Boot Sequence , Internal Component & Cisco 3 Layer Model

1

Router Power-On Boot Sequence1. Perform power-on self-test (POST):- When the router is powered up,

microcode stored in ROM performs a POST sequence. This is used to ensure that elements such as the CPU, memory, and interfaces are capable of functioning correctly.

2. Load and run bootstrap code :- The bootstrap program is used to initialize the CPU and boot functions of the router. The bootstrap program is responsible for locating and loading the router's IOS.

3. Find the Cisco IOS Software.4. Load the Cisco IOS Software.5. Find the configuration.6. Load the configuration.7. Run the configured Cisco IOS Software.

Router Internal Components

RAM

- Temporary storage for router config files- RAM content is lost on power down

RAM Stores - Routing tables

- ARP cache - Fast switching cache

- Packet buffering - Packet hold queues

4

NVRAM - Non-volatile RAM

- Stores backup/startup configuration files

- Content is not lost when router is powered down or restarted.

Flash :-

- EEPROM (Electronically Erasable Programmable Read-Only Memory)- Holds the Cisco IOS

- Allows updating of software without replacing the Flash chip- Store Multiple versions of IOS

- Retained on power down

5

ROM- Contains POST (Power On Self Test)

- A bootstrap program (loads the Cisco IOS)

ROM Functions

Contains microcode for basic functions

Network Structure Defined by Hierarchy

Distribution Layer

Core Layer

Access Layer

The Hierarchy of Connectivity

Access Layer CharacteristicsAccess Layer Characteristics

End station entry point to the network

Access Layer

Distribution Layer Characteristics

Distribution Layer Characteristics

• Access Layer Aggregation Point

• Routes traffic

• Broadcast/Multicast Domains

• Media Translation

• Security

• Possible point for remote access

Distribution Layer

Core Layer Characteristics

• Fast transport to enterprise services

• No packet manipulation

Core Layer