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IP Concepts, Terminology, and Features
Route Weights
Route-weight calculation is an internal tool that IP uses to facilitate selection of
the best route among alternative routes to the same destination. Route-selection
criteria are encoded into the route weight in a way that allows IP to compare
routes simply by comparing their weight values, regardless of route sources.
Appendix D contains a worksheet that you can you use to calculate route weights
in your configuration.
Route-weight calculation increases the efficiency of the route-selection process.
It also reduces the size of the routing database because all route selection
parameters for each route are encoded in a single integer -- the weight value -rather than stored in separate variables.
Using selection criteria encoded in the route weight, IP chooses routes in the
following order:
1. The route with the highest preference value (see “Route Preferences” on
page 1-12)
2. A direct or OSPF intra-area route with the lowest metric
3. A direct route with the lowest metric
4. An OSPF intra-area route with the lowest metric
5. An OSPF interarea route with the lowest metric
6. An OSPF Type 1 external route with the lowest metric
7. A BGP route with the highest LOCAL_PREF value
8. A RIP route with the lowest metric
9. An EGP route with the lowest metric
10. A static route with the lowest metric
11. An OSPF Type 2 external route with a metric type earlier than Router
Software Version 8.00
Note: If OSPF is configured to propagate external routes using the route
weight as the type 2 metric, routes that are received as OSPF ASE type 2
routes are evaluated according to their respective origins (for example, RIP or
BGP).
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