gated preferences and route selection

Preference is the value gated uses to order preference of routes from one protocol or peer over another. Preference can be set in the gated configuration files in several different configuration statements. 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 preference can be set at many places in the configuration file. Simply, the last or most specific preference value set for a route is the value used.

The 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.

gated route selection criteria

gated uses the following to select a best route:

• The route with the best (numerically smallest) preference is preferred.

• A route learned from an igp is preferred to a route learned from an egp. Least preferred is a route learned indirectly by an igp from an egp.

• If AS path information is available, it is used to help determine the most preferred route. AS path information is specific to the BGP protocol and is described at ``Border Gateway Protocol (BGP)''.

  • A route with an AS path is preferred over one without an AS path.

  • If the AS paths and origins are identical, the route with the lower metric is preferred.

  • A route with an AS path origin of igp is preferred to a route with an egp AS path origin. Least preferred is an AS path with an unknown origin.

  • A route with a shorter AS path is preferred.

• If both routes are from the same protocol and AS, the one with the lowest metric is preferred.

• The route with the lowest numeric next-hop address is used.

Assigning preferences in gated

A default preference is assigned to each source from which gated receives routes. Preference values range from 0 to 255 with the lowest number indicating the most preferred route.

``Default preferences set by gated'', summarizes the default preference values for routes learned in various ways. The table lists the statements (some of these are clauses within statements) that set the preference and shows the types of routes to which each statement applies. The default preference for each type of route is listed. Comparison of default preferences in the table shows 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.

Default preferences set by gated

Preference of	Defined by statement	Default
directly connected networks	interface	0
OSPF routes	ospf	10
internally generated default	gendefault	20
redirects	redirect	30
static routes from config	static	60
RIP routes	rip	100
point-to-point interface		110
routes to interfaces that are down	interfaces	120
OSPF AS external routes	ospf	150
BGP routes	bgp	170
EGP	egp	200

Following are some sample preference specifications.

   interfaces {
           interface 138.66.12.2 preference 10 ;
   } ;
   rip yes {
       preference 90 ;
   } ;
   import proto rip gateway 138.66.12.1 preference 75 ;