Autonomous Systems and Process IDs

Autonomous System:
An autonomous system (AS) is a collection of networks under the administrative control of a single entity that presents a common routing policy to the Internet. In the figure, companies A, B, C, and D are all under the administrative control of ISP1. ISP1 "presents a common routing policy" for all of these companies when advertising routes to ISP2.
The guidelines for the creation, selection, and registration of an autonomous system are described in RFC 1930. AS numbers are assigned by the Internet Assigned Numbers Authority (IANA), the same authority that assigns IP address space. You learned about IANA and its Regional Internet Registries (RIRs) in a previous course. The local RIR is responsible for assigning an AS number to an entity from its block of assigned AS numbers. Prior to 2007, AS numbers were 16-bit numbers, ranging from 0 to 65535. Now 32-bit AS numbers are assigned, increasing the number of available AS numbers to over 4 billion.

Who needs an autonomous system number? Usually ISPs (Internet Service Providers), Internet backbone providers, and large institutions connecting to other entities that also have an AS number. These ISPs and large institutions use the exterior gateway routing protocol Border Gateway Protocol, or BGP, to propagate routing information. BGP is the only routing protocol that uses an actual autonomous system number in its configuration.

The vast majority of companies and institutions with IP networks do not need an AS number because they come under the control of a larger entity such as an ISP. These companies use interior gateway protocols such as RIP, EIGRP, OSPF, and IS-IS to route packets within their own networks. They are one of many independent and separate networks within the autonomous system of the ISP. The ISP is responsible for the routing of packets within its autonomous system and between other autonomous systems.
Process ID:
Both EIGRP and OSPF use a process ID to represent an instance of their respective routing protocol running on the router.

Router(config)#router eigrpautonomous-system

Although EIGRP refers to the parameter as an "autonomous-system" number, it actually functions as a process ID. This number is not associated with an autonomous system number discussed previously and can be assigned any 16-bit value.
Router(config)#router eigrp 1
In this example, the number 1 identifies this particular EIGRP process running on this router. In order to establish neighbor adjacencies, EIGRP requires all routers in the same routing domain to be configured with the same process ID. Typically, only a single process ID of any routing protocol would be configured on a router.
Note: RIP does not use process IDs; therefore, it can only support a single instance of RIP. Both EIGRP and OSPF can support multiple instances of each routing protocol, although this type of multiple routing protocol implementation is not usually needed or recommended.


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