BGP4+ confederations

Each sub-autonomous system must be uniquely identified within the confederation AS by a sub-autonomous system number. Within each sub-autonomous system, all the rules of internal BGP (IBGP) apply. For example, all BGP routers inside the sub-autonomous system must be fully meshed. Although EBGP is used between sub-autonomous systems, the sub-autonomous systems within the confederation exchange routing information like IBGP peers. Next hop, Multi Exit Discriminator (MED), and local preference information is preserved when crossing sub-autonomous system boundaries. To the outside world, a confederation looks like a single AS.

The AS path list is a loop-avoidance mechanism used to detect routing updates leaving one sub-autonomous system and attempting to re-enter the same sub-autonomous system. A routing update attempting to re-enter a sub-autonomous system it originated from is detected because the sub-autonomous system sees its own sub-autonomous system number listed in the update's AS path.