What happens to the MED value in BGP when it is advertised to neighbor AS?

When a Multi-Exit Discriminator (MED) value is advertised by a BGP router to a neighboring Autonomous System (AS), it is indeed propagated to all routers within that AS. The MED is an attribute used in BGP to provide a hint to external BGP peers about the preferred path for incoming traffic. By communicating the MED value, the sending AS indicates which entry point they prefer for receiving traffic from the neighbor AS.

This propagation occurs because routers within the same AS utilize BGP to exchange routing information and attributes. The MED helps influence the routing decisions made by those routers, allowing them to choose the best path based on the preference indicated by the originating AS.

The importance of the MED value lies in its role as a guideline rather than a directive; it influences but does not dictate the path selection process. Routers in the receiving AS consider the MED value alongside other attributes when making routing decisions. This behavior is a crucial part of BGP’s flexibility in managing routing preferences between different networks.