KB2823958 – Virtual Switch Extension Can’t Send Packets Separately Through Each NIC In WS2012 Hyper-V

A lot of posts appeared last night that WS2012 Hyper-V admins might want to pay attention to.  This one deals with a situation where virtual switch extension cannot send packets separately through every individual network adapter in Windows Server 2012.

Symptoms

Consider the following scenario:

  • You have a virtual switch extension installed on a Windows Server 2012-based Hyper-V server.
  • The extension sends packets that contain one or more of the following flags: 

    NDIS_SEND_FLAGS_SINGLE_QUEUE
    NDIS_SEND_FLAGS_SWITCH_DESTINATION_GROUP
    NDIS_SEND_FLAGS_SWITCH_SINGLE_SOURCE
    NDIS_SEND_COMPLETE_FLAGS_SINGLE_QUEUE
    NDIS_SEND_COMPLETE_FLAGS_SWITCH_SINGLE_SOURCE
    NDIS_RECEIVE_FLAGS_SINGLE_VLAN
    NDIS_RECEIVE_FLAGS_SINGLE_QUEUE
    NDIS_RECEIVE_FLAGS_SINGLE_ETHER_TYPE
    NDIS_RECEIVE_FLAGS_SWITCH_DESTINATION_GROUP
    NDIS_RECEIVE_FLAGS_SWITCH_SINGLE_SOURCE
    NDIS_RETURN_FLAGS_SINGLE_QUEUE
    NDIS_RETURN_FLAGS_SWITCH_SINGLE_SOURCE

In this scenario, the packets are sent by using the same network adapter in the virtual network.

Cause

This issue occurs because Network Driver Interface Specification (NDIS) chains the Net Buffer Lists (NBLs) incorrectly.

A supported hotfix is available from Microsoft.

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