<html>
<!--
Public Release 3
$Id: pref.html,v 1.2 1997/07/17 22:40:07 chopps Exp $
------------------------------------------------------------------------
Copyright (c) 1996, 1997 The Regents of the University of Michigan
All Rights Reserved
Royalty-free licenses to redistribute GateD Release
3 in whole or in part may be obtained by writing to:
Merit GateDaemon Project
4251 Plymouth Road, Suite C
Ann Arbor, MI 48105
THIS SOFTWARE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER
EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE REGENTS OF THE
UNIVERSITY OF MICHIGAN AND MERIT DO NOT WARRANT THAT THE
FUNCTIONS CONTAINED IN THE SOFTWARE WILL MEET LICENSEE'S REQUIREMENTS OR
THAT OPERATION WILL BE UNINTERRUPTED OR ERROR FREE. The Regents of the
University of Michigan and Merit shall not be liable for
any special, indirect, incidental or consequential damages with respect
to any claim by Licensee or any third party arising from use of the
software. GateDaemon was originated and developed through release 3.0
by Cornell University and its collaborators.
Please forward bug fixes, enhancements and questions to the
gated mailing list: gated-people@gated.merit.edu.
------------------------------------------------------------------------
Copyright (c) 1990,1991,1992,1993,1994,1995 by Cornell University.
All rights reserved.
THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT ANY
EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT
LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY
AND FITNESS FOR A PARTICULAR PURPOSE.
GateD is based on Kirton's EGP, UC Berkeley's routing
daemon (routed), and DCN's HELLO routing Protocol.
Development of GateD has been supported in part by the
National Science Foundation.
------------------------------------------------------------------------
Portions of this software may fall under the following
copyrights:
Copyright (c) 1988 Regents of the University of California.
All rights reserved.
Redistribution and use in source and binary forms are
permitted provided that the above copyright notice and
this paragraph are duplicated in all such forms and that
any documentation, advertising materials, and other
materials related to such distribution and use
acknowledge that the software was developed by the
University of California, Berkeley. The name of the
University may not be used to endorse or promote
products derived from this software without specific
prior written permission. THIS SOFTWARE IS PROVIDED
``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES,
INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
-->
<head>
<TITLE>GateD Configuration Guide - Preference</TITLE>
</head>
<body>
<H1>Preference</H1>
<P>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 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 <STRONG>igp</STRONG>, this is accomplished automatically by the
protocol based on metric. Preference may be used to select routes
from the same <STRONG>egp</STRONG> 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. (See <A HREF="glossary.html#preference">Glossary:
Preference</A>.) 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.
Some protocols implement a second preference (preference2), sometimes
refered to as a tie-breaker.
<H3>Selecting a route</H3>
<P><UL>
<LI> The route with the best (numerically smallest) preference is prefered.
<LI> If the two routes have the same preference, the route with the
best (numerically smallest) preference2 (also known as a tie-breaker)
is prefered.
<LI> A route learned from a <STRONG>igp</STRONG> is prefered to a
route learned from an <STRONG>egp</STRONG>. Least prefered is a route
learned indirectly by an <strong>igp</strong> from an
<strong>egp</strong>.
<LI> If AS path information is available it is used to help determine
the most prefered route.
<UL>
<LI> A route with an AS path is prefered over one without and AS path
<LI> If the AS paths and origins are identical, the route with
the lower metric is prefered.
<LI> A route with an AS path origin of <STRONG>igp</STRONG> is
prefered over a route with an AS path origin of
<STRONG>egp</STRONG>. Least prefered is an AS path with
an <STRONG>unknown</STRONG> origin.
<LI> A route with a shorter AS path is prefered.
</UL>
<LI> If both routes are from the same protocol and AS, the one with
the lowest metric is prefered.
<LI> The route with the lowest numeric next-hop address is used.
</UL>
<H3>Assigning preferences</H3>
<P> 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.
<P> The following table 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 preference, and shows the types of
routes to which each statement applies. The 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.
<P><PRE>
<STRONG>Preference Of</STRONG> <STRONG>Defined by Statement</STRONG> <STRONG>Default</STRONG>
direct connnected networks interface 0
OSPF routes ospf 10
IS-IS level 1 routes isis level 1 15
IS-IS level 2 routes isis level 2 18
internally generated default gendefault 20
redirects redirect 30
routes learned via route socket kernel 40
static routes from config static 60
ANS SPF (SLSP) routes slsp 70
HELLO routes hello 90
RIP routes rip 100
point-to-point interface 110
routes to interfaces that are down interfaces 120
aggregate/generate routes aggregate/generate 130
OSPF AS external routes ospf 150
BGP routes bgp 170
EGP egp 200
</PRE>
<HR>
<H2>Sample Preference Specifications</H2>
<PRE>
interfaces {
interface 138.66.12.2 preference 10 ;
} ;
rip yes {
preference 90 ;
} ;
import proto rip gateway 138.66.12.1 preference 75 ;
</PRE>
<P>In these statements the preference applicable to routes learned via RIP
from gateway 138.66.12.1 is 75. The last preference applicable to routes
learned via RIP from gateway 128.66.12.1 is defined in the accept
statement. The preference applicable to other RIP routes is found in the
rip statement. The preference set on the interface statement applies only
to the route to that interface.
<hr>
Last updated 1994/03/16 21:38:35.
<p><ADDRESS>gated@gated.cornell.edu</ADDRESS>
</body>
</html>
