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Unicast Routing Protocols: RIP, OSPF, and BGP

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1. Unicast Routing Concepts & Architecture

Routing is the process of selecting the best path across an internetwork to deliver a packet from source to destination.

The global Internet is structured as a collection of Autonomous Systems (AS):

• Autonomous System (AS): A collection of connected routers and subnets under the administrative control of a single organization (such as an ISP, university, or enterprise) that presents a unified routing policy.

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2. Distance Vector Routing: RIP

Distance Vector Routing is based on the classical Bellman-Ford Algorithm:

• Philosophical Principle: "Each router tells its immediate neighbors about the entire world."
• Routers periodically broadcast their entire routing table to their directly connected neighbors.

Routing Information Protocol (RIP):

1. Metric: Hop Count (number of routers traversed).
   • Maximum allowed hop count = 15.
   • A hop count of 16 denotes infinity (network unreachable). This restricts RIP to small-to-medium networks.
2. Periodic Updates: Broadcasts full routing table updates every 30 seconds over UDP Port 520.
3. Slow Convergence & The Count-to-Infinity Problem:
   • If a link fails, routers may pass outdated routing information in a loop, incrementally incrementing the hop count up to 16 before discovering the link is down.

Solutions to Count-to-Infinity:

• Split Horizon: A router never advertises a route back out the same interface through which it learned that route.
• Poison Reverse: A router advertises unreachable routes back out the learned interface with a metric of 16 (infinity), actively "poisoning" the route.
• Hold-down Timers: When a link fails, routers refuse updates for that destination for a cooldown period (e.g., 180 seconds).

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3. Link-State Routing: OSPF

Link-State Routing is based on Dijkstra's Shortest Path First (SPF) Algorithm:

• Philosophical Principle: "Each router tells the entire world about its immediate neighbors."

Open Shortest Path First (OSPF):

1. Link State Advertisements (LSAs): Each router floods small LSA packets describing only the state and cost of its directly connected links.
2. Link State Database (LSDB): Every router accumulates all LSAs, constructing an identical, complete mathematical topology graph of the entire network.
3. Dijkstra's Algorithm: Each router runs Dijkstra's SPF algorithm independently, with itself as the root node, calculating the shortest path tree to all destination networks.
4. Metric: Cost, inversely proportional to link bandwidth:

(A $100\text{ Mbps}$ Fast Ethernet link has cost 1; a $10\text{ Mbps}$ link has cost 10).
5. Hierarchical Areas: Large OSPF networks are divided into Areas to contain LSA flooding. All secondary areas must connect to Area 0 (Backbone Area) via Area Border Routers (ABRs).
6. Designated Router (DR) & Backup DR (BDR): On shared multi-access networks (switched Ethernet), routers elect a DR and BDR to reduce adjacency mesh overhead from $O(n^2)$ to $O(n)$.

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4. Path-Vector Routing: BGP

Path-Vector Routing is the inter-domain protocol of the global Internet, implemented by BGP-4 (Border Gateway Protocol Version 4) (RFC 4271):

• Exterior Gateway Protocol (EGP): Operates between distinct Autonomous Systems.
• Runs over TCP Port 179: Provides reliable, connection-oriented session exchange without requiring periodic broadcast flooding.
• Path Vector Concept: When BGP advertises a route to a destination network, it prepends its own Autonomous System Number (ASN) to the AS-Path Attribute:

• Loop Prevention: If an AS receives a BGP advertisement containing its own ASN in the AS-Path list, it immediately discards the route, completely eliminating inter-domain routing loops!
• Policy-Based Routing: Path selection is based on political, economic, and security business agreements rather than raw physical latency.

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5. Comprehensive Comparison: RIP vs. OSPF vs. BGP

Feature	RIP (v2)	OSPF (v2/v3)	BGP (BGP-4)
Type	Distance Vector (IGP)	Link-State (IGP)	Path Vector (EGP)
Algorithm	Bellman-Ford	Dijkstra (SPF)	Path Vector / Policy
Metric	Hop Count (Max 15)	Cost ($\propto \frac{1}{\text{Bandwidth}}$)	AS-Path & Policy Attributes
Scope	Small, simple LANs	Large enterprise campus / AS	Global Internet Core
Convergence	Slow (Count-to-infinity)	Very Fast	Moderate
Transport Protocol	UDP Port 520	IP Protocol 89 (Raw IP)	TCP Port 179
Update Mechanism	Periodic (every 30s)	Event-triggered LSAs	Incremental event-triggered
Hierarchy	Flat	Areas (Area 0 Backbone)	Autonomous Systems

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