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why we are not using one routing protocol like OSPF for example for all autonumus systems.

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    Because different protocols do different things, required for different scenarios. – Jesse P. Jul 7 at 18:18
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    Because they're autonomous. That means they make their own decisions about how to operate their networks. – Ron Trunk Jul 7 at 21:28
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To be blunt, in my network, I will run whatever I want; nobody outside my network has any say in it. When my network connects to someone else's network, we have to come to an agreement. For decades now, The Internet™ has agreed on BGP as the inter-AS routing protocol.

(You might find a few odd locales using IS-IS, but that's still a local agreement.)

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I could give you a very thorough long-winded explanation that you may/may-not understand... Instead I will say the following:

  • Scalability: OSPF, EIGRP, and other routing protocols do not scale to levels needed efficiently, only BGP does this.
  • Control: No routing protocol offers more means of manipulation than BGP.
  • Security: Protocols like OSPF running globally across the internet would introduce significant security risks.

That's just a few reasons.

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