Introduction to Routing and Packet Forwarding

Router as Computer

• Describe

Computers that specialize in sending packets over the data network. They are responsible for interconnecting networks by selecting the best path for a packet to travel and forwarding packets to their destination

• Routers are the network center

Routers generally have 2 connections:

- WAN connection (Connection to ISP)

- LAN connection

• Data is sent in form of packets between 2 end devices
• Routers are used to direct packet to its destination
• Routers examine a packet’s destination IP address and determine the best path by enlisting the aid of a routing table
• Router components

- CPU

- Random access memory (RAM)

- Read-only memory (ROM)

- Non-volatile RAM (NVRAM)

- Flash memory

- Interfaces

• Major phases to the router boot-up process

- test router hardware

- Locate & load Cisco IOS software

- Locate & load startup configuration file or enter setup mode

• Verify the router boot-up process:

The show version command is used to view information about the router during the bootup process

• Routers and the Network Layer

Routers use destination IP address to forward packets

- The path a packet takes is determined after a router consults information in the routing table.

- After router determines the best path

- Packet is encapsulated into a frame

- Frame is then placed on network medium in form of Bits

• Routers Operate at Layers 1, 2 & 3

- Router receives a stream of encoded bits

- Bits are decoded and passed to layer 2

- Router de-encapsulates the frame

- Remaining packet passed up to layer 3

-Routing decision made at this layer by examining destination IP address

- Packet is then re-encapsulated & sent out outbound interface