Connected: An Internet Encyclopedia
5.2.4.4 Administrative Preference
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RFC 1812
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5.2.4.4 Administrative Preference
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.
Next: 5.2.4.5 Load Splitting
Connected: An Internet Encyclopedia
5.2.4.4 Administrative Preference