1. Why we need iBGP ?
  2. Why can’t we use IGPs rather than iBGP?
  3. Why AD value of iBGP more than eBGP and other IGPs ?
  4. Why OSPF protocol is not used in Internet rather than BGP ? I believe link-state protocols invented earlier to Path Vector protocols like EGP and BGP ? But still BGP is chosen instead of OSPF/IS-IS ? why ?
  1. iBGP is used for connecting the routers inside an Autonomous System. Also can be seen in the Transit Autonomous system.
  2. iBGP advertises the global prefixes within the AS to the other edge routers because IGPs are not designed to scale to handle the number of global inetrnet prefixes that exist in the global internet routing table.
  3. The AD values are decided by the Protocol. By default eBGP is preferred over iBGP.
  4. BGP adds multiple features like flexibility in selecting the best path using PATH ATTRIBUTES, also it provides scalability that means millions of routes can be installed in the BGP routing table but the IGPs are not designed to handled such large number of routes.

hope this helps!