Understanding BGP Attribute Local Preference

Local Preference is a BGP attribute that is pivotal in influencing route selection within an autonomous system (AS). Each BGP router within an AS assigns a Local Preference value to incoming BGP routes. This value indicates the preference for routes and helps determine which route will be chosen as the best path for outbound traffic. The higher the Local Preference value, the more preferred the route.

Key Points to Know:

1. Local to the Autonomous System: Local Preference is a local attribute, meaning it is only significant within the confines of a single AS. It is not advertised to external ASes, making it ideal for influencing outbound routing decisions.
2. Default Value: If Local Preference is not explicitly set, the default value is 100. Routes with a higher Local Preference are preferred over routes with a lower Local Preference.
3. Decision Making: In BGP, the best route to a destination is determined based on several attributes, with Local Preference being a crucial factor. It is typically considered before other attributes, such as AS Path and Weight, making it a primary means of route selection within an AS.

Local Preference is a valuable tool for network administrators because it allows them to control the selection of outbound routes, which can be crucial for optimizing network traffic. Here are some key reasons why Local Preference is significant:

1. Load Balancing: Local Preference is often used to load balance traffic across multiple exit points or service providers. By adjusting Local Preference values, administrators can influence which routes are chosen for specific traffic, ensuring balanced utilization of available paths.
2. Traffic Engineering: It enables fine-grained control over how traffic flows out of an AS. Network Admins can steer traffic over preferred links, optimizing performance and cost.
3. Path Selection: Local Preference is a critical factor in determining the best path for outbound traffic. It complements other BGP attributes like AS Path and Weight to ensure that the chosen route aligns with the organization’s policies and requirements.
4. Redundancy and Resilience: Local Preference can be used to define primary and backup routes, ensuring that traffic can be automatically redirected in case of link failures, thereby enhancing network resilience.

Local Preference can be applied in various scenarios to achieve specific network objectives:

1. Multi-Homed Networks: In multi-homed networks, Local Preference can control the distribution of outbound traffic between different service providers, helping achieve load balancing and redundancy.
2. Route Aggregation: It can be employed to favor aggregated routes over more specific routes, simplifying the BGP routing table and reducing memory and processing overhead.
3. Geographical Traffic Engineering: By assigning higher Local Preference values to routes that traverse preferred geographical regions or network links, you can optimize traffic paths for lower latency and better performance.

The biggest takeaways with local-pref are:

-Higher preference always wins
-BGP is a two-way street. Local pref influences outbound traffic, not necessarily inbound. Your BGP view of a destination may not be the same as the other side of the connection. In other words, they may send the traffic back a different route.

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