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@ -229,3 +229,32 @@ system.
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\XXX{netsplit, multi links w/ same and diff costs, v2 vs v3, mixed segments,
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multiple prefixes. napsat na kolejích s gennetem..}
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\section{Area structure}
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It is also worth to consider the expected size and structure of an OSPF areas
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where Birdvisu might run. While it is up to the administrator and they may be
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very creative, there are some limits to such creativity.
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The largest system which can be spanned by a single OSPF instance is the whole
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autonomous system (AS). The largest ASes only have about several hundred
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thousand routers\cite{as-topologies}. The average degree also seems to be rather
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low.
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We can derive another limit from IPv4 address allocations. A /8 block (i.e.
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Class A) has 16 777 216 addresses. Very few ISP would be assigned such a large
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block, but they might be using the 10.0.0.0/8 private block. Even if an ISP
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wants to use all those addresses, majority of them will likely not be assigned
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to routers, but to some end devices that actually provide \uv{useful} services.
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Those devices are also likely to be grouped into non-trivial networks, thus
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reducing the number of vertices (i.e. routers and networks) in the OSPF
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topology. (While IPv6 allocates many more addresses, we assume that the overall
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topology will not be different from the IPv4 one.)
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It is probably not practical to have a single OSPF area span all the routers in
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a large AS, since any link state change results in an LSA being flooded
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throughout the area.
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While we cannot be sure about particular administrative decisions, given the
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observations above, we expect that a single area contains at most few thousand
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vertices and probably much less than that.
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