Posts Tagged ‘Routing’
August 18th, 2008 | 
Author: Routing & Packet Forwarding
Routing Protocols & Concepts
Router as a Computer:
Computers that specialize in sending packets over the data network.
Responsible for interconnecting networks.
Routers are the network center.
Generally have a min of 2 connections:
WAN connection (Connection to ISP).
LAN connection.
In addition to packet forwarding, routers provide other services as well.
A router connects multiple networks.
This means that it has multiple interfaces that each belong to a different IP network.
Each network that a router connects to typically requires a separate interface.
Routing Table:
Used to determine the best path.
Incoming packets’ dest IP address is looked up in the table.
The table includes an exit interface or next hop.
Once a match is found, the router encapsulates the IP packet into the L2 frame of the exit interface.
Router as a computer:
Router components and their functions:
CPU – Executes OS instructions.
RAM – Contains the running copy of config, routing table, buffers, etc. Contents lost when pwr is off.
ROM – Holds diagnostic software used @ startup and the bootstrap prgm.
NVRAM – Stores startup config.
Flash – Contains the OS, non-volitile.
Interfaces – usually multiple physical interfaces. Examples of interface types:
Ethernet / Fast Ethernet interfaces.
Serial interfaces.
Management interfaces (concole).
Internetwork Operating System:
IOS
A multitasking OS integrated with routing, switching, internetworking, and telecomm functions.
Stored as an image. Many different IOS images available.
IOS interface is CLI
On boot, the startup-config file in NVRAM is copied into RAM and stored as the running-config file.
Changes entered by the admin are stored in the running-config and are immediately implemented by the IOS.
Grouping Devices into Networks and Hierarchical Addressing :
The original Internet – a few trusted users, no security issues.
Security needs have changed dramatically.
Dividing networks based on ownership means access can be prohibited, allowed, or monitored.
E.G. college network – admin, research, and student networks.
Security between networks is implemented in a router or firewall at the perimeter of the network.
The Internet = millions of hosts identified by a unique IP.
If each host had to know the address of all hosts it would severely degrade their performance.
Dividing networks so hosts who need to communicate are together reduces overhead (o/h) of hosts needing to know all IPs.
For all other destinations, hosts only need to know the address of a router.
This router is called a gateway. It serves as an exit from that network.
Hierarchical Addressing:
A hierarchical address uniquely identifies each host.
It also has levels that assist forwarding packets across internetworks, enabling division based on those levels.
So, hierarchical addressing supports communications between internetworks.
Postal addresses are examples of hierarchical addresses.
Refering only to the relevant address level (country, state, city, etc.) at each stage makes this process very efficient.
No need at each stage to know the all details (except Last).
IP addresses work in a similar way.
IP addresses have a network portion and a host portion.
Routers forward packets between networks using only the network portion.
Once delivered, the whole address will have been used to deliver the packet.
If necessary, additional layers of addressing can be created (subnet portion).