Hello @giorgiortavidze,
Many thanks for your question. It is not necessary to configure next-hop-self on all iBGP routers and can have undesirable results. Any time we configure this we are modifying the original routes and modifying routes should always be done with care. Even better we try to avoid the need to modify routes by appropriate design choices.
Next hop self is one possible solution if a router receiving the unmodified route would be unable to understand how to reach the specified next hop. Although we can modify the route in BGP, a common alternative is to provide the router with an additional piece of information about how to reach the specified next-hop, for example providing a static route to that next hop or having another routing protocol (such as OSPF) operating more locally sharing information about exit points from your network.
If we apply next hop self by default a common issue is that traffic will flow but follow a sub-optimal path such as looping via a transit AS (it might be some remote branch site) rather than taking a more direct route with more bandwidth available. Connectivity can feel “slow” but not broken in this case.
So you have several options you can design into your network. Have fun choosing!
Kind regards,
Jon