The AS-Path is an essential attribute used in the Border Gateway Protocol (BGP), which is the primary Exterior Gateway Protocol (EGP) facilitating routing on the global internet. BGP is used to exchange routing information between different autonomous systems (AS), where an AS is a collection of IP networks and routers under the control of a single organization.
Here’s a closer look at the AS-Path attribute:
- Function: The AS-Path is essentially a list of Autonomous System (AS) numbers that a BGP route has traversed to reach a destination.
- Loop Prevention: One of the primary purposes of the AS-Path is to prevent routing loops in BGP. When a BGP router receives a route advertisement, it checks the AS-Path for its own AS number. If it finds its AS number in the path, it discards the advertisement, preventing the route from looping back into its own network.
- Route Selection: BGP uses the AS-Path as one of the criteria for its path selection algorithm. If multiple paths to a destination are available, BGP typically prefers the route with the shortest AS-Path, assuming other attributes are equal.
- Path Prepending: Network administrators can manipulate the AS-Path to influence routing decisions. One common method is “AS path prepending,” where an AS number is repeated multiple times in the AS-Path to make a route appear less desirable (longer) to other BGP routers.
- Visualization: An AS-Path might look something like this:
64512 65001 65100, where each number represents an AS that the route advertisement has passed through. The rightmost number is the originating AS, and as you move to the left, you’re seeing the ASes the route advertisement has traversed in sequence. - Filtering & Policy: The AS-Path can also be used for filtering routes and applying routing policies. For instance, an ISP might have a policy to reject or give preference to routes that have passed through specific ASes, based on the AS-Path attribute.
In summary, the AS-Path is a critical BGP attribute that serves various functions, from preventing loops to facilitating routing decisions and policies.