2 SITES in AREA 100

[john.dejesus]

Hi,

Please refer to the attached topology. I have R1 which is in AREA 100 and it is connected to AREA 0 via R2 and R5. And I have a client which is on R3 and assigned its interfaces in AREA 100. R3 is redistributing subnets from BGP. My first goal here is to perform summarization in ABRs (R1, R2 and R5). Secondly, I want to assign my client/s in the same OSPF AREA regardless of the location just for uniformity purposes. For example, if I have another client (client B for example) I will put them in AREA 150. Would this pose any issue/s? I've simulated it in GNS3 and found none. I just want to validate if this is feasible. Thank you in advance.

[Joseph W. Doherty]

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Posting

I believe, when using ABR summaries, traffic might be sent to an ABR that doesn't have reachability to an non-zero area subnet.  If it is, the ABR will not forward the traffic back to another ABR across area 0 if it's in the summarized area.  For example, if traffic is sent to R2 or R5 for 192.168.10.0/26, because of the summary for 192.168.10.0/24, it will be dropped.  This isn't a problem when the non area 0 subnets are not summarized.

[john.dejesus]

Hi Joseph,

Got it! the ideal router to perform the summarization would be on R1. What do you think about my second objective?

[JohnTylerPearce]

Secondly, I want to assign my client/s in the same OSPF AREA regardless of the location just for uniformity purposes

This goal will work, as long as each Area 100 has access to Area 0. Since they the nonbackbone areas are disconnected they will appear to each other as Network Summary LSAs os Type 3 LSAs.

[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 whatsoever (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

Got it! the ideal router to perform the summarization would be on R1. What do you think about my second objective?

If you only summarize routes behind an ABR that is the transit to those subnets, that should be okay.

Again, I believe you can partition the same area as long as specific subnets are advertised.

What areas really control is what neighbor/adjacent routers will share the link state database.  Once you get to an ABR, it's routes that are exchanged.  This is why you can partition the same area or other areas, area zero doesn't really care what the non-zero area value is.

If you ping R2's interface to R3 from R5's interface to R3, it will transit via R3 even if the R3 to R2 area 0 link has a better metric because intra-area paths are preferred over inter-area paths.  Conversely if R3's two interfaces were in different areas (e.g. areas 100 and 150), the R5's interface to R3 could not ping R2's interface to R3 unless one of the area 0 paths to R2 was good; i.e. R3 would never be used for transit.

Although you can do this, I would recommend you don't intentionally partition any area as it can lead to "surprises".  If your network isn't too large, just use one area (the old rule of 50 routers per area is rather "elastic" especially on newer equipment).  If you need to use different partitioned areas, number differently, unlikely you're going to run out of area numbers.