Routing Protocols

Cisco classifies its routing protocols into two categories, Interior gateway protocols (IGP) and Exterior gateway protocol (EGP).

Gateway protocols means that path to the another network , in other words it is a route to another network. The term router and gateway are interchangeable.

Interior Gateway Protocol

This is the routing protocol in a typical LAN environment. The key points to notice that you hold all the routers and all the routers know all the paths to different networks. RIP is an example of an interior gateway protocol where all the neighbor share the routing information.


This is the typical example showing the picture of a network running interior gateway routing protocol. Notice that all the routers are connected together and they know the route to each and every network.
RIP, OSPF, EIGRP are some of the most common interior gateway routing protocol.

IGP is further divided into two main categories; one is Distance Vector and other is Link-State.

Distance Vector

This protocol is only sees the next neighbor of it and that is all. The router does not know the each and every router it just knows its neighbors. RIP and EIGRP are distance vector.


In a distance vector the router A have complete knowledge about router B and D but it does not have a full knowledge of all the routers. It knows something about router C and E but it is not that useful. This mechanism results in a loop condition and there are several mechanism to prevent loop. RIP is pure distance vector protocol and it uses hop-count as its metric. However, EIGRP is called advanced distance vector because it does not uses the hop-count metric and it has some powerful features.

The hop-count metric counts the total number of the hops to the destination and the least number of hops path is chosen. In this case if router A has to get to router E by using a pure distance vector protocol it will count the hops to the destination, and if it chooses the path from router B to C to E it has 3 hops to get to the destination but if it chooses the path from D to E it has only 2 hops so the path with the minimum hops will be chosen. This is a true RIP working.

It is not the case with EIGRP because it does not use hop-count metric it uses bandwidth and delay as its metrics and the path with more bandwidth and least delay is chosen.


If we use EIGRP in above case the router A will choose the path from router B to C to E because it has more bandwidth and less delay. However, it has more hops but it does not matter if data gets to the destination quickly with more hops then go with more hops. This is how EIGRP works. Thinks of this protocol as road, a destination has to roads one has more bumps and crust but is shorter and other has crystal clear but a bit larger and it takes less time then shorter road then the best road is larger and this is it.

Link-State Protocol

In this protocol each router knows completely about all the router in its network. This protocol consumes more power and resources but it is very powerful protocol and mostly used in industries. OSPF is link-state protocol it uses bandwidth as its metric. OSPF is open standard and almost all vendors implement this where in other hands EIGRP is Cisco proprietary, if you have to run this you have to have Cisco equipments.

Exterior Gateway Protocol

It is another world we do not know the path of all the routers to the network we only know the router to our next hop.


This is the big picture of exterior gateway protocol where each router is owned by different companies and you send your data to next known router only and then the next router has to deal with the data. In interior gateway protocol you have complete control on your routers and it works in a company, but in exterior you do not have complete control on all routers you only have control on your own router.

When you were holding all the router in interior protocol that is considered only one Autonomous system no matter how many routers you hold, but when you go to exterior protocol each next hop is a separate Autonomous system.


Notice here I wanted to know the route to the particular domain, I my data is carried out by all the shown routers. I only control the 1st router but all the other are not in my hand, this is exterior routing protocols that carry my data to the destination. Each of the router shown above is considered a separate autonomous system.
Border gateway protocol (BGP) is an exterior gateway routing protocol.

Path Vector

It is type of exterior gateway and it works on autonomous number, an autonomous system could have hundred of routes in its LAN but it is considered as one. The internet traffic is tracker by these numbers and each organization is assigned an unique autonomous number if it becomes part of large network to transfer Internet data. BGP falls in this type and this protocol works on a mechanism that is choose the shortest path to the destination that is available.



Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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