In routing terminology, what is an Interior Gateway Protocol (IGP)?

Introduction / Context:
Interior Gateway Protocols (IGPs) are an essential building block in the hierarchical routing structure of the Internet and large enterprise networks. They handle routing inside a single autonomous system, where one administrative entity controls the routing policies. Knowing what an IGP is and how it differs from an Exterior Gateway Protocol is fundamental for network design and certification exams.

Given Data / Assumptions:

Networks are grouped into autonomous systems, each with its own internal routing.
Routing inside an AS focuses on efficiency and fast convergence rather than external business policies.
IGPs are distinct from EGPs, which operate between ASes.
The question asks for a definition of IGP, not for names of specific protocols.

Concept / Approach:
An Interior Gateway Protocol is any routing protocol designed to distribute routing information within a single autonomous system or routing domain. Examples include RIP, OSPF, IS-IS and EIGRP (in Cisco networks). These protocols maintain a view of the internal network topology and compute best paths between internal routers. They do not directly control how routes are exchanged with other ASes; that is the job of Exterior Gateway Protocols like BGP. IGPs are optimized for relatively fast convergence and manageability within one administrative control.

Step-by-Step Solution:
Step 1: Recognize that an autonomous system may contain many routers and subnets that all need to know how to reach one another. Step 2: An IGP runs on these routers and exchanges information about reachable networks and metrics such as cost or hop count. Step 3: The routers use the IGP's algorithm (distance vector, link state or hybrid) to compute best paths and fill their routing tables for internal destinations. Step 4: Since this process is entirely within one AS, the protocol is called an Interior Gateway Protocol. Step 5: Protocols that exchange routes across AS boundaries are instead classified as Exterior Gateway Protocols and are not IGPs.

Verification / Alternative check:
Network design guides and textbooks divide routing protocols into IGPs (RIP, OSPF, IS-IS, EIGRP) and EGPs (BGP). They describe IGPs as handling intra AS routing and stress their role in building the internal topology. None of these sources describe IGPs as application layer login protocols or as physical layer standards, confirming the definition in option A.

Why Other Options Are Wrong:
Routing protocols that operate between ASes, such as BGP, are classified as EGPs, not IGPs.
Remote login protocols like Telnet or SSH operate at the application layer and are unrelated to gateway routing inside an AS.
Physical layer standards define signaling and media properties, not routing information exchange.

Common Pitfalls:
A frequent mistake is to confuse IGP with IP, assuming that IGP is simply another name for the Internet Protocol. They are different: IP is a network layer protocol for packet delivery, while IGPs are routing protocols that decide which path IP packets should take within a domain. Another pitfall is to forget that an AS can run multiple IGP instances or multiple areas (as in OSPF), but they all fall under the IGP category.

Final Answer:
An Interior Gateway Protocol (IGP) is a routing protocol used to exchange routing information within a single autonomous system or routing domain.