Routing and Switching

Switching moves data within a local network (Layer 2), while routing moves data between different networks (Layer 3). Together, they form the backbone of how data travels from source to destination across the Internet and private networks.

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Switching — Layer 2

A switch connects devices on the same local area network (LAN) and forwards frames based on MAC addresses.

How a Switch Works

1. A frame arrives at a switch port
2. The switch reads the source MAC address and records it in the MAC address table (also called the CAM table)
3. The switch looks up the destination MAC address in the table
4. If found, the frame is forwarded to the specific port (unicast)
5. If not found, the frame is flooded to all ports except the source (unknown unicast)

MAC Address Table (CAM Table):

| MAC Address        | Port |
|--------------------|------|
| AA:BB:CC:DD:EE:01  | 1    |
| AA:BB:CC:DD:EE:02  | 3    |
| AA:BB:CC:DD:EE:03  | 5    |

Switch Features

Feature	Description
Full duplex	Devices can send and receive simultaneously
Microsecond latency	Switches forward frames very quickly
Collision-free	Each port is its own collision domain
Broadcast domain	All ports on a switch share a single broadcast domain (unless VLANs are used)

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VLANs — Virtual Local Area Networks

VLANs allow you to logically segment a single physical switch into multiple isolated broadcast domains.

Why Use VLANs?

• Security — Isolate sensitive traffic (e.g., finance VLAN, HR VLAN)
• Performance — Reduce broadcast traffic by limiting broadcast domains
• Organisation — Group users by function rather than physical location
• Compliance — Separate regulated data (e.g., PCI DSS for payment data)

VLAN Example

Physical Switch:

Port 1-8:    VLAN 10 (Engineering)
Port 9-16:   VLAN 20 (Marketing)
Port 17-24:  VLAN 30 (Management)

Devices in VLAN 10 cannot communicate with VLAN 20
without a router (inter-VLAN routing).

Trunk Ports

A trunk port carries traffic for multiple VLANs between switches using 802.1Q tagging:

Switch A ══════ Trunk (carries VLAN 10, 20, 30) ══════ Switch B

Each frame on the trunk is tagged with a VLAN ID
so the receiving switch knows which VLAN it belongs to.

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Routing — Layer 3

A router connects different networks and forwards packets based on IP addresses. Every time data crosses from one network to another, a router makes the forwarding decision.

How Routing Works

1. A packet arrives at the router
2. The router examines the destination IP address
3. The router consults its routing table to find the best path
4. The packet is forwarded to the next hop (another router or the final destination)
5. The TTL is decremented by one — if it reaches 0, the packet is dropped

The Routing Table

A routing table contains entries that map destination networks to next hops:

# View the routing table on Linux
ip route show

# Example output:
default via 192.168.1.1 dev eth0
192.168.1.0/24 dev eth0 proto kernel scope link
10.0.0.0/8 via 192.168.1.254 dev eth0

Destination	Next Hop	Interface	Metric
0.0.0.0/0 (default)	192.168.1.1	eth0	100
192.168.1.0/24	Directly connected	eth0	0
10.0.0.0/8	192.168.1.254	eth0	50

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Static vs Dynamic Routing