OSPF ABR should be directly connected in an Area, which is not Area0?

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Sep 22nd, 2010
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Hi,


I'm planning to introduce OSPF Multiarea in a Campus Backbone based on the traditional network layer Core-Dsitribution-Access, the L3 is transported until the Access-Switches.


The Area 0 will include the Core Switches and the Distribution ones, the other areas are "mapped" on each Campus Access Block and they will include the relative Distribution switches and the Access ones.


Seeing the architecture defined due to the L1 infrastructure limitations I was wondering if the Distribution switches, which are playing the ABR roles since are connected to the Core and Access switches in a L3 redundant way have to be directly connected between each other or this is not an must point for OSPF....for a visual explanation please check the attached file...


Thanks for the feedback

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Jon Marshall Wed, 09/22/2010 - 11:24
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oguarisco wrote:


Hi,


I'm planning to introduce OSPF Multiarea in a Campus Backbone based on the traditional network layer Core-Dsitribution-Access, the L3 is transported until the Access-Switches.


The Area 0 will include the Core Switches and the Distribution ones, the other areas are "mapped" on each Campus Access Block and they will include the relative Distribution switches and the Access ones.


Seeing the architecture defined due to the L1 infrastructure limitations I was wondering if the Distribution switches, which are playing the ABR roles since are connected to the Core and Access switches in a L3 redundant way have to be directly connected between each other or this is not an must point for OSPF....for a visual explanation please check the attached file...


Thanks for the feedback


No, the 2 distribution switches acting as ABRs do not need to be interconnected,


Jon

Atif Awan Wed, 09/22/2010 - 22:11
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Just a point to note is that in case you are doing summarization on the ABRs (distribution switches in this case) if you want to avoid sub-optimal routing to the access in case of an access uplink failure, there needs to be an interconnect between the ABRs in the respective Area. In cases where the ABRs are in close physical proximity a physical link is usually used for this, however, I have seen designs where ABRs are spread across sites and logical links (GRE tunnels for example) are used. This is something not mandatory in your case but I thought it will be good to share it anyway.

oguarisco Wed, 09/22/2010 - 23:07
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HI all,


thanks a lot for sharing with me your thoughts regarding my OSPF question!


Atif, I'll keep in mind your excellent suggestion regarding Area summarization on ABR, as you pointed out our ABRs are geographically separated and unfortunately the L1 infrastructure has some limitations :-(((... moreover I would like not to use Tunnels to virtually interconnect the ABRs...


Basically if I want to introduce the OSPF concept defined in the attached file I would not summarize on the ABR so I will be sure that the best path to a destination will be always used...


Regards

Omar

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