Default Preferences Assigned to Routes by GateD

Introduction

GateD should assign preferences to all incoming routes; if there is no more-specific information in the configuration file, a set of compiled-in default preferences are used so that there is a "preference of last resort"--all routes accepted by import policy should have a preference value assigned, but the proper assignment of preferences is up to the protocol coder, as of the time of this writing. (There is more information about preferences in GateD available.)

Current Values

The GateD distribution includes a set of default preference values which are available to routing protocol programmers for use; as of this writing, not all of the values listed below are actually used in the code (as symbols; the values themselves may be hardcoded in, but there are #define'd symbols which are not found in any of the source files). Currently, the default preferences are found in the rt_table.h file of the GateD distribution; as of this writing, the default preferences are:

/*
 *      Preferences of the various route types
 */
#define RTPREF_KERNEL_TEMP      0       /* For managing the forwarding table */
#define RTPREF_DIRECT           0       /* Routes to interfaces */
#define RTPREF_OSPF             10      /* OSPF Internal route */
#define RTPREF_ISIS_L1          15      /* IS-IS level 1 route */
#define RTPREF_ISIS_L2          18      /* IS-IS level 2 route */
#define RTPREF_SLSP             19      /* NSFnet backbone SPF */
#define RTPREF_DEFAULT          20      /* defaultgateway and EGP default */
#define RTPREF_REDIRECT         30      /* redirects */
#define RTPREF_KERNEL           40      /* learned via route socket */
#define RTPREF_SNMP             50      /* route installed by network management */
#define RTPREF_STATIC           60      /* Static routes */
#define RTPREF_IGRP             80      /* Cisco IGRP */
#define RTPREF_HELLO            90      /* DCN Hello */
#define RTPREF_RIP              100     /* Berkeley RIP */
#define RTPREF_DIRECT_AGGREGATE 110     /* P2P interface aggregate routes */
#define RTPREF_DIRECT_DOWN      120     /* Routes to interfaces that are down */
#define RTPREF_AGGREGATE        130     /* Aggregate default preference */
#define RTPREF_OSPF_ASE         150     /* OSPF External route */
#define RTPREF_IDPR             160     /* InterDomain Policy Routing */
#define RTPREF_BGP_EXT          170     /* Border Gateway Protocol - external peer */
#define RTPREF_IDRP_INT         180     /* IDRP - internal peer */
#define RTPREF_IDRP_EXT         190     /* IDRP - external peer */
#define RTPREF_EGP              200     /* Exterior Gateway Protocol */
#define RTPREF_KERNEL_REMNANT   254     /* Routes in kernel at startup */

Implications

What do the above numbers mean? In viewing them, and referring to this explanation of GateD preferences, it is clear that, since GateD uses lower preference values to indicate more desirable or better routes, the default preferences express the following ideas about routing:

Routes which are directly reachable via the router's directly-attached media (interfaces) are the best possible routes; this makes sense, because the router need not depend upon, e.g., IP-level packet routing to reach these destinations.
Routes which are left over from any kernel entries which existed prior to the startup of the gated process--routes which are, therefore, prior to any new information learned via the routing protocols which GateD will start up and prior to the routing policies described in the GateD configuration file--are the worst possible routes.
Routes learned via an IGP are better than routes learned via an EGP.
Static routes--the "#define routes" of routing, for those familiar with the C programming language--are better routes than those learned via an EGP.
Aggregate routes are better than those learned via an EGP, but worse than static or IGP-learned routes.
For routing protocols which have a concept of internal vs. external peers (e.g., IDRP) or a local vs. a more global area (e.g., OSPF), the former is usually considered to be a better source of routes than the latter.

Summary

Using the mathematical "<" comparison operator to express preference value relationships:
```
	direct <\ IGP/Static <\ Aggregate <\ EGP <\ "leftovers"
```
Using the mathematical ">" comparison operator to express route desirability relationships:
```
	direct > IGP/Static > Aggregate > EGP > "leftovers"
```