When a server migrates from one site to another, it must return the traffic to its default gateway because its IP address schema remains the same regardless of its physical location. Since there is one IP address (or virtual IP addresses (VIP)) for a given default gateway per subnet, this implies that after the migration of a logical server, the traffic must be returned to the original site where the active default gateway stands. In a complex multi-tier architecture, routers and firewalls are usually enabled to improve the communication and security between the tiers.
If, for example, a solution built with a 3-tier application (e.g. Web Server, Application and Database tiers) is moved from one data center to another, the traffic between each tier will have to return to the site where the gateways or firewalls are active. If we add to that the different network services required for optimization and data security (load balancer, SSL termination, IPS) enabled at different tiers, then up to ten round trips for a simple query may occur. Consequently, depending on the distance between the data centers, the latency for a request may be significantly affected (i.e. additional 10 to 20 ms for 100 km using dedicated fiber for a 10 round trips).
It is therefore crucial that the inter-application-tier or server-to-server traffic is better controlled to minimize the “ping-pong” effect.
Emerging solutions such as EMC VPLEX Geo, which support active/active data over thousands of km with no service interruption, separate performance and distance considerations.
Cisco supports deployment options for enabling the same default gateway functionalities in different data center sites (FHRP localization). This functionality is completely transparent to the application layer as well as the network layer. By activating this IP localization service, after the migration of VMs it is possible to use a local default gateway configured with the same IP identification (same virtual MAC addresses and virtual IP) that were defined on the original site.