AS_PATH

Overview of AS_PATH

• AS_PATH is a critical attribute in BGP (Border Gateway Protocol) that plays a central role in the Best Path Selection Algorithm, where it is ranked fourth in importance.
• AS_PATH is a Well-Known Mandatory attribute, which means it must be included in all BGP UPDATE messages, and all BGP speakers must recognize and propagate it.
• One of the primary functions of AS_PATH is to serve as a loop prevention mechanism in BGP.

Loop Prevention and Traffic Flow

• AS_PATH ensures loop prevention in BGP routing. It keeps track of the ASes a BGP route has traversed, and this information is used to prevent routing loops.
• Lower numbers of ASes in the AS_PATH attribute are preferred when multiple paths to the same destination exist. This preference for shorter AS_PATHs helps optimize the routing process.
• AS_PATH is a powerful tool to influence the inbound flow of traffic to an autonomous system (AS). By manipulating the AS_PATH attribute, network administrators can guide incoming traffic through specific paths within the network.

Setting AS_PATH

• AS_PATH is set in a per-neighbor fashion. This means that BGP speakers can influence the AS_PATH attribute for routes received from specific BGP neighbors.
• A common practice is to always prepend your own AS number (AS_PATH) when advertising routes. However, manipulating someone else's AS number in the AS_PATH is considered incorrect and should be avoided to maintain BGP integrity.

Manipulating AS_PATH

• AS_PATH can be manipulated through the use of a route map. Network administrators can use route maps to prepend or modify AS numbers in the AS_PATH attribute, thus influencing routing decisions.

AS_PATH Field Values

• The AS_PATH attribute contains several field values, including:

1. Path Segment Type:

• AS_SET: Represents an unordered list of AS numbers.
• AS_SEQUENCE: Represents an ordered list of AS numbers.

2. Path Segment Length:

• Indicates the number of ASes in a specific path segment.

3. Path Segment:

• Contains the list of ASNs (Autonomous System Numbers) that the route has traversed. This information is crucial for loop detection and prevention.

Summary

• AS_PATH is a Well-Known Mandatory attribute in BGP, serving as a critical loop prevention mechanism. It influences routing decisions based on the number of ASes traversed. AS_PATH can be set in a per-neighbor fashion and is commonly manipulated using route maps. By controlling the AS_PATH attribute, network administrators can guide the inbound flow of traffic to an AS.

Note:

• Proper and ethical use of the AS_PATH attribute is essential in maintaining BGP routing integrity and adhering to BGP best practices.