Autonomous Systems
An autonomous system (AS) is a portion of an internetwork under common administrative authority that is regulated by a particular set of administrative guidelines. Autonomous systems divide global external networks into individual routing domains, where local routing policies are applied. This organization simplifies routing domain administration and simplifies consistent policy configuration. Each autonomous system can support multiple interior routing protocols that dynamically exchange routing information through route redistribution.
The autonomous system number assignment for public and private networks is governed by the Internet Assigned Number Authority (IANA). A public autonomous system can be directly connected to the Internet. This autonomous system number (AS number) identifies both the routing process and the autonomous system. Private autonomous system numbers are used for internal routing domains but must be translated by the router for traffic that is routed out to the Internet. You should not configure routing protocols to advertise private autonomous system numbers to external networks. By default, Cisco NX-OS does not remove private autonomous system numbers from routing updates.
Administrative Distance
An administrative distance is a rating of the trustworthiness of a routing information source. A higher value indicates a lower trust rating. Typically, a route can be learned through more than one protocol. Administrative distance is used to distinguish between routes learned from more than one protocol. The route with the lowest administrative distance is installed in the IP routing table.
Table 6-1 shows the default administrative distance for selected routing information sources.
Table 6-1 Default Administrative Distance
| Route Source | Default Administrative Distance |
| Connected network | 0 |
| Static route | 1 |
| EIGRP | 90 |
| OSPF | 110 |
| RIPv2 | 120 |
| External EIGRP | 170 |
| Unknown or unbelievable | 255 (will not be used to pass traffic) |
Routing protocols can use load balancing or equal-cost multipath (ECMP) to share traffic across multiple paths. If the router receives and installs multiple paths with the same administrative distance and cost to a destination, load balancing can occur. Load balancing distributes the traffic across all the paths, sharing the load. The number of paths used is limited by the number of entries the routing protocol puts in the routing table. ECMP does not guarantee equal load balancing across all links. It guarantees only that a particular flow will choose one particular next hop at any point in time.
If you have multiple routing protocols configured in your network, you can configure these protocols to share routing information by configuring route redistribution in each protocol. The router that is redistributing routes from another protocol sets a fixed route metric for those redistributed routes, which prevents incompatible route metrics between the different routing protocols. For example, routes redistributed from EIGRP into OSPF are assigned a fixed link cost metric that OSPF understands. Route redistribution also uses an administrative distance to distinguish between routes learned from two different routing protocols. The preferred routing protocol is given a lower administrative distance so that its routes are picked over routes from another protocol with a higher administrative distance assigned.