Default Gateways and Routing

Another point of confusion when one is configuring the network adapters is where to place the default gateway. Many administrators try to place gateways on both network adapters, which causes extremely unreliable traffic flows. It does not matter whether the adapters are all on the same or different subnets; only one adapter should have a default gateway assigned.

For an Edge Server, always place the default gateway on the external-facing network adapter associated with the Access Edge Server IP address. This applies whether a single external adapter is used for all three roles or whether multiple adapters are used. This ensures that all requests the server does not know how to route are passed out through the Access Edge IP address.

The tricky part of configuring an Edge Server is to make sure that it always uses the internal-facing network adapter to communicate with any internal servers or clients. Administrators can accomplish this by creating persistent static routes on each Edge Server. Routing is something generally associated with network devices, but in the case of a multihomed server, an administrator must configure the routing table to act appropriately. For a Lync Server Edge role, the administrator must manually enter route statements to use that internal adapter for internal subnets. Figure 31.14 shows how the external adapter has a default gateway associated, but routing statements for internal networks make use of the internal adapter.

Figure 31.14. Edge Server gateways and routing.

It’s possible to use the netsh.exe command to add static routes for all of the private IP ranges using the following syntax:

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netsh interface ipv4 add route 10.0.0.0/8 "<Internal-Facing Interface Name>"
<Internal-Facing Network Gateway>

netsh interface ipv4 add route 172.16.0.0/12 "<Internal-Facing Interface Name>"
<Internal-Facing Network Gateway>

netsh interface ipv4 add route 192.168.0.0/16 "<Internal-Facing Interface Name>"
<Internal-Facing Network Gateway>

Be sure to run this command again for any additional publicly routable IP addresses that are part of the internal network.