Question

Give an example of an arrangement of routers grouped into autonomous systems so that the path with the fewest hops from a point A to another point B crosses the same AS twice. Explain what BGP would do with this situation.

Short answer

An example involves AS1 connected to AS2 and AS3, forming a loop. BGP prefers fewer AS hops routes.

Step-by-step solution

Define Autonomous Systems (AS)

An Autonomous System (AS) is a collection of IP networks and routers under the control of a single organization that presents a common routing policy to the internet. Each AS is identified by a unique number called an AS number (ASN).

Create the Network Topology

Consider AS1, AS2, and AS3. Assume AS1 connects point A and point B directly. AS1 also connects to AS2, and AS2 connects to AS3, which connects back to AS1.

Define the Route

The direct path from point A to point B in AS1 might have multiple hops due to internal configurations. The path through AS1 → AS2 → AS3 → AS1 might have fewer hops, even though it crosses through AS1 twice.

Explain BGP Behavior

The Border Gateway Protocol (BGP) prefers routes with fewer AS hops. In this case, BGP would preferentially select the path with fewer hops, even if it means crossing the same AS twice.

Key concepts

Autonomous Systems (AS)

An Autonomous System (AS) is a collection of IP networks and routers managed by a single organization, presenting a unified routing policy to the internet. Each AS is unique and is identified by an Autonomous System Number (ASN), a numerical identifier assigned by a global organization such as IANA (Internet Assigned Numbers Authority).

This AS structure is crucial for making the organization of the internet feasible. Without it, managing routes on a global scale would be chaotic. ISPs (Internet Service Providers) and large organizations usually operate their own AS.

The primary role of an AS is to facilitate efficient routing within itself and coordinate with other ASes. This coordination ensures data can travel across the numerous networks that make up the internet.

BGP (Border Gateway Protocol)

The Border Gateway Protocol (BGP) is the standardized exterior gateway protocol used to exchange routing information between Autonomous Systems (AS) on the internet. BGP facilitates the decision-making process for choosing the best path for data packets, ensuring connectivity between disparate parts of the internet.

BGP makes routing decisions based on path, network policies, and rules set by the network administrator. It uses AS path information to avoid routing loops and to select the most efficient route for data to travel.

One significant characteristic of BGP is its preference for routes with fewer AS hops. This protocol has made the modern internet scalable and robust by enabling complex multi-organizational networks to communicate seamlessly.

Network Topology

Network topology refers to the arrangement of different elements (links, nodes, etc.) in a computer network. In the context of routing and BGP, understanding the network topology is essential.

Imagine three Autonomous Systems: AS1, AS2, and AS3. Each AS contains multiple routers. Let's say AS1 connects two points, A and B, directly. However, AS1 also connects to AS2 and AS3, and AS3 connects back to AS1.

This arrangement can lead to a situation where the shortest or most efficient route between points A and B, as identified by BGP, might traverse multiple ASes and even pass through the same AS twice due to fewer hops or reduced latency. This kind of intricate network topology allows for redundancy and optimized routing paths, ensuring reliability and efficiency.

AS Paths

AS Paths are sequences of AS numbers that data packets traverse to reach their destination. This sequence is crucial for routing decisions made by the Border Gateway Protocol (BGP).

Consider a route from point A to point B in AS1. The direct path might involve several internal hops within AS1. Alternatively, a path through AS1 → AS2 → AS3 → AS1 could be shorter in terms of AS hops. Despite crossing AS1 twice, BGP might select this path if it has fewer hops.

This is because BGP prioritizes routes with the shortest AS Path. The AS Path list, propagated by BGP, helps prevent routing loops and ensures data packets are routed through the most efficient path.

Understanding AS paths is vital for network engineers for optimizing routing policies and ensuring efficient and reliable data transmission across networks.