The Global Site Selector (GSS) is an Intelligent Domain Name Server that distributes the user’s requests to the remote sites where the applications are active. The GSS has already been described at the beginning of this article (Global Site Load Balancing Services) in a traditional routed environment. The same equipment can be used for LAN extension in conjunction with other network services such as the SLB devices, as well as with centralized management tools used for migrating VMs.
GSS with KAL-AP: In conjunction with load balancing equipment such as the Cisco Application Content Engine (ACE), the GSS periodically sends probes (Keep Alive Appliance Protocol, KAL-AP) to the load balancing device in order to determine the status of VMs and services distributed throughout the cloud.
When a VM migration is complete, the GSS locates the active application based on regular keep-alive probing, and immediately associates it with the public address of the hosting data center.
The existing sessions are maintained via the original data center to avoid interruption, while all the DNS requests from new clients for this application are updated with the public IP address used for the new site. This mechanism supports load distribution across multiple WANs (different active applications distributed between multiple locations).
In the meantime, this mechanism optimizes traffic by sending all new sessions directly to the location hosting the active service of the cloud without using the Layer 2 extension path.
By correlating the keep-alive function (KAL) from the GSS with other egress path optimization functions such as FHRP localization as described above, new incoming sessions established at the new data center will be optimized for direct return flow using their local default gateway.
To keep the existing sessions secure, the traffic for those current sessions must return to the original site via a mechanism of source-NAT activated at the upstream layer. This allows both scenarios to be used while ensuring symmetrical flows for all sessions.
vCenter and GSS: A module script is added to the central management of VMs from VMware (vCenter) so that when the VM management migrates a VM on a remote host, manually or dynamically, it informs the upstream GSS about the new location of this particular VM in real time.
The GSS then immediately assigns a public address associated with the new site to the VM. Similarly with the KAL-AP probes described previously, the established sessions remain directed to the same site of origin to maintain the workflows.