Routing Protocols are one of the most important topics in CCNA Training. To clear the CCNA exam or to study beyond CCNA, one must have good command over routing protocols. Depending on our need, we can use different types of Routing Protocols. There are also some Routed Protocols that are built in almost every device that can participate in network. Both are different concepts and both are used in a network. So let’s take a look at basics.
We have already discussed why we need routing in a network? Protocols are set of rules over which communication happens. Take it this way, what will happen if you try to talk with a person who doesn’t speak the same language that you do? Will the communication happen? I guess not. Both need some common language in which both can talk. Same here. If two devices want to transfer some data, they need to agree on some common protocols and when it come to connecting two different network, these are Routing Protocols.
Routing Protocols vs. Routed Protocols
Routing Protocols are responsible of finding best path from source to destination while Routed Protocols are used to carry the data over the selected path. This data can be files, web data, e-mail data and more. Most popular Routed Protocol is IP. Every device that participates in network have a build-in IP protocol in protocol suite.
Routing protocols use static or dynamic advertisement messages to learn about other devices or protocols in a network. These work at layer 3 and only layer 3 devices are capable of running routing protocols. You will not see them in common devices like a PC or laptops, Printer.
There are mainly 3 types of Routing Protocols in a network. Static, Dynamic and Default. We can use them as per our need. There are some things that should be considered while choosing a Routing Protocol like use, size of network, reliability and convergence time. It’s hard to tell which is best routing protocol among all. If network is small, nothing is better than a static route. But if it is covering a large area, there is no way using and maintaining static route is easy. So choose carefully.
What is AD Value?
AD stands for Administrative Distance. It measure how trustworthy a network really is. Varying from protocol to protocol, lower AD is always best. As you can see in the image, Static can be considered as best Routing Protocol. AD value varies from 0 to 255: 0 means most reliable and 255 means “Never to be consider ever.” AD values can be used for Path Selection too. If you set AD=255 for certain path, that will never be installed in Routing Table.
So remember, if you have multiple paths for same destination, path with lower AD will always be preferred.
Static Routing Protocol
As the name Static indicates, here, you have to manually give the path information to the router. We use IP Route command to enable static routing in a network.
Advantages of this routing is that it is most reliable because you provide information yourself. Router doesn’t need to rely on updates from other router to make the routing table. There is no overhead on router and it is easy to implement.
Disadvantage is that if network is large, it is hard to implement static routing. Plus if link fails, static route can not re-route the network traffic.
Dynamic Routing Protocol
Unlike the static routing, here router learns about other network with some update packets. These packets are sent in some interval. It is better than Static Routing as it can work great with large network too. Also if some link fails and there is one more path, traffic will be redirected to other path automatically. Cool, right?
Dynamic Routing Protocols are classified in Interior Gateway Routing Protocols and Exterior Gateway Routing Protocols. There are three categories in IGP,
Distance Vector Routing Protocols work on distance (how far the destination is?) and Vector (in which direction). DVRP measure distance in terms of hop counts which means how many networks it has to cross if it want to reach the destination. Means in which direction, hop count is less? Traffic will always be forwarded to lesser hop count path, doesn’t matter if another path is getting better transfer speed.
RIP and IGRP comes under this category.
Link State Routing Protocols work on Shortest Path First algorithm. They also have complete information of network like a map. There is no limit of size. Network can be big or small, it will work efficiently. Most popular LSRP is OSPF. Other one is IS-IS. These are also much better than DVRP in term of convergence
Hybrid Routing Protocols have the property of both Distance Vector and Link State. EIGRP routing protocols comes under this category. It uses properties of distance vector protocol to reduce the processing power requirement, but exchanges routing table information only when their tables change to reduce transmission overhead. EIGRP also insure that routing loop never occurs in a network.
Default routing can be used with Static Routing. Default route (also called as gateway of last resort) specifies a path that a router must select if it doesn’t know how to reach to the destination. If router receives a packet with a destination that is not in it’s routing table, it will normally drop the packet. But if there is a default route, that unknown packet will be forwarded to the address associated with default route.
It is useful to have default route if there are a lot of routes to be handled.
We will configure them one by one in upcoming posts.