OSPF is an industry-standard classless IP routing protocol available on most networking hardware
that switches packets at layer 3. It is a link-state routing protocol used to route traffic
within a routing domain, and we assume that you know how OSPF operates. OSPF allows for
aggregation of addresses but only at
area border routers (ABRs)
autonomous system boundary
. Routes within a single area cannot be aggregated. OSPF in multiple areas
requires a hierarchical network addressing design because it uses one backbone area, with individual
areas directly connected to the backbone area, and ABRs advertise summary routes, at
best. Not designing hierarchically for multiple areas tends to break the OSPF routing domain.
Because of the way OSPF operates, you need to make sure that the borders of your OSPF
areas correspond to the transition points from one layer to another. For example, Figure 11.7
shows how the border between Areas 0 and 1 and Areas 2 and 0 occurs at the distribution router
where summaries are sent into Area 0.
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