🌐 OSPF (Open Shortest Path First)

OSPF is a link-state dynamic routing protocol that automatically exchanges routing information between routers and calculates the shortest path using the SPF algorithm.

This is dynamic routing.

Until now in static routing:

👉 You manually added routes.

Now with OSPF:

👉 Routers automatically learn routes from each other.

This is heavily used in:

• Enterprise networks
• ISPs
• Data centers
• Corporate environments

🎯 What You Will Learn

• ✔ Dynamic route learning
• ✔ Neighbor relationships
• ✔ Route advertisement
• ✔ Automatic path selection
• ✔ Scalable routing

🔥 Difference: Static vs OSPF

🧪 Practical Topology

In Cisco Packet Tracer create:

PC0 --- R1 --- R2 --- R3 --- PC1

🌐 IP Addressing Plan

🔹 LAN Networks

🔹 Router-to-Router Links

🔌 Wiring (IMPORTANT)

PC Connections

Router Connections

⚙️ Step 1: Configure Router Interfaces

🔹 Router1

enable
configure terminal

interface g0/0
ip address 10.0.0.1 255.255.255.0
no shutdown

interface g0/1
ip address 192.168.1.1 255.255.255.252
no shutdown

🔹 Router2

enable
configure terminal

interface g0/0
ip address 192.168.1.2 255.255.255.252
no shutdown

interface g0/1
ip address 192.168.2.1 255.255.255.252
no shutdown

🔹 Router3

enable
configure terminal

interface g0/0
ip address 192.168.2.2 255.255.255.252
no shutdown

interface g0/1
ip address 20.0.0.1 255.255.255.0
no shutdown

⚙️ Step 2: Configure PCs

PC0

IP: 10.0.0.10

Gateway: 10.0.0.1

PC1

IP: 20.0.0.10

Gateway: 20.0.0.1

❌ Test Before OSPF

From PC0:

ping 20.0.0.10

❌ Fail

Because routers don’t know remote networks yet.

🔥 Step 3: Configure OSPF

🔹 Router1

router ospf 1

network 10.0.0.0 0.0.0.255 area 0
network 192.168.1.0 0.0.0.3 area 0

🔹 Router2

router ospf 1

network 192.168.1.0 0.0.0.3 area 0
network 192.168.2.0 0.0.0.3 area 0

🔹 Router3

router ospf 1

network 192.168.2.0 0.0.0.3 area 0
network 20.0.0.0 0.0.0.255 area 0

🧠 Important Concept

What is Area 0?

👉 Backbone area of OSPF

All routers communicate inside Area 0.

🔥 Step 4: Verify Neighbor Relationship

Run:

show ip ospf neighbor

You should see neighboring routers.

🔥 Step 5: Verify Routing Table

Run:

show ip route

You’ll see routes marked:

O

👉 Means learned through OSPF

🧪 Final Test

From PC0:

ping 20.0.0.10

✔ SUCCESS

Disclaimer

🌐 DHCP (Dynamic Host Configuration Protocol)

DHCP is a network protocol used to automatically assign IP configuration to clients in a network.

This is used in:

• Offices
• Companies
• Schools
• WiFi routers
• ISPs
• Enterprise networks

Almost every modern network uses DHCP.

🎯 What DHCP Does

Instead of manually assigning IP addresses to every PC:

❌ Manual:

PC1 = 192.168.1.10
PC2 = 192.168.1.11
PC3 = 192.168.1.12

✅ DHCP automatically gives:

• IP Address
• Subnet Mask
• Default Gateway
• DNS Server

🔥 Real-Life Example

When you connect:

Mobile to WiFi

Laptop to office LAN

👉 Router automatically gives IP

That is DHCP.

🎯 What You Will Learn

• ✔ Automatic IP allocation
• ✔ DHCP pool creation
• ✔ Default gateway assignment
• ✔ DNS assignment
• ✔ Lease process
• ✔ Enterprise networking basics

🧪 Practical Topology

In Cisco Packet Tracer create:

PC0 ----\
PC1 ----- Switch ---- Router
PC2 ----/

🌐 IP Plan

🔌 Wiring

PCs → Switch

Use:

Copper Straight-Through

Switch → Router

Use:

Copper Straight-Through

⚙️ Step 1: Configure Router Interface

On router:

enable
configure terminal

interface g0/0
ip address 192.168.1.1 255.255.255.0
no shutdown

🔥 Step 2: Configure DHCP Service

ip dhcp pool OFFICE

👉 Creates DHCP pool named OFFICE

Configure Network

network 192.168.1.0 255.255.255.0

Configure Default Gateway

default-router 192.168.1.1

Configure DNS

dns-server 8.8.8.8

🔥 Full DHCP Configuration

enable
configure terminal

interface g0/0
ip address 192.168.1.1 255.255.255.0
no shutdown

ip dhcp pool OFFICE
network 192.168.1.0 255.255.255.0
default-router 192.168.1.1
dns-server 8.8.8.8

🔥 Step 3: Exclude Router IP

VERY IMPORTANT

ip dhcp excluded-address 192.168.1.1

🧠 Why?

👉 Prevents DHCP from assigning router IP to PCs.

🔥 Step 4: Configure PCs

On every PC:

Desktop → IP Configuration

Select:

DHCP

🎯 Result

PC automatically gets:

🧪 Step 5: Verify

From PC:

ipconfig

You’ll see DHCP-assigned IP.

🧪 Step 6: Test Connectivity

ping 192.168.1.1

✔ Success

🔍 DHCP Process (VERY IMPORTANT)

DORA Process

🧠 How It Works

• 1️⃣ PC broadcasts DHCP Discover
• 2️⃣ Router replies DHCP Offer
• 3️⃣ PC sends Request
• 4️⃣ Router sends Acknowledgement

✔ IP assigned

🔥 Verification Commands

On router:

show ip dhcp binding

👉 Shows assigned IPs

show ip dhcp pool

👉 Shows DHCP pool details

🧠 Important Interview Questions

❓ What port does DHCP use?

UDP 67 & 68

❓ What is APIPA?

If DHCP fails:

169.254.x.x

❓ Why exclude addresses?

Prevent important IP assignment conflicts.

❓ What is lease time?

Duration client can use IP.

🔥 Common Mistakes

🎯 What You Learned

• ✔ Automatic IP assignment
• ✔ DHCP pool
• ✔ DORA process
• ✔ Enterprise IP management
• ✔ Router as DHCP server

Disclaimer

🌐 Static Routing — Practical + Interview Guide

📌 What is Static Routing?

Static routing is a method where routes are manually configured on routers to define how packets should reach different networks.

👉 Simple:

“Router ko manually batate hain traffic kahan bhejna hai.”

🎯 What You Will Learn

• Communication between different networks
• How routers forward packets
• Real-world routing logic

🧪 Lab Setup (in Cisco Packet Tracer)

🔹 Topology

PC1 --- Router1--- Router2 --- PC2

Lab Topology & IP Addressing Plan

Step 1 — Build topology in Packet Tracer

• Drag 2 x Router 2911 and 2 x PC
• Connect PC1 → Router1 with Copper Straight-Through
• Connect Router1 → Router2 with Copper Cross-Over (or Serial DCE)
• Connect Router2 → PC2 with Copper Straight-Through

Step 2 — Configure IP addresses

Router 1:

Router> enable
Router# configure terminal
Router(config)# hostname R1
R1(config)# interface GigabitEthernet 0/0
R1(config-if)# ip address 192.168.1.1 255.255.255.0
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface GigabitEthernet 0/1
R1(config-if)# ip address 10.0.0.1 255.255.255.252
R1(config-if)# no shutdown
R1(config-if)# end

Router 2:

Router> enable
Router# configure terminal
Router(config)# hostname R2
R2(config)# interface GigabitEthernet 0/0
R2(config-if)# ip address 10.0.0.2 255.255.255.252
R2(config-if)# no shutdown
R2(config-if)# exit
R2(config)# interface GigabitEthernet 0/1
R2(config-if)# ip address 192.168.3.1 255.255.255.0
R2(config-if)# no shutdown
R2(config-if)# end

Step 3 — Add Static Routes ⭐ (Most important!)

ip route [destination network] [subnet mask] [next-hop IP]
On Router 1 — tell it how to reach Network 3:
R1(config)# ip route 192.168.3.0 255.255.255.0 10.0.0.2

On Router 2 — tell it how to reach Network 1:
R2(config)# ip route 192.168.1.0 255.255.255.0 10.0.0.1

Step 4 — Configure PCs

On PC1 → Desktop → IP Configuration:

IP: 192.168.1.10, Mask: 255.255.255.0, Gateway: 192.168.1.1

On PC2 → Desktop → IP Configuration:

IP: 192.168.3.10, Mask: 255.255.255.0, Gateway: 192.168.3.1

Step 5 — Test with Ping

• On PC1 → Desktop → Command Prompt:
• ping 192.168.3.10
• You should see 4 replies ✅

Might be possible first time when you will ping it will only 2 replies came and others packets loss but second time if you try definitely ping successfully and winner winner !!

Verify on Router

R1# show ip route

R1# show running-config

Common mistakes to avoid:

• Forgetting no shutdown on interfaces
• Wrong next-hop IP (must be the directly connected neighbor's IP)
• Missing the return route (both routers need static routes!)
• Wrong subnet mask in the ip route command

Inter-VLAN Routing (Router-on-a-Stick)

📌 What is Inter-VLAN Routing?

Inter-VLAN routing is the process of enabling communication between different VLANs using a Layer 3 device (router or L3 switch).

👉 Simple:

“Different VLANs can talk using a router.”

🧪 Lab Topology (Build in Cisco Packet Tracer)

Devices:

• 1 Switch (2960)
• 1 Router (2911)
• 4 PCs

🔹 VLAN Plan

⚙️ Step 1: Assign IP Addresses

⚙️ Step 2: Create VLANs (Switch)

enable
configure terminal

vlan 10
name HR

vlan 20
name IT

⚙️ Step 3: Assign Access Ports

interface fa0/1
switchport mode access
switchport access vlan 10

interface fa0/2
switchport mode access
switchport access vlan 20

interface fa0/3
switchport mode access
switchport access vlan 10

interface fa0/4
switchport mode access
switchport access vlan 20

⚙️ Step 4: Connect Switch → Router (IMPORTANT)

Use Straight-through cable

Connect:

Switch Fa0/23 → Router G0/0

⚙️ Step 5: Make Switch Port TRUNK

• interface fa0/23
• switchport mode trunk

⚙️ Step 6: Configure Router (CORE STEP 🔥)

enable
configure terminal

interface g0/0
no shutdown

interface g0/0.10
encapsulation dot1Q 10
ip address 192.168.10.254 255.255.255.0

interface g0/0.20
encapsulation dot1Q 20
ip address 192.168.20.254 255.255.255.0

🧪 Step 7: Test Connectivity

Same VLAN

ping 192.168.10.2

✔ Works

🔹 Different VLAN (MAIN TEST)

From PC0:

ping 192.168.20.2

✔ Now it will WORK

Important Note:

Ethernet switches remember MAC addresses (Layer 2) to map devices to physical ports, while routers remember IP addresses (Layer 3) to map devices to logical network addresses. Switches use a temporary CAM table, while routers use DHCP leases to "remember" these assignments.

Key Takeaways:

Switch (MAC): Learns source MAC addresses from incoming frames and associates them with a port. These entries are temporary (typical 300-second aging timer).

Router (IP): Assigns IP addresses via DHCP to specific MAC addresses. These assignments ("leases") can be configured as static/persistent.

Function: Switches enable local network communication (same network), while routers manage traffic between different networks.

🔍 What Happens Internally

• PC0 → sends packet to gateway (router)
• Router receives via VLAN 10 sub-interface
• Router routes to VLAN 20
• Sends back to PC3

👉 “Packet goes:

PC → Switch → Router → Switch → Destination”

👉 “Router performs Layer 3 decision”

⚠️ Common Mistakes

• ❌ Forgetting trunk on switch
• ❌ Missing encapsulation dot1Q
• ❌ Wrong default gateway
• ❌ Router interface shutdown

🧠 Key Concepts

• Router uses sub-interfaces
• Each VLAN = one sub-interface
• Router acts as gateway

🎯 Interview Questions

❓ What is Router-on-a-Stick?

👉 Using one physical router interface with multiple sub-interfaces for VLAN routing

❓ Why trunk is required?

👉 To carry multiple VLANs between switch and router

❓ Why ping was failing earlier?

👉 No Layer 3 routing between VLANs

🧠 Real-World Use

Used in:

• Small office networks
• Labs and training
• Basic enterprise setups