Routing Fundamentals

• Routing is the process of forwarding packets from one network to another.
• This is sometimes referred to as a relay system.
• When IP packets travel over the Internet, routing information is exchanged between the devices that control the flow of information over the Internet.
• These devices are known as routers, and they use the IP address as the basis for controlling the traffic. 
• A router passes data between multiple networks. 
• It works at the OSI Network Layer (Layer3), which means that it must be able to understand the data packets so that it can route them to their destination.
•  They are essentially computers optimized for handling packets that have to be transferred between separate networks.
•  Routers attempt to send packets from their source to their destination in the fastest way possible which is not always the absolute shortest path.
• These devices need to talk the same language to function properly, though they belong to different administrative domains. 
• For example, one router may be in India, and the receiving router may be in USA. It is necessary that a routing protocol is followed for smooth flow of traffic. 

Explanation of Routing

When a router takes a packet and sends it closer to its final destination, we say it has forwarded a packet. 

In the simplest terms, that's what a router does; it forwards packets toward their destination and it tries to do so at the least possible "cost".

Cost, for a router, is not measured in dollars and cents, but in "hops". 

Every time a packet is routed between one router and another, a number in the data packet called the hop count increases by 1. 

If the hop count reaches certain preset limits (for example RIP allows a maximum of 16 hops), the packet may be discarded as undeliverable since in 16 hops, the routers have not been able to deliver the packet to the destination address.

For routers, however, cost is not the absolute variable it is in the physical world because it's not much more expensive to go "the long way" from an origin to a destination when dealing with the Internet. This is because:

• Data moves at the speed of light (or very close to it over copper wires), so any additional distance is not very relevant.
• The Internet was designed to be redundant. If the first route fails, try a second or third.

The design of the Internet is solid because it reroutes data packets all the time as a result of events like natural disasters and power failures.

Routing Tables:

All the routing information needed for a router to forward packets to a next hop relay device can be found in the router’s routing table. 
Again, if a destination logical address is not found in the table, the router discards the packets. 
A gateway of last resort can be set on the router to forward packets not listed in the routing table. This is called setting the default route.
To see a routing table:

RouterA#sh ip route


Administrative Distances:

When configuring routing protocols, you need to be aware of administrative distances. These are used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.
Table  shows the default administrative distances that a Cisco router

will use to decide which route to take to a remote network.

Default Administrative Distances
Route Source                                         Default Distance
Connected interface                                  0
Static route                                               1
EIGRP summary                                      5
External BGP                                           20
EIGRP                                                     90
IGRP                                                       100
OSPF                                                       110
IS-IS                                                        115
RIP                                                          120
EDP                                                        140
External EIGRP                                      170
Internal BGP                                           200

Unknown 255                                         (This route will never be used.)

Given below are the widely used routing protocols for routing Internet traffic:

• RIP v1
• RIP v2
• OSPF
• IGRP
• EIGRP
• BGP

Full Forms: Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Border Gateway Protocol (BGP).

Routing protocols are primarily distinguished into three types:

• Distance Vector Protocols
• Link State Protocols
• Hybrid Protocols

The table below provides the routing protocol used with different routed protocols:

Routing Protocol	Routed Protocol
RIP, OSPF,IS-IS, BGP,EIGRP	IP
RIP, NLSP, EIGRP	IPX
RTMP, EIGRP	AppleTalk