Cabletron Systems SmartSwitch Router, Руководство пользователя

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Chapter 11: Routing Policy Configuration Guide
164 SmartSwitch Router User Reference Manual
Preference
Preference is the value the SSR 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 SSR 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