This situation can occur if routing policies are not consistently deployed through the network. Close calls occur when the best path isn't being followed to an external destination, which, of course, can be nettlesome. Verifying the configuration and checking for alternate routes is the first step in sorting out the problem. The problem could be a result of the fact that a router is a route reflector with two clients, and one of them is not physically attached to the reflector but has another router in the path. Retana recommends that unless bgp cluster-ids are needed for administration, they shouldn't be used, so network operators can maintain greater network flexibility.
Simplicity is the key.
It is commonly used in large Internet service provider backbones and is a standard specified in IETF RFC 1771. IGBP peers may use loopback interfaces to eliminate a dependency that would otherwise occur with the use of an IP address of a physical interface. BGP decision algorithm comes into play when a BGP speaker receives updates that describe different paths to the same destination. After choosing the best path, BGP propagates the path to its neighbors based on attributes such as next hop, local preference, and administrative policies. BGP updates exchange routing information among network devices.