BGP - Accumulated IGP metric

The Accumulated IGP metric attribute, or AIGP, is a is a BGP attribute introduced to influence the path selection process. This attribute allows BGP to take into account the IGP metric to an AS (Autonomous System) boundary router for its best path selection.

This attribute was specifically designed to resolve issues with "hot potato routing". In hot potato routing, traffic is sent out of an AS as soon as possible, without considering the path inside the AS. This could lead to sub-optimal paths being chosen. By considering the IGP metric, AIGP provides a way to control routing based on the shortest internal path to the exit point of the AS.

To use AIGP in BGP path selection, the AIGP attribute is inserted into a route by the AS border router. This router calculates the IGP cost from itself to all other border routers in the AS. It then inserts the maximum of these costs into the AIGP attribute of the route before advertising the route to a neighboring AS.

The AIGP attribute is optional and non-transitive, which means it doesn't have to be included in a BGP update, and if it's not recognized, it's ignored and not passed on.