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I am currently reviewing the OSPF configuration of an existing network in an attempt to streamline it.

From a Layer 3 standpoint, this network consists of two core routers in Area 0 (CORE1 and CORE2) as well as ten other routers (BB1 through BB10 where BB stands for backbone), each an Area Border Router between Area 0 and its own area. These additional areas are standard areas and there are no OSPF routers in them apart for the ABR. (This might have not always been the case, but it sure is now. There are a few more OSPF routers on the network, e.g. a few Anycast DNS servers, but those are all in Area 0.) Each ABR routes some networks directly and also redistributes a few static routes (which OSPF considers external).

Here's an illustration where two out of the ten areas and ABRs are shown as an example:

illustration of the network described above

My question is: does it make sense to keep using areas in this situation? Does using areas simplify shortest-path calculation on the other nine routers if a new directly-routed network is added to or removed from one of the ABRs? What about if a static route is added/removed?

  • Where there is only one router today, may be two tomorrow. – Ricky May 5 at 16:56
  • True, although we're currently trending downward and don't expect to ever surpass Cisco's recommendation of 50 routers per area in Area 0 -- unless we ditch the current design completely, in which case we'd redesign the OSPF adjacency too. – RavuAlHemio May 5 at 17:14
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    It really depends on the number of routes. Areas were developed to minimize the churn from changing topologies. When OSPF was developed, routers had much slower CPUs and much less memory than they do today. – Ron Trunk May 5 at 17:15
  • Certainly. My question is if areas are of any use in this particular situation: one router per area, all standard areas, no real potential for summarization. – RavuAlHemio May 5 at 18:24
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    All of those redistributed static routes are being propagated as type-5's, which are flooded everywhere. In theory you could summarize the few directly connected routes (..thus possibly reducing LSA count somewhat). Since you're not summarizing there's really nothing being added with making these routers ABR's. – rnxrx May 6 at 3:31

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