6. Area and Domain Partition

Dieser Abschnitt bewahrt den RFC-Text fuer Unreachable Prefix Announcement (UPA), einschliesslich IS-IS and OSPF signaling, U-Flag and UP-Flag semantics, propagation rules, IANA assignments und security considerations.

6.  Area and Domain Partition

   UPA is not meant to address an area/domain partition.  When an area
   or domain partitions, while multiple ABRs or ASBRs advertise the same
   summary, each of them can only reach a portion of the summarized
   prefix.  As a result, depending on which ABR or ASBR the traffic is
   using to enter a partitioned area, the traffic could be either
   dropped or delivered to its final destination.  UPA does not make the
   problem of an area partition any worse.  In case of an area
   partition, each ABR or ASBR will generate UPAs for the destinations
   for which the reachability was lost locally.  As the UPA propagates
   to the nodes outside a partitioned area, it may result in such nodes
   picking an alternative egress node for the traffic, if such a node
   exists.  If such an alternative egress node resides outside a
   partitioned area, traffic will be restored.  If such an alternative
   egress node resides in a partitioned area and is covered by the
   summary, the traffic will be dropped if it enters a partitioned area
   via an ABR or ASBR that cannot reach that node.  This will result in
   similar behavior as without the UPA.  The above statements are also
   applicable to a domain partition.