Windows 2008 / Multi-subnet clusters and using static routes.
With the enhancements in Windows 2008 to allow for multi-subnet clustering it is becoming more common to see this utilized with Exchange 2007 SP1 installations.
When implementing a clustered solution, it is a requirement that there be a minimum of two interfaces on each node, and that each node can maintain communications across those interfaces. I see administrators implement this requirement in two different fashions with multi-subnet clusters:
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The “public” interface of each node resides in different subnets with the “private” interfaces residing in a stretched subnet.
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The “public” interface of each node resides in different subnets with the “private” interfaces also residing in different subnets.
If you are the second bullet, you’ll want to continue reading this blog. (If you are the first bullet you’ll probably want to read it anyway since you’ve made it this far…)
For users that have a configuration where both network interfaces are in different subnets this will generally require routing between those two subnets. A common mis-configuration that I see in this design is the use of default gateways on both of these network interfaces.
When a user attempts to configure two network interfaces each with a default gateway, the following error is noted from the operating system:
The text in this message is specifically important as it highlights at this time that this configuration will not produce the desired results.
The most likely cluster configuration where Exchange is used, with this type of clustering, is cluster continuous replication (CCR). When multiple default gateways are defined, users may see inconsistent results in the performance and ability to replicate logs between the nodes. The replication issues between nodes are also exacerbated when continuous replication hostnames are used utilizing the secondary networks with the default gateway assigned. These issues are secondary to any issues that the cluster service many have maintaining communications between the nodes and any communications issues clients may have connecting to the nodes.
If the default gateways are removed from the “private” adapters, reliable routed communications can only occur over the “public” interface. So…if two default gateways cannot be used, how should we ensure proper communications over both the “public” interface and “private” interface where both reside in different routed subnets.
The first part of this solution is to ensure that the binding order of the network interfaces is set correctly in the operating system. To confirm the binding order:
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Open the network connections control panel.
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Choose the advanced menu (if menu is disabled, enable it by selecting Organize –> Layout –> Menu Bar).
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Select advanced settings from the advanced menu.
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On the adapters and bindings tab, ensure that the “public” interface is first in the list, with all secondary interfaces following after.
The second part of the solution is to maintain the default gateway on the “public” interface.
The third part of the solution is to enable persistent static routes on the “private” interfaces. In terms of the routes we simple need to configure routes to other “private” networks using gateway addresses that have the ability to route between those “private” networks. All other traffic not matching this route should be handled by the default gateway of the “public” adapter.
Let’s take a look at an example.
I desire to have a two node Exchange 2007 SP1 CCR cluster on Windows 2008 with each node residing in a different subnet.
NodeA:
Public
Private
NodeB:
Public:
Private
(Note that gateway on network is not the default gateway setting but is the gateway on the private interface network that can route packets to the private network on the other nodes.)
In this case I would want to establish the necessary persistent static routes on each node. In order to accomplish this, I can use the route add command. The structure of the route command:
NodeA: Route add 10.0.1.0 mask 255.255.255.0 10.0.0.254 –p
NodeB: Route add 10.0.0.0 mask 255.255.255.0 10.0.1.254 –p
The –p switch will ensure that the routes are persistent lasting after a reboot. Failure to use the –p will result in the routes being removed post a reboot operation.
You can verify that the routes are correct by running route print and reviewing the persistent route information.
By utilizing only a default gateway on the “public” adapter, and static routes on the “private” adapters, you can ensure safe routed paths for client communications, cluster communications, and replication service log shipping.
