5.2.4.4 Administrative Preference

One suggested mechanism for the Vendor Policy Pruning Rule is to use administrative preference, which is a simple prioritization algorithm. The idea is to manually prioritize the routes that one might need to select among.

Each route has associated with it a preference value, based on various attributes of the route (specific mechanisms for assignment of preference values are suggested below). This preference value is an integer in the range [0..255], with zero being the most preferred and 254 being the least preferred. 255 is a special value that means that the route should never be used. The first step in the Vendor Policy pruning rule discards all but the most preferable routes (and always discards routes whose preference value is 255).

This policy is not safe in that it can easily be misused to create routing loops. Since no protocol ensures that the preferences configured for a router is consistent with the preferences configured in its neighbors, network managers must exercise care in configuring preferences.

• Address Match
  It is useful to be able to assign a single preference value to all routes (learned from the same routing domain) to any of a specified set of destinations, where the set of destinations is all destinations that match a specified network prefix.

• Route Class
  For routing protocols which maintain the distinction, it is useful to be able to assign a single preference value to all routes (learned from the same routing domain) which have a particular route class (intra-area, inter-area, external with internal metrics, or external with external metrics).

• Interface
  It is useful to be able to assign a single preference value to all routes (learned from a particular routing domain) that would cause packets to be routed out a particular logical interface on the router (logical interfaces generally map one-to-one onto the router's network interfaces, except that any network interface that has multiple IP addresses will have multiple logical interfaces associated with it).

• Source router
  It is useful to be able to assign a single preference value to all routes (learned from the same routing domain) that were learned from any of a set of routers, where the set of routers are those whose updates have a source address that match a specified network prefix.

• Originating AS
  For routing protocols which provide the information, it is useful to be able to assign a single preference value to all routes (learned from a particular routing domain) which originated in another particular routing domain. For BGP routes, the originating AS is the first AS listed in the route's AS_PATH attribute. For OSPF external routes, the originating AS may be considered to be the low order 16 bits of the route's external route tag if the tag's Automatic bit is set and the tag's Path Length is not equal to 3.

• External route tag
  It is useful to be able to assign a single preference value to all OSPF external routes (learned from the same routing domain) whose external route tags match any of a list of specified values. Because the external route tag may contain a structured value, it may be useful to provide the ability to match particular subfields of the tag.

• AS path
  It may be useful to be able to assign a single preference value to all BGP routes (learned from the same routing domain) whose AS path "matches" any of a set of specified values. It is not yet clear exactly what kinds of matches are most useful. A simple option would be to allow matching of all routes for which a particular AS number appears (or alternatively, does not appear) anywhere in the route's AS_PATH attribute. A more general but somewhat more difficult alternative would be to allow matching all routes for which the AS path matches a specified regular expression.