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I am currently learning about BGP at the moment and I have a question about the traffic flow and redistribution.

So ISPs typically run some sort of IGP within their AS. If I was to send a packet to Facebook, for example, this traffic would have to pass through my ISP's AS, right?

However, wouldn't this mean that there would be the need for redistribution from BGP to their IGP and then back to BGP? If that's the case, wouldn't this slow down the packet forwarding process and also add unnecessary complexity? It also seems a little nonsentical. Am I understanding this incorrectly?

Thank you in advance for your help

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  • Whatever the actual flow of routing information, routing protocols exchange and distribute that routing information between (participating) routers, they are not used to route the actual data packets.
    – Zac67
    Jun 22, 2023 at 9:54

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However, wouldn't this mean that there would be the need for redistribution from BGP to their IGP and then back to BGP? If that's the case, wouldn't this slow down the packet forwarding process and also add unnecessary complexity?

You have a fundamental misunderstanding about routing: routing protocols (BGP, OSPF, etc.) do not route packets. They simply provide information to tell the router which interface to forward the packet out of. Routers learn routes either by routing protocols, directly connected networks, or manually (added (static routes). We sometimes call this set of functions the "control plane."

The process of actual forwarding of packets is usually done in hardware and is called the "data plane."

Also, as you're learning about BGP, you will learn about BGP devices called "route servers" and "route reflectors." These devices exchange routing information, but may not be in the data path at all.

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