Introduction to Routing and Packet Forwarding

§Router components and their functions”
§CPU - Executes operating system instructions
§Random access memory (RAM) - Contains the running copy of configuration file. Stores routing table. RAM contents lost when power is off
§Read-only memory (ROM) - Holds diagnostic software used when router is powered up. Stores the router’s bootstrap program.
§Non-volatile RAM (NVRAM) - Stores startup configuration. This may include IP addresses (Routing protocol, Hostname of router)
§Flash memory - Contains the operating system (Cisco IOS)
§Interfaces - There exist multiple physical interfaces that are used to connect network. Examples of interface types:
-Ethernet / fast Ethernet interfaces
-Serial interfaces
-Management interfaces


§Routers are computers that specialize in sending data over a network.
§Routers are composed of:
-Hardware i.e. CPU, Memory, System bus, Interfaces
-Software used to direct the routing process
§IOS
§Configuration file
§Routers need to be configured. Basic configuration consists of:
-Router name
-Router banner
-Password(s)
-Interface configurations i.e. IP address and subnet mask
§Routing tables contain the following information
-Directly connected networks
-Remotely connected networks
-Network addresses and subnet masks
-IP address of next hop address


§Routers determine a packets path to its destination by doing the following
§Receiving an encapsulated frame & examining destination MAC address.
§If the MAC address matches then Frame is de-encapsulated so that router can examine the destination IP address.
§If destination IP address is in routing table or there is a static route then Router determines next hop IP address. Router will re-encapsulate packet with appropriate layer 2 frame and send it out to next destination.
§Process continues until packet reaches destination.
§Note - only the MAC addresses will change the source and destination IP addresses do not change.

source : cisco ; ccna2 router and routing basic