The Origin attribute is a well-known
mandatory attribute (type code 1) that 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.
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