OSPF area boundary question

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Aug 31st, 2010
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We have the following network topology:

- MAN ring (4 6500 switches connected in ring) in OSPF area 0

- remote location with 3750 stack connected via two fast ethernet uplinks to two of the cat 6500 in the MAN ring - this 3750 is in the different OSPF area.

The question is: where should we have area 0 boundary?

1. On the remote 3750 - making it ABR router, and placing fast ethernet uplinks in the area 0 ?

2. On the 6500's belonging to the main MAN ring, placing fast ethernet uplinks in the "remote" OSPF area ?

Basically: should the remote OSPF area reach the core switches, or should it end in the remote location?



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Peter Paluch Tue, 08/31/2010 - 05:50
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Hello WM,

I believe that the 2nd alternative would be more correct - make the 6500s the ABRs, and the 3750 should be internal routers in non-backbone areas. Consider the fact that OSPF on backbone routers is potentially very memory-intensive. In addition to all the LSA1 and LSA2 local to the backbone area, backbone routers need to store all LSA3, LSA4 and LSA5 generated for all inter-area routers, ASBRs and external networks. That could put unnecessary memory and processing burden on your 3750 switches. With the 3750 switches being internal in non-backbone areas, you can then designate those areas as stubby or totally stubby and decrease the size of link-state database on the 3750 switches.

This my rationale - but I am very much looking forward to hearing opinions of other friends here.

Best regards,


Richard Burts Tue, 08/31/2010 - 05:58
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I agree with Peter that the 6500 is a better choice to be ABR than 3750. If the network were larger, with many remote areas and many router/switches I might suggest keeping the core routers as internal area 0 only and distribute the ABR function to router/switches in the remote areas, thus allowing the core switches to focus on core routing. But in a network the size that you describe I agree that making the 6500 to be ABR is preferable.

I might also suggest that you consider making the non zero area to be a stub (or even totally stubby) area. This will reduce even more the processing load on the 3750 for processing OSPF.




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