GoKawiilBorder Gateway Protocol ( BGP ) is the de facto routing protocol of the Internet. It offers built-in mechanisms to allow entities, represented by Autonomous Systems (ASes), to express how they want to send and receive traffic on the Internet. One such mechanism is path attributes , which carry essential routing information and metadata for their associated route. The path selection algorithm processes some of these path attributes in a deterministic sequence to calculate the best path for this specific prefix.
Using our unique position on the Internet, we took an investigative look at one of the well-known mandatory attributes in BGP, the ORIGIN attribute. ORIGIN must be present in every BGP prefix announcement and should not be modified by any router after being set by the originating one. What we found through our own experiments was a dramatic ~70% of observed paths in numerous vantage points have a different ORIGIN value compared to what was set by the originating Autonomous System. This ORIGIN attribute manipulation has a significant impact on the way traffic is forwarded on the Internet, as we’ll explore in this post.
BGP ORIGIN and its operational history
The ORIGIN attribute indicates how a route was injected into BGP — not to be confused with the origin AS that indicates which AS announced a route. It has three possible values:
(0) IGP: Indicates the route is interior to the originating AS
(1) EGP: A historical value indicating that the route was learned via the old Exterior Gateway Protocol (EGP) , which is obsolete and not intended to be used in the modern Internet
(2) INCOMPLETE: Indicates the route was learned via an unknown or external source
Among total observable routes from all the public BGP collectors of RIPE RIS and RouteViews , 89.8% have ORIGIN set to IGP, 3.5% to EGP, and 6.7% are INCOMPLETE. As mentioned above, EGP is meant to be deprecated entirely and INCOMPLETE carries a minority share of total routes. These figures suggest that while IGP is by far the most popular value for ORIGIN, more than 10% of routes have an EGP or INCOMPLETE value that could make a difference in routing decisions.
As a part of the BGP path selection process , a router evaluates the ORIGIN if two routes have equal Local Preference and AS_PATH length, selecting and installing the path with the lower ORIGIN value.
Aside from the path selection decision, it is worth noting that RFC4271 states the following about ORIGIN:
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