Internet Protocol (IP) routing protocols have one primary goal: to fill the IP routing table with the current best routes it can find. The goal is simple, but the process and options can be complicated. Routing protocols define various ways that routers chat among themselves to determine the best routes to each destination.
As networks grew more complex over time, routers gained both processing power and Random Access Memory (RAM). As a result, engineers designed newer routing protocols, taking advantage of faster links and faster routers, transforming routing protocols.
Routing protocols help routers learn routes by having each router advertise the routes it knows. Each router begins by knowing only directly connected routes. Then, each router sends messages, defined by the routing protocol, that list the routes. When a router hears a routing update message from another router, the router hearing the update learns about the subnets and adds routes to its routing table. If all the routers participate, all the routers can learn about all subnets in an internetwork.
When learning routes, routing protocols must also prevent loops from occurring. A loop occurs when a packet keeps coming back to the same router due to errors in the routes in the collective routers’ routing tables. These loops can occur with routing protocols, unless the routing protocol makes an effort to avoid the loops.
As you pursue your CCNA studies you will find that different authors mix and match the terms routing protocols, routed protocols, and routable protocols. The concepts behind these terms are not difficult but because the terms are so similar, they can be a bit confusing. In all of my posts, as well all Cisco documentation, these terms are generally defined as follows:
- Routing Protocol – A routing protocol is defined as a set of messages, rules, and algorithms used by routers for the overall purpose of learning routes. This process includes the exchange and analysis of routing information. Each router chooses the best route to each subnet in a process known as path selection and finally places those best routes in its IP routing table. Examples of a routing protocol include RIP, EIGRP, OSPF, and BGP.
- Routed Protocol and Routable Protocol – Both of these terms refer to a protocol that defines a packet structure and logical addressing, allowing routers to forward or route the packets. Routers forward or route packets defined by routed and routable protocols. Examples include IP and IPX. IPX is a part of the Novell NetWare protocol model.
Even though routing protocols such as RIP are different from routed protocols such as IP, they do work together very closely. The routing process forwards IP packets, but if a router does not have any routes in its IP routing table that match a packet’s destination address, the router discards the packet. Routers need routing protocols so that the routers can learn all the possible routes and add them to the routing table so that the routing process can forward routable protocols such as IP.
A routing protocol’s underlying algorithm determines how the routing protocol does its job. The term routing protocol algorithm simply refers to the logic and processes used by different routing protocols to solve the problem of learning all routes, choosing the best route to each subnet, and converging in reaction to changes in the internetwork. There are three main branches or families of routing protocol algorithms; Distance Vector, Link-State, and Balanced Hybrid.
Continue reading ‘Routing Protocols Overview’
