Chapter 2 Advanced Routing for FortiOS 5.0 : Dynamic Routing Overview : What is dynamic routing? : Comparing static and dynamic routing
  
Comparing static and dynamic routing
A common term used to describe dynamic routing is convergence. Convergence is the ability to work around network problems and outages — for the routing to come together despite obstacles. For example, if the main router between two end points goes down, convergence is the ability to find a way around that failed router and reach the destination. Static routing has zero convergence beyond trying the next route in its limited local routing table — if a network administrator doesn’t fix a routing problem manually, it may never be fixed, resulting in a downed network. Dynamic routing solves this problem by involving routers along the route in the decision-making about the optimal route, and using the routing tables of these routers for potential routes around the outage. In general, dynamic routing has better scalability, robustness, and convergence. However, the cost of these added benefits include more complexity and some overhead: the routing protocol uses some bandwidth for its own administration.
Table 7: Comparing static and dynamic routing
Feature
Static Routing
Dynamic Routing
Hardware support
Supported by all routing hardware
May require special, more expensive routers
Router Memory Required
Minimal
Can require considerable memory for larger tables
Complexity
Simple
Complex
Overhead
None
Varying amounts of bandwidth used for routing protocol updates
Scalability
Limited to small networks
Very scalable, better for larger networks
Robustness
None - if a route fails it has to be fixed manually
Robust - traffic routed around failures automatically
Convergence
None
Varies from good to excellent