Why Does BGP Work on Application Layer?

BGP Overview:

Border Gateway Protocol or BGP is considered as an inter-domain routing protocol which would be designed to provide loop-free routing between separate routing domains that would be containing several independent routing policies (autonomous systems). The Cisco software implementation of BGP v4 which would include support for 4-byte autonomous system numbers as well as multiprotocol extensions in order to allow BGP to carry routing information for IP multicast routes as well as multiple Layer 3 protocol address families including IP Version 4 (IPv4), IP Version 6 (IPv6), Connectionless Network Services (CLNS), Virtual Private Networks Version 4 (VPNv4), and Layer 2 VPN (L2VPN). This module would be containing a conceptual material in order to help you understand how BGP would be implemented in Cisco software. Before we discuss its application, if you wish to do it practically, I would suggest you join the prep courses offered by the SPOTO.

The application for BGP:

Autonomous Systems

Within the Internet, an autonomous system otherwise known as (AS) is considered to be controlled by the network by a single entity typically an ISP (Internet Service Provider) or a very large organization with independent connections so as to multiple networks. These Autonomous Systems must have an officially registered Autonomous System Number or ASN, which they get from their Regional Internet Registry: ARIN, APNIC, LACNIC, AFRINIC or RIPE NCC. A unique ASN (AS Number) is allocated to each of the AS for use in BGP routing. AS numbers would be considered as quite important because the ASN is going to uniquely identify each network on the Internet.

Peering

Two routers that would be establishing a connection for exchanging BGP information, would be referred to as BGP peers. Such BGP peers exchange routing information between them by the means of BGP sessions that run over TCP, which it would be quite reliable, connection-oriented & error free protocol.

Selecting the Best Path

Once the BGP Session has been established, the routers would be able to advertise a list of network routes that they could have access to and would be scrutinizing them to find the route with the shortest path.

Of course, BGP doesn’t make sense when you are connected to just one other peer, such as your ISP, because he is always going to be the best and only path to other networks. However, when you would be connected to multiple networks at the same time, then certain paths would be faster, shorter or more reliable than others. Like Google's AS15169 peers with 270 other networks, one of which is Digital Ocean Inc. AS14061. They are both going to be connected to other ISPs for the internet, however in this way, since they would now have peered together, they could exchange routing information, so now that their router would be able to choose a shorter path of connectivity which they might have between themselves. 

Misconfiguring or Abusing BGP

Since BGP is believed to be at the absolute core of the internet when it is misconfigured or abused it could wreak havoc across the large portions of the internet. Apart from misconfiguration, BGP could be also abused for malicious purposes. By taking advantage of unsecured BGP peering or not verifying routes that would be announced from your peers, attackers might announce IP ranges that they don’t actually own and thus routing internet traffic would be towards their links, essentially which would be creating a MITM attack. 

I would say that you are going to need you to have more and more practical practices, so as to make learning it more precisely and for that, I would suggest you to gain the preparation courses offered by the SPOTO.

if you want to know more news and improve your understanding of BGP, you can join SPOTO's WhatsApp: http://bit.ly/2IRmvF2, and you can learn much basic knowledge of Cisco certification.