The accumulated IGP (AIGP) metric is an optional non-transitive attribute that can be attached to selected routes (using route policies) to influence the BGP decision process to prefer BGP paths with a lower end-to-end IGP cost, even when the compared paths span more than one AS or IGP instance. AIGP is different from MED in several important ways:
AIGP is not intended to be transitive between completely distinct autonomous systems (only across internal AS boundaries)
AIGP is always compared in paths that have the attribute, regardless of whether they come from different neighbor AS or not
AIGP is more important than MED in the BGP decision process (see the section titled BGP decision process)
AIGP is automatically incremented every time there is a BGP next-hop change so that it can track the end-to-end IGP cost. All arithmetic operations on MED attributes must be done manually (for example, using route policies)
In the SR OS implementation, AIGP is supported only in the base router BGP instance and only for the following types of routes: IPv4, label-IPv4, IPv6 and label-IPv6. The AIGP attribute is only sent to peers configured with the aigp command. If the attribute is received from a peer that is not configured for aigp or if the attribute is received in a non-supported route type the attribute is discarded and not propagated to other peers (but it is still displayed in BGP show commands).
When a 7450, 7750, or 7950 router receives a route with an AIGP attribute and it re-advertises the route to an AIGP-enabled peer without any change to the BGP next-hop the AIGP metric value is unchanged by the advertisement (RIB-OUT) process. But if the route is re-advertised with a new BGP next-hop the AIGP metric value is automatically incremented by the route table (or tunnel table) cost to reach the received BGP next-hop and/or by a statically configured value (using route policies).