Riverstone Networks WICT1-12 - Manual (Page 431)

Riverstone Networks RS Switch Router User Guide Release 8.0 18-1
18 ROUTING POLICY CONFIGURATION
The RS family of routers supports extremely flexible routing policies. The RS allows the network administrator
to control import and export of routing information based on criteria including:
• Individual protocol
• Source and destination autonomous system
• Source and destination interface
• Previous hop router
• Autonomous system path
• Tag associated with routes
• Specific destination address
The network administrator can specify a preference level for each combination of routing information being
imported by using a flexible masking capability.
The RS also provides the ability to create advanced and simple routing policies. Simple routing policies provide
a quick route redistribution between various routing protocols (RIP, OSPF, IS-IS and BGP). Advanced routing
policies provide more control over route redistribution.
18.1 PREFERENCE
Preference is the value the RS routing process uses to order preference of routes from one protocol or peer over
another. Preference can be set using several different configuration commands. Preference can be set based on one
network interface over another, from one protocol over another, or from one remote gateway over another.
Preference may not be used to control the selection of routes within an Interior Gateway Protocol (IGP). This is
accomplished automatically by the protocol based on metric.
Preference may be used to select routes from the same Exterior Gateway Protocol (EGP) learned from different
peers or autonomous systems. Each route has only one preference value associated with it, even though the
preference can be set at many places using configuration commands. The last or most specific preference value
set for a route is the value used. A preference value is an arbitrarily assigned value used to determine the order of
routes to the same destination in a single routing database. The active route is chosen by the lowest preference
value.
A default preference is assigned to each source from which the RS routing process receives routes. Preference
values range from 0 to 255 with the lowest number indicating the most preferred route.
The following table summarizes the default preference values for routes learned in various ways. The table lists
the CLI commands that set preference, and shows the types of routes to which each CLI command applies. A
default preference for each type of route is listed, and the table notes preference precedence between protocols.
The narrower the scope of the statement, the higher precedence its preference value is given, but the smaller the
set of routes it affects.