BGP advertises routes by exchanging routing information between border routers in Autonomous Systems (AS). When a router wishes to advertise a route to a destination network, it sends an update message to its neighboring routers, which in turn propagate the information to their neighboring routers. The update message includes information such as the network prefix, the AS path, and any attributes associated with the route.
The AS path is a sequence of AS numbers that indicates the path that the update message has taken to reach the advertising router. This information is used to prevent routing loops and to ensure that the path is optimal.
BGP also uses various attributes to adjust the selection of the best path for a particular destination network. These attributes include the weight, local preference, origin, and next hop. By adjusting these attributes, routers can influence the path selection process and ensure that traffic is routed through the optimal path.
Overall, BGP advertises routes by exchanging routing information between routers and using a combination of attributes to determine the best path for each destination network.
