Routing

Routing is the process of selecting paths in a network along which to send data packets. It operates at Layer 3 (Network Layer) of the OSI model and is one of the most critical functions in computer networking.

Overview

Routing determines how data travels from source to destination across potentially multiple intermediate networks (hops). A router examines the destination IP address of each packet and consults its routing table to decide the best next hop.

Why Routing Matters

Scalability: The internet has billions of devices; routing hierarchies make this manageable
Fault tolerance: Dynamic routing can reroute traffic around failed links
Performance: Choosing optimal paths reduces latency and congestion
Policy enforcement: Organizations can control traffic flow for cost, security, or compliance

Key Concepts

Concept	Description
Routing Table	Database of known networks and how to reach them
Next Hop	The next router in the path to the destination
Metric	A value (hop count, bandwidth, delay) used to compare routes
Administrative Distance	Trustworthiness rating of a routing source
Convergence	Time for all routers to agree on the network topology
Hop Count	Number of routers a packet must traverse

Routing Types

graph TD
    A[Routing] --> B[Static Routing]
    A --> C[Dynamic Routing]
    C --> D[Distance Vector]
    C --> E[Link State]
    C --> F[Path Vector]
    D --> G[RIP]
    E --> H[OSPF]
    E --> I[IS-IS]
    F --> J[BGP]
    A --> K[Default Route]
    A --> L[Policy-Based Routing]

How a Router Makes Decisions

Receives a packet on an interface
Examines the destination IP address
Looks up the routing table for the longest prefix match
If a match is found, forwards to the next hop
If no match, sends to the default route (if configured) or drops the packet

Administrative Distance (Trustworthiness)

Source	AD Value
Directly connected	0
Static route	1
eBGP	20
EIGRP (internal)	90
OSPF	110
IS-IS	115
RIP	200
EIGRP (external)	170
iBGP	200

Interview Questions

Q: What is the difference between routing and forwarding? A: Routing is the control plane process of determining paths (building routing tables). Forwarding is the data plane process of moving packets from input to output interface based on the routing table.
Q: What is longest prefix match? A: When multiple routes match a destination, the router selects the one with the longest subnet mask (most specific). For example, 10.0.0.0/24 is preferred over 10.0.0.0/16 for destination 10.0.0.5.
Q: What happens when no route matches a packet? A: If a default route (0.0.0.0/0) exists, the packet is sent there. Otherwise, the router drops the packet and may send an ICMP “Destination Unreachable” message.

Common Mistakes

Confusing routing (control plane) with forwarding (data plane)
Assuming all routing protocols use the same metric
Forgetting that BGP uses AS-path, not hop count, as its primary metric
Not understanding that AD only matters when multiple sources provide routes to the same destination

Summary

Routing is the backbone of internetworking. Static vs. dynamic routing, the various protocols (RIP, OSPF, IS-IS, BGP), and the concepts of convergence, metrics, and administrative distance are all essential interview topics.

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