Description: The BGP Multi-Exit Discriminator (MED) is an optional attribute of the Border Gateway Protocol (BGP) used to influence the selection of the preferred route when multiple routes exist to a destination. This attribute allows network administrators to specify the preference of one route over another, thus facilitating traffic management in complex networks. The MED is presented as a numerical value attached to BGP routes, and generally, a lower value indicates a higher preference. Unlike other BGP attributes, such as AS Path or Local Preference, the MED is considered by BGP routers only when the routes come from the same autonomous system (AS). This means that MED is particularly useful in situations where an Internet Service Provider (ISP) has multiple links to a customer and wants to control how traffic is routed to its network. Implementing MED can help optimize network performance and ensure that traffic follows the most efficient path, which is crucial in environments where latency and bandwidth are critical factors.
History: The MED attribute was introduced in the BGP-4 specification in 1994, as part of an effort to enhance BGP’s ability to handle multiple routes to a destination. Over the years, MED has evolved in its implementation and use, being adopted by many Internet service providers to manage traffic more efficiently. Its design is based on the need to provide network administrators with a tool to influence route selection without needing to change the configuration of the involved autonomous systems.
Uses: MED is primarily used in Internet service provider networks to manage how traffic is routed to their customers. It allows ISPs to influence route selection when they have multiple links to a customer, ensuring that traffic follows the most efficient path. It is also used in interconnection scenarios between different autonomous systems, where there is a desire to control the flow of traffic between them.
Examples: A practical example of using MED is when an ISP has two links to a customer and wants certain types of data traffic to follow a specific link. By configuring a lower MED value on the route they want to prefer, the ISP can influence the customer’s router’s route selection, ensuring that traffic follows the desired path. Another example is in enterprise networks where multiple connections to different service providers are used, allowing organizations to manage traffic more effectively.
