AIGP, Accumulated Interior Gateway Protocol, is an optional nontransitive path attribute that is included with advertisements throughout an AS. It is the fourth decider in a BGP best path after ‘Locally Originated via Network or Aggregate Advertisement’
Interior Gateway Protocols, OSPF or EIGRP, use the lowest path metric to identify the shortest path to a destination. They can not other scale to the same magnitude as BGP.
AIGP provides an ability for BGP to maintain and compute a path metric in an environment that utilises multiple ASs with a unique IGP routing domain in each AS.
There are some guidelines to AIGP metrics:
- A path with a AIGP metric is preferred to a path without one
- If a next hop address requires a recursive look-up, the AIGP path needs to calculate the metric to include the distance to the next-hop address. The metric is the original AIGP metric plus the next-hop AIGP metric
- A path with a lower AIGP metric is preferred