Understanding the Effects of Slow Links on Group Policy

A slow link is the speed it takes for a packet to get from one site to another. If the time the packet takes to reach the other site exceeds Microsoft's preconfigured slow link threshold, the link is determined to be slow.

What is the Effect of a Slow Link on a Site?

Microsoft Windows Server 2003 has a default determination of what constitutes a slow link between sites and automatically changes what Group Policies are provided to a user on the receiving end of a slow link. Security policies and administrative templates are always loaded, no matter what the link speed. However, group policies such as Login scripts, software pushes, and folder redirection are not pushed to the user who is accessing GP via a slow link. This can be problematic for sites that don't have local domain controllers and receive authentication across a slow WAN link.

If you have unreliable or saturated bandwidth you might want to change the configuration of what is considered a slow link in the site or disable slow link detection completely.

Determining Slow Link Speed

By default, a slow link has an average ping time of greater than 32ms using 2048 byte packets, or a time greater than 500Kbps. Microsoft uses the following formula to convert ping times to Kbps. The formula is as follows :

 16,000 / ping = Kbps

Therefore, the default value of a 32ms ping times equals the following when the formula is applied:

 16,000 / 32ms = 500Kbps

To determine whether a site has a slow link, perform a ping from that location to the nearest DC it would use to authenticate and obtain its Group Policy. Use the following format for the ping command to make sure the test packet is a 2048KB packet:

 ping -l 2048  servername  (where  servername  is the closest domain controller)

The time it takes to return the ping will show if the link is more than 500Kpbs and is thus a slow link and subject to the slow link restrictions.

Configuring a Unique Slow Link Speed

To override Microsoft's default definition of a slow link, change slow link behavior, or otherwise change slow link configuration, go to the following areas in Group Policy:

Computer Configuration, Administrative Templates, System, Group Policy, Group Policy Slow Link Detection Properties. (Set to 0 to disable slow link detection or set a unique slow link time period.)
User Configuration, Administrative Templates, System, Group Policy, Group Policy Slow Link Detection. (Set to 0 to disable slow link detection or set a unique slow link time period.)

Group Policy also allows for changing the behavior of processes such as scripts, folder redirection, software installation, and security when slow links are in effect. These can be changed by choosing Computer Configuration, Administrative Templates, System, Group Policy and editing the Policy Processing Group Policies.