Content

| 3.5 |  | Route Calculation |
| 3.5.1 |  | Route calculation fundamentals |

The capability of a routing protocol to update and calculate routes efficiently is based on several factors:

- Whether the protocol calculates and stores multiple routes to each destination
- The manner in which routing updates are initiated
- The metrics used to calculate distances or costs

The following sections discuss these three factors in detail.

Multiple routes to a single destination
Some routing protocols allow the router to install only a single route to a destination network in its routing table. Other routing protocols permit the router to store multiple routes to each destination, at the cost of additional overhead. One advantage of multiple routes is that equal-cost load balancing or unequal-cost load balancing may be used. Another advantage is that maintaining multiple routes to a single destination reduces the network vulnerability to routing loops and dropped packets when a link fails. If a router maintains two different routes to 10.0.0.0 and one route fails, the router can continue to route to 10.0.0.0 using the second route, without waiting for an alternate route to propagate. Maintaining multiple routes does not reduce convergence time, but it can insulate a router from instabilities during the convergence process.

| Web Links Routing Basics http://www.cisco.com/univercd/cc/td/doc/ cisintwk/ito_doc/routing.htm