Chapter 11: Routing Policy Configuration Guide
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DIGITAL GIGAswitch/Router User Reference Manual
Preference
Preference is the value the GSR 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 GSR 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.
Table 5. Default Preference Values
Preference
Defined by CLI Command
Default
Direct connected networks
ip-router global set interface
0
OSPF routes
ospf
10
Static routes from config
ip add route
60
RIP routes
rip set preference
100
Point-to-point interface
110
Routes to interfaces that are
down
ip-router global set interface
down-preference
120
Aggregate/generate routes
aggr-gen
130
OSPF AS external routes
ospf set ase-defaults preference
150
BGP routes
bgp set preference
170
