Riverstone Networks WICT1-12 - Manual (Page 433)

Riverstone Networks RS Switch Router User Guide Release 8.0 18-3
Routing Policy Configuration Preference
In some cases, a combination of the associated attributes can be specified to identify the routes to be imported.
Note It is quite possible for several BGP import policies to match a given update. If
more than one policy matches, the first matching policy will be used. All later
matching policies will be ignored. For this reason, it is generally desirable to order
import policies from most to least specific. An import policy with an
optional-attributes-list will match any update with any (or no) communities.
The importation of RIP routes may be controlled by source interface and source gateway. RIP does not support the use
of preference to choose between RIP routes. That is left to the protocol metrics.
Due to the nature of OSPF, only the importation of ASE routes may be controlled. OSPF intra-and inter-area routes
are always imported into the routing table with a preference of 10. If a tag is specified with the import policy, routes
with the specified tag will only be imported. It is only possible to restrict the importation of OSPF ASE routes when
functioning as an AS border router.
Like the other interior protocols, preference cannot be used to choose between OSPF ASE routes. That is done by the
OSPF costs.
Route-Filter
This component specifies the individual routes which are to be imported or restricted. The preference to be associated
with these routes can also be explicitly specified using this component.
The preference associated with the imported routes are inherited unless explicitly specified. If there is no preference
specified with a route-filter, then the preference is inherited from the one specified with the import-source.
Every protocol (RIP, OSPF, and BGP) has a configurable parameter that specifies the default-preference associated
with routes imported to that protocol. If a preference is not explicitly specified with the route-filter, as well as the
import-source, then it is inherited from the default-preference associated with the protocol for which the routes are
being imported.
18.1.2 Export Policies
Export policies control the redistribution of routes to other systems. They determine which routes are advertised by the
Unicast Routing Process to other systems. Every export policy can have up to three components:
• Export-Destination
• Export-Source
• Route-Filter