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+6, Multisite Scenario
+The Multisite scenario refers to the cases when VNFs are deployed on multiple VIMs.
+There could be three typical usecases for such scenario.
+One is in one DC, multiple openstack clouds are deployed. Taking into consideration that the
+number of compute nodes in one openstack cloud are quite limited (nearly 100) for
+both opensource and commercial product of openstack, multiple openstack clouds will
+have to be deployed in the DC to manage thousands of servers. In such a DC, it should
+be possible to deploy VNFs accross openstack clouds.
+Another typical usecase is Geographic Redundancy (GR). GR deployment is to deal with more
+catastrophic failures (flood, earthquake, propagating software fault, and etc.) of a single site.
+In the Geographic redundancy usecase, VNFs are deployed in two sites, which are
+geographically seperated and are deployed on NFVI managed by seperate VIM. When
+such a catastrophic failure happens, the VNFs at the failed site can failover to
+the redundant one so as to continue the service. Different VNFs may have specified
+requirement of such failover. Some VNFs may need stateful failover, while others
+may just need their VMs restarted on the redundant site in their initial state.
+The first would create the overhead of state replication. The latter may still
+have state replication through the storage. Accordingly for storage we don't want
+to loose any data, and for networking the NFs should be connected the same way as
+they were in the original site. We probably want also to have the same number of
+VMs on the redundant site coming up for the VNFs.
+The other usecase is the maintainance. When one site is planning for a maintaining,
+it should first replicate the service to another site before it stops them. Such
+replication should not disturb the service, nor should it cause any data loss. The
+service at the second site should be excuted, before the first site is stopped and
+began maintenance. In such case, the multisite schemes may be used.
+The multisite scenario is also captured by the Multisite project, in which specific
+requirements of openstack are also proposed for different usecases. However,
+the multisite project mainly focuses on the requirement of these multisite
+usecases on openstack. HA requirements are not necessarily the requirement
+for the approaches discussed in multisite. While the HA project tries to
+capture the HA requirements in these usecases. The following links are the scenarios
+and Usecases discussed in the Multisite project.