High Availability Driver

Background

Starting from Windows Server 2008 and Windows Vista, and applied to all later operating systems so far, Microsoft Windows has changed the way it handles ARP packets that are sent when an IP address is added on a computer: the SPA field in the initial ARP request is set to 0.0.0.0, instead of the added IP-address. Due to the change, the packets are not exactly Gratuitous ARP packets anymore, but rather Probe ARP packets.

The change may cause routers, firewalls, or Windows 2003 Servers in that subnet to lose track of where the IP-address really resides, and that is critical to high availability (HA) solutions that rely on dynamic IP-addresses. There seems to be differences between specific firmware versions of routers, and there is no information available, which are the affected router models and firmware versions.

The problem arises when a virtual unit is switched over from one Windows Server to another. Due to the change in operating system, a router, firewall, or Windows 2003 Server might not see that the IP address has been moved to another computer, and continues sending IP datagrams to the original computer where the service is not running anymore. A typical example of this problem is that users are unable to open their softphones after the virtual unit, where the Connection Server is running, has been switched over, either manually or automatically due to some kind of failure.

The change in operating system does not affect Windows Servers or Windows computers in the same subnet. For example, server-to-server communications within switched server network are not affected. Also, not all routers (or firewalls) are affected.