The basic tc-htb interface is limited to 32bit rate values due to the 'struct tc_ratespec'. The value is in bytes per second, so the user is not able to specify rates higher than 34.359.738.360 bps, which is roughly 34 Gbps. Modern NICs are capable of much higher speeds. The extension to the API was added in kernel 3.13 that included 2 new netlink attributes: TCA_HTB_RATE64 and TCA_HTB_CEIL64, which are 64bit values that can be used to break the 32bit barrier. OVS so far didn't adopt the use of these attributes, so it doesn't support QoS at high speeds. Implementation in iproute2: https://github.com/shemminger/iproute2/commit/8334bb325d5178483a3063c5f06858b46d993dc7
Closing BZ as this is fixed upstream.