In Internet routing, what is the main difference between interior and exterior neighbor gateways?

Introduction / Context:
The global Internet is composed of many interconnected networks, each administered by an organization such as an Internet Service Provider, enterprise or university. These large administrative domains are called autonomous systems (AS). Routers that exchange routing information inside an AS behave differently from routers that exchange routing information between ASes. Understanding the distinction between interior and exterior neighbor gateways is fundamental to interdomain routing.

Given Data / Assumptions:

The Internet is divided into multiple autonomous systems, each with its own internal routing policies.
Routers that connect networks within the same AS are interior neighbors.
Routers that connect one AS to another AS and exchange routing information across that boundary are exterior neighbors.
Interior and exterior gateways use different classes of routing protocols.

Concept / Approach:
Interior Gateway Protocols (IGPs), such as RIP, OSPF and IS-IS, are used for routing within a single autonomous system. Routers that run these protocols and share routes only with routers inside the same AS are interior gateways. Exterior Gateway Protocols (EGPs), most notably BGP, are used to exchange routing information between autonomous systems. Routers that peer with routers in a different AS using an EGP are exterior gateways. The key distinction is therefore the administrative boundary: interior gateways operate within one routing domain, while exterior gateways connect different routing domains and must respect interdomain policies.

Step-by-Step Solution:
Step 1: Identify the scope of routing. Inside an AS, the goal is usually to find shortest or least cost paths and ensure fast convergence. Step 2: Routers that participate only in this internal routing, using IGPs, are interior neighbor gateways and share a common administration. Step 3: At the edge of the AS, border routers connect to other autonomous systems and run BGP or other exterior protocols to exchange reachability information. Step 4: These border routers, when peering with routers in a different AS, are exterior neighbor gateways and must enforce policies such as route filtering and path selection based on business relationships. Step 5: Therefore, the main difference is whether routing information is exchanged inside a single AS or across AS boundaries.

Verification / Alternative check:
Routing architecture descriptions for the Internet consistently state that IGPs handle intra AS routing, while BGP, as the de facto EGP, handles inter AS routing. Border routers often run both an IGP internally and BGP externally, illustrating that a single physical router can act as both an interior gateway (toward the internal network) and an exterior gateway (toward other autonomous systems). This reinforces the boundary based definition in the correct option.

Why Other Options Are Wrong:
Whether a router is hardware or software based has nothing to do with being interior or exterior; both types can serve in either role.
The choice of wireless versus wired links is also independent of routing role; interior and exterior gateways can use many media types.
The distinction is not based on multicast versus unicast traffic, but on administrative and routing domain boundaries.

Common Pitfalls:
Students sometimes mistakenly associate interior and exterior with physical location, thinking interior routers are physically inside a building and exterior routers are outside. In reality, the terms refer to routing domains and administrative control, not building geography. Another pitfall is to assume that the same protocol, such as OSPF, could be used across the entire Internet, ignoring the need for interdomain policies that BGP provides.

Final Answer:
Interior neighbor gateways are routers that exchange routing information within a single autonomous system using interior gateway protocols, whereas exterior neighbor gateways exchange routes between different autonomous systems using exterior gateway protocols.