Intra-Domain Routing (Interior Gateway Protocols - IGPs)

• Scope: This refers to the routing protocols and processes that operate entirely within a single Autonomous System (AS). All routers participating in intra-domain routing are under the same administrative control and adhere to a common set of policies and metrics.
• Primary Objective: To find the most efficient and optimal paths for packets to travel between any two points within that specific AS. The emphasis is on fast convergence, loop-free paths, and granular control over internal traffic flow.
• Routing Algorithms: IGPs typically employ well-defined, mathematically rigorous routing algorithms, either Link-State (like OSPF or IS-IS) or Distance-Vector (like RIP), or a hybrid (like EIGRP). These algorithms are designed to compute optimal paths based on link metrics.
• Policy: While internal policies can influence metrics (e.g., preference for certain links), the fundamental goal is connectivity and optimality within the AS.
• Information Exchange: Routers exchange detailed topological information (Link-State) or full routing tables (Distance-Vector) within the AS. This information is typically not shared with external ASes in its raw form.
• Examples of IGPs:
• OSPF (Open Shortest Path First): A widely adopted Link-State protocol.
• IS-IS (Intermediate System to Intermediate System): Another robust Link-State protocol, often favored by large ISPs.
• EIGRP (Enhanced Interior Gateway Routing Protocol): A Cisco proprietary hybrid protocol.
• RIP (Routing Information Protocol): An older, simpler Distance-Vector protocol, generally used in smaller, less complex networks.

Inter-Domain Routing (Exterior Gateway Protocols - EGPs)

• Scope: This refers to the routing protocols and processes that operate between different Autonomous Systems (ASes). It is the mechanism by which different ISPs and large organizations connect and exchange reachability information, forming the global fabric of the Internet.
• Primary Objective: To propagate reachability information (which network prefixes are reachable through which ASes) across the entire Internet and, crucially, to enforce routing policies. Policy considerations, such as business agreements (e.g., customer-provider relationships, peering agreements), traffic engineering (influencing ingress/egress points), security, and political considerations, are paramount. The goal is not necessarily to find the globally "shortest" path (as defined by a simple metric) but the "best" path according to the policies of the involved ASes.
• Routing Algorithms: Inter-domain routing does not use traditional Link-State or Distance-Vector algorithms. Instead, it uses a Path-Vector approach. Routers exchange the full AS path (the sequence of ASNs that a route has traversed) to reach a destination. This path information is vital for loop prevention (if an AS sees its own ASN in the path, it's a loop) and for policy enforcement.
• Policy: Extremely critical. ASes use extensive policy rules (based on AS path, communities, origin AS, local preference, etc.) to filter, prioritize, and select routes. This allows an AS to control which routes it advertises to neighbors, which routes it accepts from neighbors, and how it prefers one path over another, even if it's "longer" in terms of hops.
• Information Exchange: Border routers (known as BGP Speakers) exchange network prefixes (IP address blocks) and their associated AS paths, along with various BGP attributes that carry policy information.
• Examples of EGPs:
• BGP (Border Gateway Protocol): The only inter-domain routing protocol used on the global Internet. There is no alternative for exchanging routes between ASes.
• Key Characteristics: Decentralized administration, diverse policies, focus on policy enforcement, stability, and loop prevention across a vast, untrusted environment.

Why are Two Tiers (Intra- and Inter-Domain) Necessary for the Internet?

1. Scalability: The sheer size of the Internet (hundreds of thousands of ASes, millions of network prefixes) makes it impossible for a single routing protocol to manage.
2. A single Link-State protocol would require every router to have an LSDB of the entire Internet, which is computationally and memory-wise infeasible.
3. A single Distance-Vector protocol would suffer from extreme convergence issues and "count-to-infinity" problems across a global scale.
4. The two-tiered approach localizes complexity: IGPs manage detailed internal topology within an AS, while EGPs manage the coarser-grained connectivity between ASes.
5. Administrative Autonomy and Policy Control: Different ASes are managed by different organizations with distinct business models, security requirements, and traffic engineering objectives.
6. ISPs need to control how traffic enters and exits their networks (e.g., preferring cheaper peering links over paid transit, prioritizing traffic from certain customers).
7. Large enterprises need to control their own internal routing without external interference.
8. Inter-domain routing (BGP) provides the necessary flexibility for ASes to implement and enforce these complex, often conflicting, administrative and business policies. IGPs, focused on pure shortest-path optimality, would not suffice for these policy requirements.
9. Privacy and Abstraction: Intra-domain protocols maintain a detailed view of the internal network topology. This detailed internal structure is not (and should not be) exposed to other ASes on the Internet. Inter-domain routing abstracts this away, simply advertising which network prefixes are reachable through a given AS, without revealing the internal routing mechanisms or specific internal link costs.
10. Convergence and Stability: While IGPs prioritize rapid convergence within an AS for internal efficiency, inter-domain routing (BGP) prioritizes stability over extremely fast convergence. Changes in the global routing table are carefully propagated to avoid cascading instabilities across the Internet.