Routing Protocols – main aspects of dynamic routing protocols

Dynamic routing protocols are used to maintain newest routing information in communications between network devices, discover other networks and choose the best paths to forward messages. One of the earliest dynamic routing protocols was RIPv1 (Routing Information Protocol). This protocol used hop count as a communication element and it was not appropriate for larger network implementations. RIPv1 does not support VLSM implementations. Newer dynamic protocols were invented to cover RIP limitations, RIPv2, EIGRP (Enhanced Interior Gateway Routing Protocol – Cisco proprietary protocol), OSPF (Open Shortest Path First) and IS-IS (Intermediate System-to-Intermediate System). These type of protocols were called Interior Gateway Protocols because they were implemented inside a network. Another type of dynamic routing protocols is the Exterior Gateway Protocols, these kind of dynamic routing protocols are used to interconnect networks. A well known exterior gateway protocol is BGP (Border Gateway Protocol).

   In network communications, routers are used to forward packages between one host to another. Routers need to exchange network updates between each other. Whenever dynamic routing protocols are used, changes in the network are automatically noticed by all routers involved in communication. For exchanging routing messages more network resources are used (memory and CPU, network bandwidth etc.). It is easier to implement and maintain networks that use dynamic routing protocols rather than static routing, because once devices are configured, networks will work independently. Dynamic Protocols use tables and databases to maintain network information, algorithms to determine the best path and messages to be exchanged between them.
   There are some main differences between static and dynamic routing. Scalability is affected with static routing, while networks that use dynamic routing can be easily scaled. In Static routing, a network topology change must be manually configured and in dynamic routing this is done automatically. Static routing requires less configuration knowledge while dynamic routing requires more complex knowledge. Static routing is more secured, the complexity of static routing increases with network size and network resources are less used than in dynamic routing. Paths remain the same in static routing while in dynamic routing they can change frequently (also paths must be changed manually in static configurations).
   The following image will display the main routing protocols used in today’s networks:
Dynamic Routing Protocols

When talking about Interior Gateway Protocols (IGP) or Exterior Gateway Protocols (EGP), we have to say something about Autonomous Systems, also known as AS. Autonomous Systems are basically networks that have one ownership and a single point of administration. IGPs are implemented inside networks while EGPs are implemented outside of them (used to interconnect Autonomous Systems).

   There are two types of Interior Gateway Protocols:
Distance vector protocols – use hop count for metric and Belman-Ford algorithm to determine best paths between devices. Routers that use distance vector protocols do not have a whole image of the network topology (they know only the distance and the path they need to follow to reach remote networks), they usually send the complete routing tables to their neighbours and are not used in larger network implementations. These type of protocols are usually easier to configure than link-state routing protocols.
Link-state routing protocols – devices share a complete image of network topology. They also exchange only the newest network information, but after the whole network is converged (when all routers share the same routing information). These kind of protocols can be used in large network implementations and the convergence is achieved very fast. Link-state routing protocols are harder to configure and require more knowledge then distance vector protocols.
When working with dynamic routing protocols, you’ll have to remember the following elements:
Load balancing  is a computer networking method to distribute workload across multiple computers or a computer cluster, network links, central processing units, disk drives, or other resources, to achieve optimal resource utilization, maximize throughput, minimize response time, and avoid overload (Wikipedia :
 Metric– is used by routing protocols to evaluate paths for packets to follow. Dynamic routing protocols use different metrics, RIP uses hop count, EIGRP uses bandwidth and delay and OSPF uses bandwidth etc. A network can use multiple routing protocols and each one would have it’s own routing methods, paths and metrics. These are the metrics used in IP networks:
Hop count – used by RIP, it counts the number of routers a packet must travel to reach it’s destination.
Bandwidth – used by different routing protocols, the path with the highest bandwidth is chosen.
Cost – a numeric value that can be configured by Administrators to determine the priority of a certain path.
Delay – counts the time that a packet will take to travel from source to destination.
Load – measures the amount of traffic of a path.
Reliability – measures the probability of link failure.
   Classful Routing Protocols – do not send network mask in routing update messages. Cannot be used when implementing VLSM (when using different network mask).
   Classless Routing Protocols – sent network mask in routing update messages. Support for VLSM implementations.
Administrative distance – is the measure used by Cisco routers to select the best path when there are two or more different routes to the same destination from two different routing protocols (Wikipedia: The routing protocol with the lowest AD wins. Below is the list of dynamic routing protocols administrative distance:

Protocol Administrative distance
Directly connected
Static route 1
EIGRP summary route 5
External BGP 20
Internal EIGRP 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EGP 140
External EIGRP 170
Internal BGP 200

In the following video tutorial, I will show you how to view the administrative distance and metric on a Cisco router, how to configure static routing and other useful configuration commands. I hope this was informative folks, have a wonderful day.


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