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Hey there! I'm currently working as an Associate DevOps Engineer, and I'm diving into popular DevOps tools like Azure Devops,Linux, Docker, Kubernetes,Terraform and Ansible. I'm also on the learning track with AWS certifications to amp up my cloud game. If you're into tech collaborations and exploring new horizons, let's connect!
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DevOps Journey with M Hassan
174 posts
I am writing these blogs because I recently completed a comprehensive DevOps course where I gained in-depth knowledge of the topics mentioned. As I progressed through the course, I realized the importance of having a concise and accessible resource to revise and reinforce my understanding of each topic. Therefore, I decided to create cheat sheets in the form of blog posts. These cheat sheets will not only serve as a handy reference for myself but also benefit others who are also interested in mastering DevOps concepts. By documenting each topic and providing concise explanations, I aim to create a valuable resource that simplifies complex concepts and facilitates hands-on practice. This way, I can solidify my own understanding while helping others on their DevOps journey.
CCNA (Cisco Certified Network Associate) is a foundational networking course by Cisco.
It helps you understand:
How computers and devices communicate
How data travels from one device to another
Networking concepts used in real-world networks
CCNA is mainly focused on:
Networking fundamentals
OSI & TCP/IP models
Routing and switching basics
Protocols and troubleshooting
The OSI (Open Systems Interconnection) Model is a 7-layer reference model used to understand how data moves in a network.
📌 Important:
OSI is mostly used for learning and understanding
Data flow:
Sender: Layer 7 → Layer 1
Receiver: Layer 1 → Layer 7
This is where the user interacts with the network
Provides services to applications
Examples:
Zoom
Browser (Chrome)
📌 When you send a message on Zoom, it starts here.
Responsible for:
Encryption
Decryption
Compression
Formatting
Example:
Zoom encrypts your message so no one else can read it
Compresses data to reduce size
Creates, manages, and terminates sessions
Keeps communication alive
Example:
Maintains your Zoom meeting connection
Handles reconnects if connection drops briefly
Ensures data delivery
Decides which protocol to use
Protocols:
TCP: Reliable, ordered, slower (Zoom login, file transfer)
UDP: Fast, no guarantee (Live video/audio)
Responsible for routing
Uses IP addresses
Finds the best path for data
📌 Routers work at this layer.
Uses MAC addresses
Error detection
Creates frames
📌 Switches work at this layer.
Physical transmission of data
No logic, no addressing
Examples:
Ethernet cable
Fiber optic cable
Wi-Fi signals
📌 Hubs work at this layer.
The TCP/IP model is used in real networking.
| TCP/IP Layer | OSI Layers |
| Application | 7, 6, 5 |
| Transport | 4 |
| Internet | 3 |
| Network Access | 2, 1 |
📌 OSI = learning model
📌 TCP/IP = practical model
A protocol is a set of rules that devices follow to communicate.
Without protocols:
Devices won’t understand each other
Communication will fail
Examples:
HTTP / HTTPS → Web
TCP / UDP → Transport
FTP → File transfer
📌 Communication happens using protocols on the same layer.
A header is extra information added to data by each layer.
Header includes:
Source address
Destination address
Protocol information
Control details
Each OSI layer adds its own header.
Happens when data is sent
Data moves from Layer 7 → Layer 1
Each layer adds its header
📦 Envelope Example:
Data = letter
Each layer = one envelope
Final envelope is sent on the wire
Happens when data is received
Data moves from Layer 1 → Layer 7
Each layer removes its header
📦 Receiver opens envelopes one by one until data reaches application.
| OSI Layer | Data Name |
| Layer 7–5 | Data |
| Layer 4 | Segment |
| Layer 3 | Packet |
| Layer 2 | Frame |
| Layer 1 | Bits |
| Device | OSI Layer | Description |
| Hub | Layer 1 | Sends data to all ports |
| Switch | Layer 2 | Uses MAC addresses |
| Router | Layer 3 | Uses IP & routing |
Small network (home, office, school)
Uses switches
Large network (cities, countries)
Routers connect LANs
📌 Two or more routers connected together form a WAN
Dedicated physical server
Full hardware access
High performance
Example:
Company data center server
Multiple users share same server
Limited control
Lower cost
Example:
Cheap website hosting
One sender → one receiver
Most common communication
Example:
Sending message to one person on Zoom
Example:
Online live class to selected students
Example:
ARP request
📌 Broadcast works only in LAN, not across routers.
OSI = 7 layers
TCP/IP = 4 layers
Encapsulation = sender
Decapsulation = receiver
Hub → L1, Switch → L2, Router → L3
LAN = local, WAN = network of LANs
Bare metal = dedicated
Shared hosting = shared
Unicast / Multicast / Broadcast = delivery types
A router is a networking device mainly used to connect different networks.
1️⃣ Router is a Unicast Device
A router forwards data from one sender to one specific destination
It does not broadcast data to everyone
Uses IP addresses to make decisions
✅ (Correct: Router = Unicast device)
2️⃣ Router is Used at the Edge of LAN and WAN
Routers sit at the boundary (edge) between:
LAN (Local Area Network)
WAN (Wide Area Network)
📌 Example:
3️⃣ Router is a Layer 3 Device
Works at OSI Layer 3 (Network Layer)
Uses IP addressing
Makes routing decisions
4️⃣ Router is Used for Routing
Routing = selecting the best path for data
Router checks routing table and forwards packets
5️⃣ Routers Connect Different Networks
Router is used to communicate between two or more different networks.
If devices are on different networks, they cannot communicate directly.
A router acts as a gateway and forwards data from one network to another using IP addresses.
Example:
A device in 192.168.1.0/24 network can communicate with a device in 10.0.0.0/24 network only through a router.
If you want, I can also simplify it more for CCNA notes 📘
Your teacher showed router images and explained ports.
Serial ports are used for WAN connections
Used to connect:
Router to router
Router to ISP
📌 Common WAN media:
Fiber optic
Radio link
VSAT (Satellite)
✅ Correct term: Serial Interface / WAN Interface
Routers have Ethernet LAN ports for local networks.
There are three main Ethernet speed types 👇
| Name (Corrected) | Speed |
| Ethernet | 10 Mbps |
| Fast Ethernet | 100 Mbps |
| Gigabit Ethernet | 1000 Mbps (1 Gbps) |
❌ Wrong terms corrected:
“1 MB” ❌ → 10 Mbps / 100 Mbps / 1000 Mbps
“Super fast” ❌ → Gigabit Ethernet
📌 These LAN ports connect:
PCs
Switches
Servers
Your teacher showed inside and outside components.
Ethernet ports
Serial ports
Power port
Status LEDs
Cooling vents
CPU → Processes routing decisions
RAM → Running configuration
ROM → Bootstrap & POST
Flash Memory → IOS storage
Fan → Cooling
📌 Switches also have:
CPU
RAM
Fan
Ports
(but switches work at Layer 2 mainly)
Teacher explained that networking uses addresses.
1️⃣ MAC Address
2️⃣ IP Address
3️⃣ Serial Address (covered later)
📌 For now, focus was on MAC & IP
MAC (Media Access Control) address is a physical address
Assigned by the manufacturer
Stored in network interface hardware
Works at Layer 2
📌 Format example:00:1A:2B:3C:4D:5E
Your example is 100% correct ✅
A laptop can have:
Wi-Fi MAC address
Ethernet MAC address
Bluetooth MAC address
📌 Reason:
Teacher explained real banking infrastructure.
Central location
Contains:
Core switches
Routers
Database servers
Application servers
This forms a LAN at headquarters.
Branches connect to head office
Connection methods:
Fiber optic
Radio links
VSAT (Satellite)
This creates a WAN
When a user enters ATM PIN:
1️⃣ ATM machine
→ connects to local switch
2️⃣ Switch
→ connects to router
3️⃣ Router
→ sends data via WAN (fiber / radio / VSAT)
4️⃣ Head Office Router
→ Head Office Switch
5️⃣ Switch
→ Database servers
6️⃣ DB server
→ verifies PIN & balance
→ response sent back through same path
📌 This is a real-time network transaction
Teacher explained FinTech / Banking communication type.
One sender ↔ one receiver
Bi-directional
Very secure and reliable
📌 Used by:
Banks
FinTech apps
ATM networks
| Type | Explanation |
| Unidirectional | One-way communication |
| Bi-directional ✅ | Two-way communication |
| Omni-directional | One-to-many |
📌 Banking uses bi-directional point-to-point,
❌ NOT broadcast or omni-directional.
Router = Layer 3, Unicast, Routing device
Router connects LAN ↔ WAN
Serial ports → WAN
Ethernet / Fast Ethernet / Gigabit Ethernet → LAN
MAC address = physical address
One device can have multiple MACs
Banking networks use WAN + point-to-point
ATM → Switch → Router → WAN → Head Office → DB
MAC (Media Access Control) address is a physical address
It is assigned by the manufacturer
Stored in the network interface hardware
Works at OSI Layer 2 (Data Link Layer)
MAC address is unique worldwide
MAC address can be written in different formats, but the value remains the same.
Examples:
Colon format:
00:1A:2B:3C:4D:5E
Hyphen format:
00-1A-2B-3C-4D-5E
Cisco format:
001A.2B3C.4D5E
📌 These are just different representations, not different MACs.
A MAC address is 48 bits total.
It is divided into two main parts:
| Part | Size | Meaning |
| Vendor Code (OUI) | 24 bits | Company / Manufacturer |
| Serial Number | 24 bits | Unique device number |
✅ Corrected term:
📌 Example:
00:1A:2B | 3C:4D:5E
Vendor Serial Number
IP (Internet Protocol) address is a logical address
Works at OSI Layer 3 (Network Layer)
Used for routing and identification
Can change (unlike MAC address)
IPv4 address is 32 bits
Written in decimal dotted format
Example:
192.168.1.10
Each number is 8 bits (1 byte).
An IP address is divided into:
| Part | Purpose |
| Network Address | Identifies the network |
| Host Address | Identifies the device |
📌 Example:
192.168.1.10
Network Host
IPv4 addresses are divided into classes based on range.
| Class | Range | Usage |
| Class A | 1 – 126 | Very large networks |
| Class B | 128 – 191 | Medium networks |
| Class C | 192 – 223 | Small networks |
| Class D | 224 – 239 | Multicast |
| Class E | 240 – 255 | Experimental |
📌 Commonly used:
Home & office → Class C
Large companies → Class A / B
Networking uses three number systems:
| System | Base |
| Decimal | Base 10 |
| Binary | Base 2 |
| Hexadecimal | Base 16 |
Digits: 0–9
Used by humans
Example: 192
Digits: 0 and 1
Used by computers
Example:
11000000
Digits: 0–9 and A–F
Used to represent binary in short form
Commonly used in MAC addresses
| Hex | Binary |
| 0 | 0000 |
| A | 1010 |
| F | 1111 |
Group binary into 4 bits
Convert each group to hex
Example:
Binary: 1100 1010
Hex: C A
Example:
Hex: A
Binary: 1010
| Address | Uses |
| MAC Address | Local delivery (Layer 2) |
| IP Address | Routing between networks (Layer 3) |
| Binary | Internal computer processing |
| Hexadecimal | MAC addresses, troubleshooting |
MAC = physical, 48-bit
Vendor code = 24-bit, Serial number = 24-bit
IPv4 = 32-bit
IP = Network + Host
Classes: A, B, C, D, E
Binary = base 2
Decimal = base 10
Hex = base 16
MAC addresses often written in hex
In networking, we never directly deploy a network.
There is a proper lifecycle:
Decide:
How many sites (branches, offices)
How many devices
Which IP range to use
Which devices (routers, switches)
Good design avoids future problems
Implement the designed network
Configure:
IP addresses
Routers
Switches
Connect devices physically and logically
Fix issues after deployment
Check:
Connectivity
IP conflicts
Routing problems
📌 Rule:
👉 A good design = less troubleshooting
Before assigning IPs, we must answer:
How many sites are there?
How many devices per site?
Will the network grow in future?
📌 Based on this, we choose:
IP class
IP range
Subnet size
Home network
Small office
Few devices
Schools
Small companies
Multiple departments
Large organizations
Multiple branches
Centralized management
These are called Carrier Networks.
📌 Examples:
Jazz
Zong
Ufone
Telenor
Characteristics:
Very large IP ranges
Millions of users
Operate at national or international level
Why Class A does not use 127.x.x.x?
127.x.x.x is reserved for loopback
Used for testing the local system
Example:
127.0.0.1 → localhost
📌 This address:
Never leaves the device
Is not used in networks
✅ That’s why:
Class A usable range = 1 – 126
127 is skipped
IPv4 has 5 classes, based on leading bits and range.
The first few bits of an IP address
Used to identify the class of IP
| Class | Leading Bits | First Octet Range |
| A | 0 | 1 – 126 |
| B | 10 | 128 – 191 |
| C | 110 | 192 – 223 |
| D | 1110 | 224 – 239 |
| E | 1111 | 240 – 255 |
📌 Leading bits help routers understand:
Network size
Address type
Network bits: 8
Host bits: 24
Hosts per network: 16,777,214
Network bits: 16
Host bits: 16
Hosts per network: 65,534
Network bits: 24
Host bits: 8
Hosts per network: 254
📌 Formula:
Hosts = 2^n – 2
(–2 for Network ID & Broadcast)
Assigned to a device
Used for communication
Example:
192.168.1.10
Identifies the network
First address of the network
Cannot be assigned to a device
Example:
192.168.1.0
| Type | Purpose | Usable? |
| Network ID | Identifies network | ❌ No |
| Broadcast ID | Send data to all devices | ❌ No |
Last IP of the network
Sends data to all devices in LAN
Example:
192.168.1.255
192.168.1.0 → Network ID (Reserved)
192.168.1.1 → First usable IP
192.168.1.254 → Last usable IP
192.168.1.255 → Broadcast (Reserved)
📌 Usable IPs = 254
| Class | Range | Usable Hosts |
| A | 1–126 | 16 million |
| B | 128–191 | 65k |
| C | 192–223 | 254 |
| D | Multicast | N/A |
| E | Experimental | N/A |
Network lifecycle: Design → Deploy → Troubleshoot
Choose IP range based on sites & devices
Carrier networks = Jazz, Zong
127.x.x.x = loopback (not usable)
Leading bits identify IP class
Network ID & Broadcast are reserved
Hosts = 2^n – 2
A Public IP address is an IP address that:
Is globally unique
Is reachable from the internet
Is assigned by an ISP
Paid (you get it from ISP)
Usually static (does not change frequently)
Used for:
Websites
Servers
Public services
📌 Example use:
Google server
Bank website
Public cloud servers
A Private IP address is used inside local networks (LANs).
Not reachable directly from the internet
Can be reused in different networks
Free to use
Private IP ranges are defined by RFC 1918.
| Class | Private IP Range |
| Class A | 10.0.0.0 – 10.255.255.255 |
| Class B | 172.16.0.0 – 172.31.255.255 |
| Class C | 192.168.0.0 – 192.168.255.255 |
📌 These ranges are internationally reserved for private use.
| Feature | Public IP | Private IP |
| Scope | Global | Local |
| Unique | Yes (Worldwide) | No |
| Cost | Paid | Free |
| Internet Access | Direct | Indirect |
| Example | Bank server | Home router |
A subnet mask tells:
Which part is network
Which part is host
| Class | Subnet Mask | CIDR |
| Class A | 255.0.0.0 | /8 |
| Class B | 255.255.0.0 | /16 |
| Class C | 255.255.255.0 | /24 |
📌 These are default (classful) subnet masks.
Two devices can communicate directly if:
They are in the same network
They have the same subnet mask
📌 Example:
192.168.1.10 /24
192.168.1.20 /24
✔ Direct communication
📌 Example:
192.168.1.10 /24
192.168.2.10 /24
❌ Direct communication not possible
A router is required to:
Connect different networks
Forward packets between networks
Router acts as a default gateway
📌 Flow:
Device → Switch → Router → Other Network
Switch works only in same network
Router works between different networks
Router uses:
IP address
Routing table
📌 Without router:
Devices use private IPs
Router has:
Private IP (LAN side)
Public IP (WAN side)
📌 Router connects:
Private Network ↔ Internet
Public IP = paid, global, static
Private IP = free, local, reusable
RFC 1918 defines private IP ranges
Subnet mask defines network & host
Same network → direct communication
Different networks → router required
Router = gateway between networks
On Day 7, our focus shifted from IP addressing and routing theory to device connectivity methods and a hands-on practical lab using Cisco Packet Tracer. This day was important because it connected theory with real-world networking practice.
In networking, there are three primary ways to connect to network devices (routers/switches):
A console connection is used for initial configuration and troubleshooting of network devices.
It provides out-of-band management, meaning it works even if the network is down
Requires a console (rollover) cable
Commonly used when:
A device is new
IP configuration is not done
Remote access (SSH/Telnet) is not available
Key Points:
No IP address required
Direct physical access is needed
Very secure (local access only)
The AUX port is mainly used for remote management via modem.
Works as an out-of-band connection
Mostly used in older or backup management scenarios
Less common in modern enterprise networks
Key Points:
Requires authentication
Used when console access is not physically possible
Largely replaced by SSH today
Interface connections require a working network and an IP address. There are two main types:
Telnet allows remote login to network devices over the network.
Uses TCP port 23
Data (username/password) is sent in plain text
Disadvantages:
Not secure
Vulnerable to sniffing and attacks
SSH is the secure alternative to Telnet.
Uses TCP port 22
Encrypts all communication
Advantages:
Secure
Industry standard for device management
Preferred over Telnet in production environments
After learning the theory, we implemented a practical topology in Cisco Packet Tracer.
2 × Routers: Cisco 2811 (IOS 15)
2 × Switches: Cisco 3560-24PS
2 × PCs
2 × Servers
The two routers were connected using a serial cable (red cable with a clock sign)
One router acts as DCE (provides clock rate)
The other acts as DTE
Before connecting:
This step is required because routers do not have serial ports by default.
For each router:
One 3560-24PS switch was connected
Under each switch:
1 PC
1 Server
This created two LANs, each connected to a router, and both routers connected via a WAN serial link.
We also learned about network cable types and where to use them.
Used to connect different types of devices:
PC → Switch
Switch → Router
Server → Switch
Logic:
Used to connect same type of devices:
PC → PC
Switch → Switch
Router → Router (Ethernet)
Logic:
Note: Modern devices support Auto-MDIX, which automatically handles this, but understanding crossover cables is still important for fundamentals.
Used for console connections only:
PC → Router Console Port
PC → Switch Console Port
Key Points:
Pin order is fully reversed
Not used for data traffic
Commonly available, so no need to create it manually
Learned three device connectivity methods: Console, AUX, and Interface-based (Telnet/SSH)
Understood why SSH is preferred over Telnet
Built a complete multi-router network in Cisco Packet Tracer
Learned when to use straight, crossover, and rollover cables
Gained practical exposure to WAN serial connections using DCE/DTE concepts
Day 7 strengthened our understanding of real-world networking, combining theory, device access methods, and hands-on topology design.
Done 👍
I’ve written Day 7 as a detailed blog-style document and placed it in the canvas.
What I covered clearly and step by step:
All three device connectivity methods: Console, AUX, and Interface
Telnet vs SSH (ports, security, use cases)
Complete Cisco Packet Tracer practical:
2 × Cisco 2811 routers (IOS 15)
VIC-2D smart serial module
Serial DCE/DTE connection (red cable with clock)
2 × 3560-24PS switches
PCs and Servers under each LAN
All cable types:
Straight-through
Crossover
Rollover (console)
In Day 8, we started practical router configuration in Cisco Packet Tracer. This lecture focused on understanding the Router CLI (Command Line Interface), its different modes, and performing basic IP configuration on router interfaces.
This day is very important because it is the foundation of real router configuration.
Cisco routers and switches are mainly configured using the CLI. When we open a router and click on the CLI tab, the router starts in a default mode.
The CLI works in different modes, and each mode has a specific purpose.
This is the default mode
Prompt ends with >
Very limited access
Used only for basic checking
Example:
Router>
To move to the next mode, we use:
enable
Prompt ends with #
Full monitoring access
Required to enter configuration modes
Example:
Router#
This mode allows us to:
View configuration
Restart the device
Enter global configuration mode
Used to configure global router settings
Prompt ends with (config)#
Command to enter:
configure terminal
Example:
Router(config)#
From this mode, we can configure:
Hostname
Routing
Interfaces
Used to configure a specific interface
Prompt ends with (config-if)#
Example:
interface fastEthernet0/0
Example prompt:
Router(config-if)#
After entering interface configuration mode, we learned how to assign an IP address.
Syntax:
ip address <IP-address> <subnet-mask>
Example:
ip address 192.168.1.1 255.255.255.0
This command assigns:
IP address
Subnet mask
By default, router interfaces are administratively down.
To enable the interface, we use:
no shutdown
After this command:
Interface becomes up
Communication is possible
📌 Without no shutdown, the interface will not work even if IP is configured.
| Action | Command |
| User → Privileged | enable |
| Privileged → Global | configure terminal |
| Global → Interface | interface <name> |
| One step back | exit |
| Directly to privileged | end |
The question mark (?) is used for help.
Examples:
Router> ?
Router# show ?
Router(config)# ip ?
This shows:
Available commands
Available options
The TAB key is used to auto-complete commands.
Example:
conf<TAB>
Completes to:
configure
Benefits:
Faster typing
Fewer mistakes
All real router configuration is done using CLI
Understanding modes prevents misconfiguration
IP addressing and no shutdown are mandatory steps
This lecture prepares us for:
Router-to-router communication
Routing protocols
WAN and LAN configuration
Routers use CLI for configuration
CLI has multiple modes
enable enters privileged mode
configure terminal enters global config
Interface mode is used for IP configuration
ip address assigns IP
no shutdown activates interface
? gives help
TAB completes commands
In Day 9, we went deeper into router internals and configuration management. This lecture helped us understand how routers are physically and logically structured, how Cisco routers store configurations, and how to secure and save configurations properly.
A Cisco router is built in a hierarchical physical structure:
Modules: Hardware cards installed in a router
Slots: Locations inside the router where modules are placed
Ports / Interfaces: Actual physical connectors used to attach cables
Interfaces are named using this pattern:
<interface-type> <module>/<slot>/<port>
Example:
FastEthernet 0/1/1
Meaning:
Module number = 0
Slot number = 1
Port number = 1
📌 This numbering helps uniquely identify each physical interface on a router.
The command:
show ip interface brief
Provides a quick summary of all router interfaces.
Interface name
IP address
Interface status (up/down)
Protocol status
📌 This command displays temporary (running) information stored in memory.
Cisco routers use multiple types of memory, each with a specific role.
Volatile (data is lost on reboot)
Stores:
Running configuration
Routing tables
ARP cache
Temporary processes
📌 Commands like show running-config read data from RAM.
Non-volatile (data remains after reboot)
Stores:
📌 Used to load configuration when router boots.
Non-volatile
Stores:
📌 Router loads IOS from flash into RAM during boot.
Non-volatile
Stores:
POST (Power-On Self-Test)
Bootstrap program
📌 Used during router startup.
Stored in RAM
Active configuration
Lost after reboot if not saved
Command:
show running-config
Stored in NVRAM
Loaded when router starts
Command:
show startup-config
To save the current running configuration:
write
or
copy running-config startup-config
Copies configuration from RAM → NVRAM
Ensures configuration is not lost after reboot
📌 Always save configuration after making changes.
Stored in plain text (or weak encryption)
Less secure
Stored in hashed form
Much more secure
Overrides enable password if both are set
Command example:
enable secret mypassword
📌 Best practice: Always use enable secret.
Can be reversed
Original password can be recovered
One-way process
Cannot be reversed
📌 Cisco uses hashing for enable secret.
The command:
show version
Displays detailed system information:
IOS version
Router uptime
Hardware model
Memory size
Configuration register
📌 Very useful for troubleshooting and audits.
Router interfaces follow module/slot/port structure
show ip interface brief gives quick interface status
RAM is volatile, NVRAM and Flash are non-volatile
Running config is in RAM, startup config is in NVRAM
Always save config using write
enable secret is more secure than enable password
Hashing is one-way, encryption is reversible
show version provides system-level details
Day 9 built a strong understanding of router internals, memory, and secure configuration handling, which is essential before moving into advanced routing topics.
In Day 10, we focused on deep interface-level troubleshooting using Cisco router commands. The goal of this lecture was to understand how links behave, how to read interface status correctly, and how to identify real-world WAN issues step by step.
This lecture is extremely important because most real network problems start at the interface/link level.
Previously, we used:
show ip interface brief
which gives summary information for all interfaces.
In Day 10, we learned to use:
show interface <interface-name>
show interface serial 0/0/0
show interface serial 0/1/1
📌 This command shows detailed information for a single interface, allowing us to troubleshoot one interface at a time.
The most important line in the output is the first line.
Example output:
Serial0/0/0 is up, line protocol is up
Interface is up → Physical / hardware layer is working
Line protocol is up → Software / data-link layer is working
📌 Simple rule:
Interface = Physical side
Line protocol = Software side
This single line already tells us where the problem is.
A link is the connection between two routers.
Example:
One router belongs to Jazz (Customer)
The other router belongs to PTCL (Service Provider)
📌 PTCL provides:
Bandwidth
WAN connectivity
📌 Jazz consumes that service.
The link exists only when both sides are correctly configured and active.
There are four possible interface status combinations:
| Interface | Line Protocol | Meaning |
| up | up | Link is working perfectly |
| down | down | Physical problem or interface shutdown |
| up | down | Software / configuration issue |
| down | up | Practically not possible |
Jazz router: down / down
PTCL router: administratively down
✔ Your understanding is correct.
PTCL controls the WAN interface provided to Jazz
PTCL can shutdown the interface using:
shutdown
administratively down, line protocol down
down, down
📌 This usually means:
Service provider has disabled the link
Cable is disconnected OR
Interface is shut down on one side
To test connectivity, we learned the ping command.
ping <destination-ip>
Uses ICMP (Internet Control Message Protocol)
Sends echo request packets
Receives echo reply packets
📌 Ping helps us verify:
IP reachability
Link connectivity
Physical side is up
Line protocol is down
This means:
Cable is connected
Interface is enabled
But software/configuration problem exists
There are three common reasons:
1️⃣ Encapsulation mismatch
2️⃣ Clock rate issue (DCE/DTE)
3️⃣ Keepalive mismatch
In Day 10, we focused mainly on encapsulation.
By default, Cisco serial interfaces use:
We can verify encapsulation using:
show interface serial <interface-name>
This output shows whether the interface is using:
HDLC
PPP
Frame Relay
On the PTCL router, we changed encapsulation:
encapsulation ppp
While on the Jazz router, encapsulation remained:
Physical layer → up
Line protocol → down
📌 This confirms:
show interface gives detailed interface info
First line of output is the most important
Interface = hardware, line protocol = software
Link exists only if both sides are correctly configured
down/down usually means physical or shutdown issue
up/down usually means configuration mismatch
Ping uses ICMP echo requests
Encapsulation mismatch (HDLC vs PPP) breaks the link
Day 10 built strong real-world WAN troubleshooting skills, which are essential for service-provider and enterprise networks.