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.
In this example, four devices are configured into two sub-autonomous systems, each containing two of the devices. The sub-autonomous systems are members of confederation 10. Devices within a sub-AS must be fully meshed and communicate using iBGP. In this example, devices A and B use iBGP to communicate. devices C and D also use IBGP. However, the sub-autonomous systems communicate with one another using eBGP. For example, device A communicates with device C using eBGP. The devices in the confederation communicate with other autonomous systems using eBGP.
Devices in other autonomous systems are unaware that devices A through D are configured in a confederation. In fact, when devices in confederation 10 send traffic to devices in other autonomous systems, the confederation ID is the same as the AS number for the devices in the confederation. Thus, devices in other autonomous systems see traffic as coming from AS 10 and are unaware that the devices in AS 10 are subdivided into sub-autonomous systems within a confederation.