CCNA All in One

Maybe you will raise a question: “Why can’t we only use OSPF or EIGRP instead?” The most important reason is BGP greatly supports path control. Maybe you learned and understood about EIGRP, OSPF routing protocols. They are different but both of them have the same purpose: find the most optimal path to the destination. But when we are working in ISP level we don’t care much about this. In ISP we really want to control the path, even it is not the most optimal path, to the destination. For example, how can you choose which path to go from AS 1 to AS 3 in the topology below? Suppose an IGP (like OSPF) is using all the way. With default values and same bandwidth on all the links, OSPF will choose the path AS 1 -> AS 2 -> AS 3. In order to manipulate the path we will have to change the bandwidth of some interfaces on the edge routers of each AS. A few months later we want to change the path then we have to configure the bandwidth again. With BGP we can easily define the path...

We really want to show you why we need BGP first but it is very difficult to explain without understanding a bit about BGP. So we will learn some basic knowledge about BGP first. First we need to understand about the different between Interior Gateway Protocol and Exterior Gateway Protocol. The difference between them is shown below: –  Interior Gateway Protocol (IGP) : A routing protocol operating within an Autonomous System (AS) like OSPF, EIGRP… Usually routers running IGP are under the same administration (of a company, corporation, individual) –  Exterior Gateway Protocol (EGP) : A routing protocol operating between different AS. BGP is the only EGP used nowadays In the topology above R1, R2 and R3 should run an IGP to communicate with each other because they are in the same AS. But to connect with other routers in another AS (like a different ISP), R1 and R3 must use an EGP. With BGP, the term  autonomous system  (AS) refers to a network that oper...