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I was studying about the BGP protocol and I had this question:
Let's say we have an AS having both BGP border routers and internal routers . I know that the border routers should form a full mesh to have iBGP sessions and exchange external routes (learned by eBGP).
I was wondering how the internal routers are going to learn about these external routes. Do the internal routers also establish iBGP sessions?; do the border routers redistribute the external routes through IGP protocol?; do the internal routes have static (or not static) default routes to border routers (but how to choose which one)?
Does anyone have an idea how the current network operators deal with this issue?

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Typically (but this is not always the case), the border routers inject a default route into the IGP. This can be done either by redistributing a route from BGP into the IGP, or redistributing a static default route.

It's also possible to inject a few summary routes into the IGP if it is necessary to steer certain traffic to a particular border router.

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  • So let's assume we have injected a default route with final hop to a border router, and anything with a destination beyond our AS will be sent to this router. If this border router has learned through iBGP that another border router is "closer" to the destination AS, it should send this traffic back into our AS to reach this preferred router, right? But then doesn't this create a loop?
    – IlianaXn
    Sep 12, 2022 at 16:00
  • As I said in the second paragraph, if you have multiple exit points, and you need to steer certain traffic to a particular border router, then you would inject the required routes into the IGP.
    – Ron Trunk
    Sep 12, 2022 at 16:26

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