issue while I was redistributing between two OSPF processes. It was long time ago so the issue is a little blurry in my mind.

I had an issue with redistributing default route from one process to another. Default route was learned from an ASBR router other side of network and I was not able to redistrubte it to different process. I did not have any problem with other routes, only problem was with default route.

Is there any reason behing that? Or maybe I was doing something wrong.

Thanks,

Masoud

Peter Paluch · ‎12-02-2015

Hi Masoud,

Default route was learned from an ASBR router other side of network and I was not able to redistrubte it to different process. I did not have any problem with other routes, only problem was with default route.

This is expected. Cisco's OSPF implementation does not allow you to redistribute a default route into OSPF. Any other route can be redistributed into OSPF, however, default route will be ignored during redistribution. The only way to inject a default route into OSPF is to use the default-information originate command in OSPF. I assume that this is a sort of a security precaution built into Cisco's OSPF to prevent inadvertent advertisement or hijacking of a default route by the other routing protocol you are redistributing from.

Best regards,
Peter