The BGP Accumulated IGP feature is required to simulate the current Open Shortest Path First (OSPF) behavior of computing
the distance associated with a path. OSPF or Label Distribution Protocol (LDP) carries the prefix or label information only
in the local area. Then, Border Gateway Protocol (BGP) carries the prefix or label to all the remote areas by redistributing
the routes into BGP at area boundaries. The routes or labels are then advertised using label-switched paths (LSP). The next-hop
for the route is changed at each Area Border Router (ABR) to a local device, which removes the need to leak OSPF routes across
area boundaries. The bandwidth available on each of the core links is mapped to the OSPF cost; therefore, it is imperative
that BGP carries this cost correctly between each of the provider edge (PE) devices. This functionality is achieved by using
the BGP Accumulated IGP feature.
You need to enable accumulated interior gateway protocol (AIGP)
internal Border Gateway Protocol (iBGP) and external
Border Gateway Protocol (eBGP) neighbors to carry the AIGP
Neighbors configured with the AIGP attribute are
put in a separate update group from other iBGP neighbors. A
separate update group is required for neighbors that are enabled to
send the AIGP value to cost community. BGP needs to translate the
AIGP attribute to the cost community or multi-exit discriminator (MED)
and attach it to the route before advertising to legacy.
When BGP installs AIGP attribute routes into the routing information base (RIB), it adds the AIGP cost with the next-hop
cost. If the next-hop is a nonrecursive IGP route, BGP sets the AIGP metric to the received AIGP value and the first hop IGP
metric to the next-hop. If the next-hop is a recursive route with the AIGP metric, BGP adds the received AIGP metric to the
next-hop AIGP metric.