Cabletron Systems SmartSwitch manual Default Preference Values

Chapter 11: Routing Policy Configuration Guide

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	ip-router global set interface	120
down	down-preference
Aggregate/generate routes	aggr-gen	130
OSPF AS external routes	ospf set ase-defaults preference	150
BGP routes	bgp set preference	170

164 SmartSwitch Router User Reference Manual