Three types of traffic may be defined
in relation to areas:
Intra-area traffic consists of packets
that are passed between routers within a single area.
Inter-area traffic consists of packets
that are passed between routers in different areas.
External traffic consists of packets
that are passed between a router within the OSPF domain and a router within
another routing domain.
Area ID 0 (or 0.0.0.0) is reserved for
the backbone. The backbone is responsible for summarizing the topologies of
each area to every other area. For this reason, all inter-area traffic must
pass through the backbone; non-backbone areas cannot exchange packets directly.
Ref: Routing TCP/IP volume 1
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