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Internal router

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

An internal OSPF router is a router whose OSPF-enabled interfaces all belong to the same area.

An internal OSPF router has all OSPF-enabled interfaces in one area. That area may be area 0 or a non-backbone area. section 18.1.2

The classification applies only to OSPF-enabled interfaces. A router can have an internet-facing interface without OSPF and still be an internal router. section 18.1.2

Example classification

R1 is an internal router because its OSPF-enabled interfaces belong to one area. It is also a backbone router and an ASBR, showing that OSPF router types can overlap. section 18.1.2

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Defines the classification, contrasts related router types, and stops short of configuration procedures.
Examples
Uses R1 and a four-area topology to illustrate overlapping classifications and area membership.
What it assumes
Assumes basic OSPF and interface knowledge while defining areas, area 0, and related router roles.
What you can do after
Evaluate whether a router is internal by checking the areas assigned to its OSPF-enabled interfaces.
How it connects
Connects internal routers with OSPF areas, backbone routers, ABRs, ASBRs, LSDBs, and intra-area routes.
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Related concepts

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