Path vector routing is a routing protocol used in computer networking to determine the best path for data packets based on a vector of autonomous systems (ASes) that the packets must traverse to reach their destination. It’s a specialized routing protocol, primarily associated with the Border Gateway Protocol (BGP), which is commonly used in the global internet to make routing decisions between autonomous systems. Here are the key characteristics and principles of path vector routing, focusing on BGP:
- Autonomous Systems (ASes): BGP and path vector routing are designed for routing between autonomous systems. An autonomous system is a collection of IP networks and routers under the control of a single organization, which manages its routing policies independently.
- Path Vector: BGP uses a path vector as a routing metric. Instead of using simple hop counts or link costs, BGP keeps track of the sequence of autonomous systems that a route traverses to reach its destination. This path vector includes the AS numbers that represent each AS on the route.
- Policy-Based Routing: BGP is highly policy-based. Network administrators can define routing policies that influence route selection. These policies take precedence over other routing factors and help shape routing decisions based on business requirements.
- Prefixes and Prefix Lengths: BGP advertises routes to specific IP network prefixes and includes the prefix length (subnet mask) in its route advertisements. This allows for very granular control over routing.
- Path Selection: BGP routers maintain multiple paths to the same destination. When selecting a path, BGP routers consider various attributes and apply a set of rules to determine the best path based on administrative policies, such as AS path length, local preference, and next-hop reachability.
- Inter-AS Routing: BGP is the protocol used for inter-domain routing, enabling communication and routing between different autonomous systems. It is integral to the operation of the global internet, where numerous autonomous systems cooperate to exchange routes.
- Reliability and Stability: BGP is known for its stability and reliability, making it suitable for large-scale networks. It employs mechanisms like route dampening to mitigate routing instability.
- Route Aggregation: BGP supports route aggregation, where multiple network prefixes can be advertised as a single summarized route, reducing the size of routing tables.
- Prefix Filtering: BGP allows for granular prefix filtering, enabling network administrators to control which prefixes are accepted and advertised.
- Large Routing Tables: Due to its use in the global internet, BGP can handle very large routing tables with numerous routes and complex routing policies.
Path vector routing, particularly in the context of BGP, plays a critical role in the operation of the internet, enabling routing decisions between a vast number of autonomous systems. It is a robust and flexible protocol but requires advanced configuration and management, which is typically handled by internet service providers, large organizations, and data centers that participate in BGP routing.
