How does OSPF select the best route and does load balancing work across multiple paths?

[TCC_2]

Core Issue

Open Shortest Path First (OSPF) is a link state routing protocol that supports hierarchical routing by allowing the domain to be divided into multiple areas for better manageability and scalability. Depending on the point where a network is sourced, there are various types of routes that could be present in a OSPF domain. When there are multiple routes to a particular network in a OSPF domain, the type of the route influences the route that is selected and installed by the router in the routing table.

Resolution

In OSPF, routes that are learned by a router from OSPF sources within the same area are known as inta-area routes. Routes that originate from an OSPF router in a different area are considered as inter-area routes. Certain networks could belong to a domain outside OSPF, which could then be redistributed into the OSPF by an Autonomous System Boundary Router (ASBR). Such routes are considered external routes. They can be further divided into external type-1 or external type-2 routes, depending on how they are advertised while being redistributing on the ASBR. The difference between these two types is the way in which the metric for the route is calculated. 

OSPF-running routers use these criteria to select the best route to be installed in the routing table:

• When there are multiple routes available to the same network with different route types, routers use this order of preference (from highest to lowest):
  1. Intra-area routes
  2. Inter-area routes
  3. External Type-1 routes
  4. External Type-2 routes
• If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route. The route with the lowest value for cost is chosen as the best route.
• If there are multiple routes to a network with the same route type and cost, it chooses all the routes to be installed in the routing table, and the router does equal cost load balancing across multiple paths.

For more information, refer to theses documents:

• The How can I give preference to OSPF interarea routes over intra-area routes? section of OSPF: Frequently Asked Questions
• Route Selection in Cisco Routers
• How  does load balancing Work?

[ccie14745]

There is one caveat to keep in mind with instance of multiple routes with the same type:  when redistributing from another protocol into OSPF, OSPF will prefer an LSA received from OSPF neighbors over the route from the other protocol, even if the metric for redistribution is lower than that of the LSA received from OSPF neighbors.

When facing this multi-path, multi-protocol problem, it's best to see whether some form of summarization can occur between protocols, to allow the protocols to continue using the longest match while providing a redundant path via a shorter match.