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A Quick Dive into BGP

Welcome all, after a short intro of BGP, let us try to extend our knowledge about BGP. We all are aware that BGP is inter-Autonomous System(AS) protocol and we also know what an Autonomous System is, in BGP world. Before going further, take a look at network below.


From the network above it is quite clear that routers connecting two AS are using eBGP protocol(external BGP), on the other hand, routers used for connection within AS are using iBGP(internal BGP) protocol. Thus, BGP is of 2 types Internal BGP and external BGP. All the other routers within AS, use IGP protocol, they are not aware of iBGP running routers.

We know, how important our neighbours are, especially when you want to pass on some information 😛 Similarly, it is necessary to identify neighbours so that route updates can be forwarded in BGP. Neighbours in BGP are manually configured, also, it is not required that neighbours must be directly connected. AS are so wide in the area that, keeping a direct connection between neighbours is not feasible. But if someone is not identified as neighbour then, information won’t be passed on. That is why, all the iBGP routers within AS must be configured as neighbours of each other(just like full mesh connectivity), otherwise route updates won’t be sent to those routers.

Route updates from neighbours are stored in local BGP table, from that table best routes are entered in the Routing Table. Suppose, you have a cold war with some AS owner, and you don’t want to allow any data flow of that AS, within your AS. Then, don’t worry, BGP is smart enough to satisfy your cunning needs 😛 BGP is configurable. It has attributes like. weight, local preference, AS Path etc. and based on these attribute values, a route can be selected as the best path. Also, there might be multiple paths to reach a destination, in that case also, attributes are used to decide which path to select. For more detailed knowledge of these attributes and path selection criteria, you can refer this link. The following image makes it easy to picture the scenario.


BGP is the most significant protocol and has lot more hidden within it. This quick dive into BGP is enough to create a connection with BGP. BGP can be better understood by doing practical configurations and then analysing the BGP’s path selection process. You can use GNS application to simulate BGP networks and analyse them. So, get started and dive into BGP..!!

Happy networking with BGP..!!!

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