CCNA NOTES 4 # Cisco Router Basics: RAM, NVRAM, Flash, Boot Sequence & CLI Explained

 

CCNA / Networking Fundamentals

Router Hardware
& Configuration


ISR Series · CLI Modes

Boot Sequence · Interfaces

Memory · Passwords

01

Hardware Memory Types

ROM
ROM Read Only Memory
Stores the boot sequence (POST + Bootstrap). Factory-programmed, non-volatile. Also holds mini-IOS (RxBoot) for recovery.
RAM
RAM (DRAM) Volatile
Holds the running-config, routing table, and all active processes. Lost on reboot.
NVRAM
NVRAM Non-Volatile
Stores the startup-config. Survives reboots. Copied to RAM as running-config at boot.
FLASH
Flash Memory Non-Volatile
Like a hard drive — stores the IOS image. Common versions: 12.x, 15.x. Retained on reboot.
02

Bootstrap / Boot Sequence

1
POST ROM
Power-On Self Test — checks all hardware: CPU, RAM, interfaces, slots.
2
Bootstrap ROM
Bootstrap program runs from ROM and locates the IOS image to load.
3
IOS Loading FLASH → RAM
IOS loaded into RAM from Flash. If not found, tries TFTP server or ROM (limited IOS).
4
Config Loading NVRAM → RAM
startup-config copied from NVRAM to running-config in RAM. If absent → Setup Mode.
5
Ready State RAM
Interfaces initialised, routing processes start — router forwards traffic.
03

Router CLI Modes

ModePromptHow to EnterPurpose
User EXECRouter>Default on loginBasic monitoring only — limited show commands,
no config changes allowed.
Privileged EXECRouter#enableFull monitoring — all show & debug commands;
gateway to config mode.
Global ConfigRouter(config)#configure terminalModify router configuration — hostname, passwords,
routing protocols, interfaces.
04

Device Access Methods

→
Physical / Console
Direct cable to console port. No IP needed. Used for initial setup. CLI access only.
→
Remote — Telnet
Network-based access via IP. Not secure — sends data in plaintext.
→
Remote — SSH
Encrypted remote access. Recommended over Telnet for all secure environments.
→
AUX Port
Remote access via modem. Used as backup management path.
→
CLI vs GUI
CLI = Command Line (all config via typed commands). GUI = Graphical interface (web-based management).
05

Router Ports & Ethernet Speeds

Interface TypeSymbolSpeed
EthernetE / e10 Mbps
FastEthernetFa / Fe100 Mbps
GigabitEthernetGe / G1,000 Mbps (1 Gbps)
10 GigabitEthernetTe10,000 Mbps (10 Gbps)
Ethernet Distance: Up to 80 to 100 m
Serial (WAN) Interface
DCEProvides the clock signal — must set clock rate
DTEReceives the clock. Default encapsulation: HDLC. PPP supports PAP/CHAP auth.
T1/E1T1 = 1.544 Mbps  |  E1 = 2.048 Mbps
06

Router Passwords

Password TypeUsed ForSecurity LevelNotes
Console PasswordConsole port accessBASICPhysical direct access protection.
Enable PasswordPrivileged EXEC modePLAIN TEXTStored unencrypted — not recommended.
Legacy use only.
Enable SecretPrivileged EXEC modeENCRYPTEDMD5 (128-bit) hash. Overrides enable password.
Always preferred.
VTY PasswordRemote access (Telnet/SSH)DEPENDSTelnet = plaintext (insecure). SSH = encrypted
 (use this).
07

Bits & Bytes — Units

BITS (b)
BYTES (B)
Bit
0 or 1
Byte
= 8 bits
Kilobit
= 1,024 bits
Kilobyte
= 1,024 bytes
Megabit
= 1,024 Kb
Megabyte
= 1,024 KB
Gigabit
= 1,024 Mb
Gigabyte
= 1,024 MB
Terabit
= 1,024 Gb
Terabyte
= 1,024 GB
Note: Mbps = Mega bits per second (lowercase b). MB = Mega Bytes (uppercase B).
08

Interface / Port Numbering

Format: type line/slot/port   e.g. Fa1/1/2
Numbering Rules
▸Line card — always starts from 1
▸Slot number — always starts from 0
▸Port (Router) — starts from 0
▸Port (Switch) — starts from 1
▸Routers do not have a line number
▸Switches may or may not have a line number
Special Interfaces
LOLoopback — logical, no physical port. Used for OSPF/BGP Router-ID and testing. IP: /32 mask.
SVIVLAN Interface — used in Layer 3 switching. e.g. interface vlan 10
⚡ Quick Revision Cheatsheet
Serial → WAN, DCE/DTE
DCE → provides clock
Default encap → HDLC
PPP → PAP / CHAP auth
Console → local access
AUX → remote via modem
Loopback → logical, no port
Flash → stores IOS
NVRAM → startup-config
RAM → running-config (volatile)
Enable secret → MD5 encrypted
SSH → secure remote (use over Telnet)
ISR G2 → 1900 / 2900 series
POST → checks CPU, RAM, interfaces
NETWORKING FUNDAMENTALS  · ROUTER HARDWARE & CONFIGURATION
R. CHOUDHARY
Location: New Delhi, Delhi, India

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