8.8 Issue with Using Private AS  Numbers
8.8.9 The ORIGIN attribute
The ORIGIN attribute is a well-known mandatory attribute (type code 1), which indicates the origin of the routing update. BGP allows three types of origins:
  • IGP -- The prefix is internal to the originating AS.
  • EGP -- The prefix was learned via some EGP, such as BGP.
  • INCOMPLETE -- The prefix was learned by some other means, probably redistribution.

BGP considers the ORIGIN attribute in its decision-making process to establish a preference ranking among multiple routes. Specifically, BGP prefers the path with the lowest origin type, where IGP is lower than EGP, and EGP is lower than INCOMPLETE.