BGP ( iBGP or eBGP; How to decide? )

My question is as below:

How does the BGP decides whether to use eBGP or iBGP?
Can you list the parameters that it uses to make the above decision.

@andrew
@ReneMolenaar
@lagapides

Thanks in advance.

Best Regards.

Hi Rebecca
This all depends on whether the two BGP speakers are in the same ASN (autonomous system number). If they are, iBGP is used. If they are not, eBGP is used.

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Hi Rebecca,
I’ll add a config to illustrate what Andrew already told above. Explanations are given after “//” :
---------

Router R1 
---
router bgp 100  // R1 is in AS100
  neighbor 10.10.10.1 remote-as 100 //remote peer is declared in AS100 -> R1 will have an iBGP session with this peer
  neighbor 20.20.20.2 remote-as 200 //remote peer is declared in AS200 -> R1 will have an eBGP session with this peer
[...]
!
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