Routing

Routing

• The process of forwarding packets from one network to another network choosing the best path by using the administrative distance from the routing table
• Routing table consists of the best routes for every destination.
• Administrative Distance (AD) is a value that routers use in order to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative Distance counts the reliability of a routing protocol. Administrative Distance (AD) is a numeric value which can range from 0 to 255. A smaller Administrative Distance (AD) is more trusted by a router, therefore the best Administrative Distance (AD) being 0 and the worst, 255.

Route Source	Default Distance Values
Connected interface	0
Static route	1
Enhanced Interior Gateway Routing Protocol (EIGRP) summary route	5
External Border Gateway Protocol (BGP)	20
Internal EIGRP	90
IGRP	100
OSPF	110
Intermediate System-to-Intermediate System (IS-IS)	115
Routing Information Protocol (RIP)	120
Exterior Gateway Protocol (EGP)	140
On Demand Routing (ODR)	160
External EIGRP	170
Internal BGP	200
Unknown*	255

Types of routing

There are three types of routing.They are

• Static routing
• Default routing
• Dynamic routing

Static routing

It is configured by the administrator manually

It is secure and fast

It was mainly used by the small organization which consists of less no.  of routers

Administrative distance for static route is 0 and 1.

Disadvantages

Used for small network

Everything should be done manually

Network change effect the complete network

Configuration of Static route

Router#config terminal

Router(config)#ip routing

Router(config)#ip route  <Destination Network>  <Destination Subnet Mask>  <next-hop address>

outer(config)#ip route <Destination Network>  <Destination Subnet Mask>   <Exit interface> <administrative distance> <permanent>

Note:-Administrative distance and permanent are the optional entries

Examples

Ex-1:

Router#Config terminal

Router (config)#ip routing

Router (config)#ip route  192.168.0.1  255.255.255.0  192.168.0.3

Ex-2:

Router#Config terminal

Router(config)#ip routing

Router(config)#ip route  192.168.0.1  255.255.255.0  s0 1 P

 Default Routing

• Manually adding the single route for the entire destination is known as default route.
• Default route used when destination is unknown.
• This was the last preferred route in the routing table.
• When there was no entry for the destination network in a routing table, the router will forward the packets o its default route.
• It will help in reducing the size of our routing table.

Configuration of Default route

Router#config terminal

Router(config)#ip routing

Router(config)#ip route  0.0.0.0 0.0.0.0  <next-hop address>

outer(config)#ip route 0.0.0.0 0.0.0.0   <Exit interface> <administrative distance> <permanent>

Note:-Administrative distance and permanent are the optional entries

Examples

Ex-1:

Router#Config terminal

Router (config)#ip routing

Router (config)#ip route  0.0.0.0 0.0.0.0  192.168.0.3

Ex-2:

Router#Config terminal

Router(config)#ip routing

Router(config)#ip route  0.0.0.0 0.0.0.0  S0 1 P

Dynamic routing

Dynamic routing use Routing Protocols that dynamically discover network destinations and how to get to them. Dynamic routing allows routing tables in routers to change if a router on the route goes down. Examples of Routing Protocols are RIP, EIGRP and OSPF. 

There are three basic types of routing protocols.They are

Distance-vector

Link-state

Hybrid

Distance-vector Routing Protocols: Distance-vector Routing Protocols use simple algorithms that calculate a cumulative distance value between routers based on hop count.

Example: RIP1, RIP2, IGRP

Hop count: A routing metric that calculate the distance between a source and destination on the number of routers in the path.

Link-state Routing Protocols: Link-state Routing Protocols use sophisticated algorithms that maintain a complex database of internetwork topology.

Example: OSPF IS-IS

Hybrid Routing Protocols: Hybrid Routing Protocols use a combination of distance-vector and link-state methods that tries to incorporate the advantages of both and minimize their disadvantages.

Example: EIGRP