Routing Protocols – Basics


The routing protocol is an internal set of rules that describes the way how should a router chooses a path and make routing possible between networks. They typically using algorithms to calculate the best possible route from one network to another.

The routing protocols are divided into two main categories:
1-IGP – Internal Gateway Protocol – IGP’s are made for internal networks inside of an organization, from LAN to LAN
2-EGP – External Gateway Protocol – EGP’s are made for routing between ISP-s (Internet Service Provider), WAN to WAN

Classful routing
In the old days of routing all the routing protocols was classful protocols, that means that the router assumed the subnet mask, from the class of the network, like:
for networks from 00000001( to 01111111 (, it automatically used subnet mask, from to it used mask and so on.. The subnet mask was not included in the Ethernet frame, so it was not possible to use the VLSM ( variable length subnet mask ), which made a lot of waste of the IP addresses.
If we have to make a new department on network, we would have to use the classful boundary of /24 – to make a new department with one PC, which is a waste of 254 IP addresses.

IGP – Internal Gateway Protocol

IGP routing protocols can be Distance Vector or Link-State protocols. The difference between them is the algorithm they use to define the route,

Distance Vector is using the Bellman-Ford algorithm, and it’s based on the HOP-COUNT
Link-State protocols are using the Dijkstra algorithm, and it’s based on calculating the “cost” of a route (by calculating with bandwidth)
*** Hop-Count and Cost are the metric used by the protocols to define the quality or reliability of a route.

Distance Vector protocols can be also classful (old) and classless:
Classful: RIP and IGRP
For IPv6: RIPng and EIGRPip6

Link-State protocols ( ONLY CLASSLESS ):
OSPFv2 and IS-IS
IPv6: OSPFv3 and IS-ISIP6

Administrative Distance

Every routing protocol has an Administrative Distance that helps the router to decide which one shall be used over another when there are multiple protocols defined on the router.

The administrative distance is a value from 0 to 255, where the 0 is the most preferred and the 255 won’t be included in the routing table.

0 – Most preferred – AD 0 is rendered to the directly connected interfaces.
1 – Static routes – route configured statically on point to point connection.
5 – EIGRP summary route – This is a route where the EIGRP configured router summarises multiple networks into a bigger one for using fewer data in routing tables.
20 – External BGP – Route for interconnecting external networks
90 – Internal EIGRP
100 – IGRP
110 – OSPF
115 – IS-IS
120 – RIP
170 – External EIGRP
200 – Internal BGP

Which routing protocol to use?

*The stateful protocols are obsolete legacy protocols, and the best way is to not implement them anywhere.

*Static routes are not manageable and scalable, they shall be used for defining exit interfaces, or backup routes when the primary route fails.
>>Click here to see configuration of static routes.

For small networks ( routes do not exceed with max 10 – 15 hops )
-RIPv2 could do the job because of easy implementation.
For larger networks, the recommended protocols are EIGRP or OSPFv2
-They are scalable protocols with all the required attributes for route and best path decisions in larger networks.
-Harder to implement and manage, but reliable by nature of functioning.


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