Overview

When using Dynamic Virtual Circuits (DVCs), and enabling Global Control for Link Aggregation, Global QoS does work, and the following logging message may be found in the logs:

Global QoS cannot be applied as there are Dynamic Virtual Circuits.

Link Aggregation is a standard technology that provides the ability to logically link together multiple physical links to increase bandwidth and reliability in case of failure. Exinda appliances can support Link Aggregation as long as Global QoS is enabled in the Control tab under Configuration > System > Setup. Without enabling Global QoS, the policy set on Exinda will be applied independently to each bridge, i.e., a copy of each will be made and applied to the different bridges so that traffic coming in will follow the policy set on the bridge independently of another one. In the case of Link Aggregation, this can make it appear that two bridges, which are sharing the same bandwidth, will think that they each have the full bandwidth isolated to themselves, which is not the case.

Global QoS shares one policy set across all the bridges so that links that share a full bandwidth will know the used and remaining bandwidth accordingly.

Root Cause

• Global QoS creates a logical bridge, much like Link Aggregation creates a logical link.
• To do so, it uses a technology called Intermedia Functional Blocks (IFB), in which any traffic coming into any bridge gets the same policy settings applied to it, instead of the standard behavior of distinct policy sets on each bridge.
• Dynamic Virtual Circuits, which split bandwidth per user, also use IFB to operate.  IFB works directly on the Linux kernel, and due to the limitations of the kernel itself, multiple IFB cannot be chained together or operate in a series.
• They can work in parallel, similar to when various Dynamic Virtual Circuits are created, but in the case of Global QoS and DVCs, they would need to work one inside the other, which is not supported.

Resolution

The only workaround for this issue is to remove Dynamic Virtual Circuits from the policy set if there is a Link Aggregation planned or to upgrade the Network Interface Card (NIC) to a bandwidth that supports the full size of the Link Aggregation in a single link.