Routing

Routing is the process of selecting the best path for data packets to travel from a source device to a destination device across a network.

Key Components

Routers - Devices that forward packets between networks
Routing Tables - Store information about network paths
Routing Protocols - Rules for determining best paths

Types of Routing

Type	Description
Static Routing	Manually configured paths
Dynamic Routing	Automatic path selection

Routing Protocols Overview

Routing protocols decide HOW routers choose paths.

Two Main Categories

flowchart TD subgraph AS1["🏢 Autonomous System 1 (AS 100 - e.g., Enterprise LAN)"] R1["Router A"] <-->|"OSPF (IGP)"| R2["Router B"] R2 <-->|"OSPF (IGP)"| Border1["🚪 Border Router 1"] end subgraph AS2["🌐 Autonomous System 2 (AS 200 - ISP Backbone)"] Border2["🚪 Border Router 2"] <-->|"OSPF / IS-IS"| Core["Core ISP Routers"] Core <--> Border3["🚪 Border Router 3"] end subgraph AS3["🏢 Autonomous System 3 (AS 32934 - Facebook DC)"] Border4["🚪 Border Router 4"] <-->|"OSPF"| WebServer["🖥️ Facebook Web Servers"] end Border1 <==>|"BGP (EGP - AS-PATH: 100 → 200)"| Border2 Border3 <==>|"BGP (EGP - AS-PATH: 200 → 32934)"| Border4

Type	Example	Used For
Interior Gateway Protocol (IGP)	OSPF	Inside one network
Exterior Gateway Protocol (EGP)	BGP	Between networks

OSPF – Open Shortest Path First

Used INSIDE one organization or ISP
Calculates shortest path using Dijkstra algorithm
Based on cost, bandwidth, and speed
Fast and dynamic

Used In

Company networks
University networks
Internal ISP routing

Goal

Find the best path inside a single network.

BGP – Border Gateway Protocol

Used BETWEEN different networks and ISPs
Runs the entire internet
Policy-based routing
Does NOT care about speed or distance

What BGP Cares About

Business agreements
Policies
Stability
AS-PATH

Difference Between OSPF and BGP

Feature	OSPF	BGP
Type	Interior (IGP)	Exterior (EGP)
Used In	Inside a network	Between networks
Goal	Shortest path	Best policy path
Algorithm	Dijkstra	Path Vector
Internet Usage	No	Yes

Protocol Comparison

Protocol	Cares About
OSPF	Speed / cost / shortest path
RIP	Hop count (distance)
BGP	Policies and preferences

Autonomous System (AS)

Definition

An Autonomous System is a large network under single administrative control.

AS Number (ASN)

Unique identifier for each network on the internet
Examples:
- Google: AS15169
- Facebook: AS32934

AS-PATH in BGP

AS-PATH is a list of AS numbers a packet passes through
Used for:
- Loop prevention
- Path selection

Example AS-PATH

flowchart LR AS100["🏢 AS100 (Client ISP)"] --> AS200["🌐 AS200 (Transit Provider)"] --> AS32934["🏢 AS32934 (Facebook)"]

What Happens When You Ping a Website

Example: `ping facebook.com`

flowchart TD D1["1. DNS Lookup: facebook.com → 57.144.56.1"] --> D2["2. Build ICMP Echo Request Packet"] D2 --> D3["3. Check Routing Table (Gateway: 192.168.1.1)"] D3 --> D4["4. ARP Resolution (Find Gateway MAC)"] D4 --> D5["5. Forward to Default Router"] D5 --> D6["6. Internet Routers Forward Across BGP Paths"] D6 --> D7["7. Facebook Server Receives & Sends ICMP Echo Reply"] D7 --> D8["8. Reply Delivered to Local Device (Calculates RTT ms)"]

Traceroute

Traceroute shows the path packets take across the internet.

Command

traceroute facebook.com

What Traceroute Shows

Each router hop
Latency between hops
Network path structure

Important Notes

Some hops show * * * due to ICMP blocking
Final hops may be hidden for security

Packets

Data in network travels in small chunks called packets.
A packet contains:

Source IP
Destination IP
Data payload

Important Commands

View Routing Table

ip route

Trace Route

traceroute facebook.com

View ARP Table

ip neigh

Quick Reference

Concept	Description
Routing	Selecting best path for packets
Router	Device that forwards packets
OSPF	Interior routing protocol (shortest path)
BGP	Exterior routing protocol (policy-based)
AS	Network under single admin control
Traceroute	Shows path packets take
Static Routing	Manually configured
Dynamic Routing	Automatically determined

Summary

OSPF is used inside networks
BGP is used between networks
Traceroute shows path, not protocol
Internet routing is mostly BGP
Your PC only knows next hop, not full path