BGP - definition of Internal BGP (iBGP)

Internal BGP or iBGP is implemented between BGP routers that are in the same autonomous system. There is no specialized command to implement an iBGP peering. An iBGP peering is characterized by the fact that the AS argument in the neighbor command is the same as the AS of the router command. Note the following BGP configuration:

router bgp 2 bgp log-neighbor-changes neighbor remote-as 2

This BGP router is in AS 2, and it is peering with a neighbor with a remote AS which is also 2. Thus, this is an iBGP peering. For more information about establishing a full mesh of iBGP peerings, take a look at BGP - iBGP full-mesh peering.

This is in contrast to eBGP peering. For a general overview of the difference between the two, take a look at BGP internal vs external.