For OSPF, the nonbackbone areas can be additionally con?¬?gured as a
stub area, a totally stubby area, a not-so-stubby area (NSSA), or a totally not-so-stubby area to
help reduce the link-state database and routing table size.
Routers operating within the two-layer network hierarchy have different routing entities. The
terms used to refer to these entities are different for OSPF than IS-IS. Refer to the following
examples from Figure 3-27:
?– Router A is called the backbone router in OSPF and the L2 router in IS-IS. The backbone, or
L2, router provides connectivity between different areas.
?– Routers B and C are called area border routers (ABR) in OSPF, and L1/L2 routers in IS-IS.
ABR, or L1/L2, routers attach to multiple areas, maintain separate link-state databases for
each area to which they are connected, and route traf?¬?c destined for or arriving from other
areas.
?– Routers D and E are called nonbackbone internal routers in OSPF, or L1 routers in IS-IS.
Nonbackbone internal, or L1, routers are aware of the topology within their respective areas
and maintain identical link-state databases about the areas.
?– The ABR, or L1/L2, router will advertise a default route to the nonbackbone internal, or L1,
router.
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