4.7 大陸の集合

   Another level of hierarchy may also be used in this addressing scheme
   to further reduce the amount of routing information necessary for
   global routing.  Continental aggregation is useful because
   continental boundaries provide natural barriers to topological
   connection and administrative boundaries.  Thus, it presents a
   natural boundary for another level of aggregation of inter-domain
   routing information.  To make use of this, it is necessary that each
   continent be assigned an appropriate contiguous block of addresses.
   Providers (both direct and indirect) within that continent would
   allocate their addresses from this space.  Note that there are
   numerous exceptions to this, in which a service provider (either
   direct or indirect) spans a continental division.  These exceptions
   can be handled similarly to multi-homed routing domains, as discussed


   The benefit of continental aggregation is that it helps to absorb the
   entropy introduced within continental routing caused by the cases
   where an organization must use an address prefix which must be
   advertised beyond its direct provider.  In such cases, if the address
   is taken from the continental prefix, the additional cost of the
   route is not propagated past the point where continental aggregation
   takes place.


   Note that, in contrast to the case of providers, the aggregation of
   continental routing information may not be done on the continent to
   which the prefix is allocated.  The cost of inter-continental links
   (and especially trans-oceanic links) is very high.  If aggregation is
   performed on the `near' side of the link, then routing information
   about unreachable destinations within that continent can only reside
   on that continent.  Alternatively, if continental aggregation is done
   on the `far' side of an inter-continental link, the `far' end can
   perform the aggregation and inject it into continental routing.  This
   means that destinations which are part of the continental
   aggregation, but for which there is not a corresponding more specific
   prefix can be rejected before leaving the continent on which they


   For example, suppose that Europe is assigned a prefix of 46/8, such
   that European routing also contains the longer prefixes 46DC:0A01/32
   and 46DC:0A02/32 .  All of the longer European prefixes may be
   advertised across a trans-Atlantic link to North America.  The router
   in North America would then aggregate these routes, and only
   advertise the prefix 46/8 into North American routing.  Packets which
   are destined for 46DC:0A01:1234:5678:ABCD:8765:4321:AABB would
   traverse North American routing, but would encounter the North
   American router which performed the European aggregation.  If the
   prefix 46DC:0A01/32 is unreachable, the router would drop the packet
   and send an unreachable message without using the trans-Atlantic



Copyright (C) 2006 七鍵 key@do.ai 初版:2006年10月19日 最終更新:2006年10月30日