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
| 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
AS100 → AS200 → AS32934What Happens When You Ping a Website
Example: ping facebook.com
Steps:
- DNS Resolution
- Domain name is converted to IP address
- Example: facebook.com → 57.144.56.1
- ICMP Packet Creation
- System creates an ICMP Echo Request packet
- Routing Table Lookup
- OS checks where to send packet next
- ARP Resolution
- Finds MAC address of the gateway
- Packet Sent to Router
- Routers Forward Packet Across Internet
- Facebook Server Responds
- Reply Returns to Your Device
Traceroute
Traceroute shows the path packets take across the internet.
Command
traceroute facebook.comWhat 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 routeTrace Route
traceroute facebook.comView ARP Table
ip neighQuick 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