Why do we need BGP?

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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 like this: “to go from AS 1 to AS 3 we will go through AS 2 then AS 4 then AS 5”. Moreover we can easily control the ratio of traffic passing through each link. BGP can do this because it has a rich set of features to control the paths to the destination. One of the most popular features BGP uses to control the path is known as “attributes”. However, discussion about “attributes” is out of the scope of this tutorial, which is dedicated for CCNA learners.
In fact, BGP is a path vector protocol. Path vector protocol does not rely on the bandwidth of the links (like OSPF) or hop count (like RIP) or a group of parameters (like EIGRP). Path vector protocol relies on the number of autonomous systems it has to go through. In other words, it choose the path with least number of autonomous systems (shortest AS Path) to reach the destination, provided that the path is loop-free. Of course we can change the path easily for our purpose.
For example in the topology above, if all the values are default and same, BGP will choose the path AS 1 -> AS 2 -> AS 3 for traffic from AS 1 destined to AS 3 because it only has to go through one AS (AS 2).
Another reason to use BGP is BGP can handle very big routing tables. The ISP level of routing do had large number of routes, which IGP cannot handle. BGP handle such large routes between AS. Currently the global Internet routing table contains over 500,000 routes.
Another good reason to use BGP is because the great benefits of MPLS-based virtual private networks (MPLS VPN). MPLS VPN is beyond the scope of this tutorial but in general MPLS VPN provides traffic separation and path isolation on a shared network infrastructure and BGP is the background layer so that VPN can operate well. For example your corporation is running Voice over IP (VoIP), Video on Demand (VoD), Internet service… on the same infrastructure then you can use MPLS VPN to separate these services as if they were running on theirs own infrastructure.

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