Sunday, April 17, 2011

Part 2: Network Redundancy for Windows Server: Dynamic IP Routing

Besides NIC teaming in earlier part 1, another way to achieve network redundancy for Windows Server is to employ dynamic IP routing. Windows Servers support both static and dynamic IP routing. In dynamic routing, new routes are learned dynamically from the route peers (typically routers and L3 switches). Any changes in network topology (including failure of server NICs or switches) would trigger a routing update for all participating routing nodes. In other words, if there were any failure in any switches or NICs, traffic would have avoided the broken path and continue to route on unbroken links.

Examples of dynamic routing include Open Shortest Path First (OSPF) and Routing Information Protocol (RIP). Windows Server 2003 supports both OSPF and RIP while Windows Server 2008 only supports RIP. Besides achieving network redundancy, you need not configure routing statically, which is otherwise prone to network topology changes and configuration errors. Consider the following example:

Dual-homed ServerX is connected to 2 different switches on different subnets. RIPv2 routing is enabled on both NICs. By default, ServerX would dynamically register both DNS records with AD-integrated DNS servers. With DNS round-robin, network load-balancing can even be achieved among multiple clients. However, in the event of either NIC or switch failure, half of the clients would lose connection with ServerX, which is usually not desirable. Consider the next example:
MS Loopback adapter is installed. RIPv2 routing is enabled on two physical NICs, as well as the loopback adapter. Dynamic DNS update is also disabled. Single static A record is created with loopback IP address.  As dynamic routing supports equal-cost multi-path load-balancing, multiple clients reach ServerX loopback via both NICs. If either NIC or switch fails, dynamic routing would have to re-converge. After a short converging time, clients can regain access to ServerX on its loopback address via the sole remaining NIC.

In conclusion, with both dynamic routing and static DNS record on loopback adapter, both network load-balancing and network redundancy can be achieved. In my next post, I would summarize the steps to achieve this in part 3.

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