Multiple default gateways

Problem

While troubleshooting a networking teaming issue on a cluster, someone sent me the a link to this article about multiple default gateways on Win 2012 native teaming: http://www.concurrency.com/blog/bug-in-nic-teaming-wizard-makes-duplicate-default-routes-in-server-2012/. The post discusses a pretty specific scenario that we didn’t have on our clusters (most of them are on 2008R2), but I discovered several nodes with more than one default route in route print:

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The issue I was looking into was another, but I remembered a problem from a weekend some months ago that might be related: When a failover was triggered on a SQL cluster, the cluster lost communication with the outside world. To be specific: no traffic passed through the default gateway. As all cluster nodes were on the same subnet the cluster itself was content with the situation, but none of the webservers were able to communicate with the clustered SQL server as they were in a different subnet. This made the webservers sad and the webmaster angry, so we had to fix it. As this happened in production over the weekend, the focus was on a quick fix and we were unable to find a root cause at the time. A reboot of the cluster nodes did the trick, and we just wrote it off as fallout from the storage issue that triggered the failover. The discovery of multiple default gateways on the other hand prompted a more thorough investigation.

Analysis

The article mentioned above talks exclusively about Windows 2012’s native teaming software, but this cluster is running Windows 2008 R2 and is relying on teaming software provided by the NIC manufacturer (Qlogic). We have had quite a lot of problems with the Qlogic network adapters on this cluster, so I immediately suspected them to be the rotten apple. I am not sure if this problem is caused by a bug in Windows itself that is present in both 2012 and 2008R2, or if both MS and Qlogic are unable to produce a functioning NIC teaming driver, but the following is clear:

If your adapters have a default gateway when you add them to a team, there is a chance that this default gateway will not get removed from the system. This happens regardless if the operating system is Windows 2012 or Windows 2008 R2. I am not sure if gateway addresses configured by DHCP also triggers this behavior. It doesn’t happen every time, and I have yet to figure out if there are any specific triggers as I haven’t been able to reproduce the problem at will.

Solution A

To resolve this issue, follow the recommendations in  http://www.concurrency.com/blog/bug-in-nic-teaming-wizard-makes-duplicate-default-routes-in-server-2012/:

First you have to issue a command to delete all static routes to 0.0.0.0. NB! This will disconnect you from the server if you are connected remotely from outside the subnet.

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Configure the default gateway for the team using IP properties on the virtual team adapter:

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Do a route print to make sure you have only one default gateway under persistent routes.

Solution b

If solution A doesn’t work, issue a netsh interface ip reset command to reset the ip configuration and reboot the server. Be prepared to re-enter the ip information for all adapters if necessary.

What not to do

Do not configure the default gateway using route add, as this will result in a static route. If the computer is a node in a cluster, the gateway will be disabled at failover and isolate the server on the local subnet. See http://support.microsoft.com/kb/2161341 for information about how to configure static routes on clusters if you absolutely have to use a static route.

Author: DizzyBadger

SQL DBA Principal Analyst

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