18 – Dynamic Routing Based on the Application State.

The Cisco load balancing service module, ACE, provides real-time information about the preferred route to access a specific application or service after it has moved to a new location. The ACE continually probes the state of the applications for which  it is responsible.

When one of these services appears in its data center, it immediately sends a static route to reach the application to the adjacent router, which in turn notifies the routed wide area network (WAN) with a preferred metric. At the same time, the site of origin withdraws the IP route of the host (or application) that no longer exists from its local routers.

Unlike the GSS, information on Layer 3 routes concerns all existing sessions (current and new) and all sessions will be redirected almost in real time – according to the route table updates – to the new data center hosting the concerned application. However, the current active sessions will be kept safe during the migration because the IP address is unchanged for both the private and the public side. Nevertheless, local stateful devices such as firewalls and load balancers must initiate and validate a new session. Except for some specific protocols primarily related to maintenance purposes such as Telnet or FTP, usually for applications related to the cloud services such as IaaS and/or traditional http based software, this is not detrimental to the services supported.

In fact, an HTTP session that was established through stateful devices such as a firewall in the primary data center can be redirected to a secondary data center offering the same access and application security level and policy rules. After the migration of the concerned application on the new data center, the local stateful devices will accept and initiate a new session for that workflow according to the security policies.

Once granted, the session will be established transparently to the end user. Note that mechanisms based on cookies or SSL IDs or other identifiers used to maintain session persistence between the server supporting the application and the end-user, must be maintained.

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