Question

Suppose ASs \(\mathrm{X}\) and \(\mathrm{Z}\) are not directly connected but instead are connected by AS Y. Further suppose that \(\mathrm{X}\) has a peering agreement with \(\mathrm{Y}\), and that \(\mathrm{Y}\) hasa peering agreement with Z. Finally, suppose that Z wants to transit all of Y's traffic but does not want to transit X's traffic. Does BGP allow Z to implement this policy?

Short answer

Yes, BGP allows Z to implement this policy using route filtering mechanisms.

Step-by-step solution

Understanding BGP Policies

Border Gateway Protocol (BGP) is a standardized gateway protocol used to exchange routing information between different Autonomous Systems (ASes) on the internet. It enables ASes to enforce routing policies that decide how they announce and accept network routes. The flexibility of BGP allows for complex policies, but each AS can control only its own policies, not those of other ASes.

Analyzing the Existing Setup and Policies

Here, there are three ASes involved: X, Y, and Z. AS X has a peering agreement with AS Y, meaning they exchange routing information freely between them. Similarly, AS Y has a peering agreement with AS Z. AS Z wants to transit all traffic from Y but explicitly does not want to transit traffic originating from AS X.

BGP and Policy Control Capabilities

BGP allows an AS to filter routes it accepts from its peers. This means AS Z can choose to accept routes advertised by Y and decide if Z wants to forward that traffic further based on its own policy rules. For instance, Z can configure its BGP to omit routes from Y that originated from X via prefix filtering.

Evaluating Policy Implementation

To achieve the desired policy, Z can implement filtering on the origin of the routes it receives from Y. Using BGP route filtering techniques like prefix-lists, access-lists, or route-maps, Z can exclude any prefixes that have originated from X when accepting traffic from Y.

Conclusion on Policy Implementation

Yes, BGP provides Z the ability to implement the policy via filtering mechanisms, allowing it to transit Y's traffic while not allowing transit for X-originating traffic.

Key concepts

Autonomous Systems

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. Autonomous Systems play a crucial role in the overall routing process of the internet. Each AS is assigned a unique AS number (ASN), which is essential for identifying them during routing decisions.

ASes are the foundational building blocks of the Internet. They connect directly via links and indirectly through a series of other ASes. Each AS can use the Border Gateway Protocol (BGP) to communicate and decide where information packets should travel. Some key characteristics and terms related to Autonomous Systems include:

• Route Propagation: ASes exchange routing information to determine the most efficient path for data transmission.
• Routing Policy: Each AS defines its policies such as which other networks it will use for data paths.
• Decentralization: There is no central authority that determines the rules for data exchange; each AS makes independent decisions.

BGP Route Filtering

Border Gateway Protocol (BGP) allows networks to exchange routing information efficiently across the internet. One of BGP's powerful capabilities is route filtering, which controls which routes an AS will accept or advertise. This filtering is crucial for implementing specific policies and managing network traffic effectively.

BGP route filtering can be used when an AS wants to control the flow of traffic between itself and neighboring ASes. In the exercise scenario, AS Z needs to filter out routes originating from AS X while accepting others from AS Y. Here's how BGP route filtering works:

• Prefix Filters: ASes can use prefix lists to specify which IP address prefixes should be allowed or denied.
• Access Lists: These lists define rules that permit or deny specific paths based on attributes, such as the AS path.
• Route Maps: These are versatile tools that can match routes based on criteria and set specific actions, like filtering out unwanted paths.

By setting these filters, an AS can effectively manage traffic flows and ensure compliance with its routing policies, as seen with AS Z implementing filters to exclude traffic originating from AS X.

Peering Agreements

Peering agreements are arrangements between two or more Autonomous Systems to exchange traffic directly without involving third-party providers. These agreements foster mutual cooperation and efficient internet connectivity.

In the context of the exercise, AS X has a peering agreement with AS Y, and AS Y with AS Z, enabling them to share routing information freely. Some important aspects of peering agreements include:

• Cost Efficiency: By short-cutting the data path between ASes, peering reduces the cost of data transmission that would otherwise require transit through other providers.
• Reduced Latency: Direct exchanges minimize the number of hops, thus improving the speed and reliability of connections.
• Strategic Partnerships: Peering arrangements often reflect strategic business relationships where both parties benefit from increased coverage and performance.

While peering agreements generally promote open data exchange, ASes can enforce selective routing policies, just as AS Z opts to filter traffic to exclude that originating from AS X, aligning with specific strategic decisions.