IPv4 Routing Protocols

IPv4 Routing Protocols

1. Goals

   • To dynamically learn and fill the routing table with a route to each subnet in the internetwork.
   • If more than one route to a subnet is available, to place the best route in the routing table.
   • To notice when routes in the table are no longer valid, and to remove them from the routing table.
   • If a route is removed from the routing table and another route through another neighboring router is available, to add the route to the routing table.
   • To work quickly when adding new routes or replacing lost routes. (The time between losing the route and finding a working replacement route is called convergence time.)
   • To prevent routing loops.

2. General steps for learning routes

   • Step 1. Each router, independent of the routing protocol, adds a route to its routing table for each subnet directly connected to the router.
     • Step 2. Each router’s routing protocol tells its neighbors about the routes in its routing table, including the directly connected routes, and routes learned from other routers.
       • Step 3. After learning a new route from a neighbor, the router’s routing protocol adds a route to its IP routing table, with the next-hop router of that route typically being the neighbor from which the route was learned.