The BGP protocol requires that all iBGP peers within an AS have a connection to one another, creating a full-mesh of iBGP peering connections. (The exception to this is route reflection.) When a prefix is announced from one iBGP peer to another, the AS path is not changed. Due to the full-mesh requirement, all iBGP peers should have the same view of the BGP table, unless different routing policies have been applied to some of the peers.
When a router receives an iBGP announcement, the BGP process uses the BGP best path selection algorithm to determine whether the received announcement is the best available path for that prefix. If it is the best available path, then the BGP process uses this route as the BGP candidate route for insertion into the routing table, and the BGP process announces this path to all its peers, both iBGP and eBGP peers. If it is not the best available path, then the BGP process keeps a copy of this path in its BGP table, so that it can be used to calculate the best available path when path information for that prefix changes (for example, if the current best available path is withdrawn).
The BGP ID is a unique identifier in the format of an IP address used to identify a peer. The peering IP address is the actual IP address used for the BGP connection.
For iBGP peerings, the BGP ID and peering IP is frequently the IP address bound to that router's loopback interface. An iBGP session is usually contained within a local LAN, with multiple redundant physical links between the iBGP devices. For iBGP routes, reachability is all that is necessary, and the loopback interface is reachable so long as at least one physical interface is operational. Because of the physical and/or logical redundancy that exists between iBGP peers, iBGP peering on the loopback interface works well.
Since BGP does not provide reachability information, you must make sure that each iBGP peer knows how to reach other peers. To be able to reach one another, each peer must have some sort of Interior Gateway Protocol (IGP) route, such as a connected route, a static route, or a route through a dynamic routing protocol such as RIP or OSPF, which tells them how to reach the opposite router.