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The Origin attribute is a well-known mandatory attribute, type code 1,
that indicates the origin of the routing update. BGP allows the
following three
types of origins:
- IGP – The prefix is internal to the originating AS.
- EGP – The prefix was learned by way of 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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