Dynamic Routing

Dynamic Routing

Dynamic routing is used to update the routing table and find networks on the routers. It is easier than static routing and default routing, but it is more expensive in terms of bandwidth and CPU utilization.

The main purposes of a dynamic routing protocol are:

  • Finding the remote networks
  • Maintaining routing information
  • Choosing the best path to the destination network
  • Ability to find a new best path if the current path is not available

There are two types of routing protocols used in the internetworks:

  1. Interior Gateway Protocols (IGPs)
  2. Exterior Gateway Protocols (EGPs)

1. Interior Gateway Protocols (IGPs)

It is a distance-vector interior gateway routing protocol developed by Cisco. It is used by routers to exchange the routing information within the same Autonomous System (AS).

Note: Autonomous System (AS) is the collection of networks under the common administrative domain, which means that all routers share the same routing table information in the same AS.

2. Exterior Gateway Protocols (EGPs)

Exterior Gateway Protocols are used to exchange routing information in the different networks.

Classes of Routing Protocol

Every routing protocol defines the set of rules that how routers communicate with each other.

There are following classes of the routing protocols:

  1. Distance vector

The distance-vector protocols are used to find the best path that a data packet takes to reach the destination.  It selects the best path based on the distance.

It measures the distance by the least number of the router from which a packet has to pass to reach the destination.

Example: Routing Information Protocol (RIP).

Link state

Link-state protocols are also called shortest-path-first protocols. It uses the Dijkstra algorithm to find the shortest path. It transmits routing information to all other routers running on the same protocol, not only directly connected neighbors routers.

Example: Open Shortest Path First (OSPF)

Hybrid                                                 

Hybrid is a combination of both Distance-vector and link state.                    

Example: Enhanced Interior Gateway Routing Protocol (EIGRP)

Difference between the Distance Vector and the Link-state Routing Protocols:

Distance vector Link-state
It sends the entire routing table It sends only link-state information
It is simple to configure It is difficult to configure
It does not know the network topology. It knows the entire network topology.
It needs additional procedures to avoid the routing loops. By construction, routing loop cannot occur.
Example:RIP Example: OSPF

Administrative Distances

Administrative distance is a default value assigned to each routing protocol that will find the best route when there are two or more different routes to the same destination from two different routing protocol.

It is an integer value from 0 to 255.

Where 0 is the most trusted value and 255 means that no traffic will be passed via this route.

Route Source Default AD
Connected interface  0
Static route 1
EIGRP 90
IGRP 100
OSPF 110
RIP 120
External EIGRP 170
Unknown 255 (This route never be used)