The accumulated IGP (AIGP) metric is an optional, non-transitive attribute that can be attached to selected routes using route policies. In networks that use AIGP, BGP paths with a lower end-to-end IGP cost are preferred, even if the compared paths span more than one AS or IGP instance. AIGP differs from MED in the following important ways:
AIGP is not transitive between completely distinct autonomous systems. It is only transitive across internal AS boundaries.
AIGP is always compared in paths that have the AIGP attribute, regardless of whether they are located in different neighbor ASs.
AIGP is more important than MED in the BGP decision process.
AIGP is automatically incremented every time there is a BGP next-hop change, so that the system can track the end-to-end IGP cost. All arithmetic operations on MED attributes must be performed manually, such as by using route policies.
On the 7210 SAS, AIGP is supported only in the base router BGP instance and only for label-IPv4 and 6PE routes. The AIGP attribute is only sent to peers configured using 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. The AIGP attribute is still displayed in BGP show command output.
When the 7210 SAS receives a route with an AIGP attribute and it re-advertises the route to an AIGP-enabled peer without changes to the BGP next hop, the AIGP metric value is unchanged by the advertisement (RIB-OUT) process. However, if the route is re-advertised with a new BGP next hop, the AIGP metric value is automatically incremented, either by the route table or tunnel table cost to reach the received BGP next hop, or by a value configured using route policies.