Spanning Tree Protocol (STP) can potentially impact environments where both OSA-Express QDIO and VIPA are deployed.
With dynamic VIPA, a TCP/IP address takeover occurs when a TCP/IP stack fails, or a static or dynamic VIPA is manually moved by operator intervention. In either case, when a static or dynamic VIPA moves, the IP address and respective workloads are taken over by another TCP/IP stack through other OSA-Express Ethernet devices running on a different server. When the original TCP/IP stack and respective OSA-Express QDIO devices are returned to operation, both the IP address and respective workload traffic are taken back by the recovered TCP/IP stack.
If the network bridge or switch is not configured properly, packets can get lost in the network or be blocked by the networking equipment. This is a result of a physical looping condition identified by the STP, or expired OSA-Express QDIO timers due to the increased latencies associated with blocked ports or delayed packets. In these cases, static or dynamic VIPA connectivity can fail.
When configuring STP, use care in the bridge or switch configuration to avoid or minimize potential loop conditions. For example, if the STP is not an integral component of the overall network, disabling the STP on all of the Virtual LANs (VLANs) that connect to OSA-Express QDIO devices will help avoid the problem.
Also, some networking switches provide a mechanism for suppressing the STP's listening and learning states on specific switch ports. For example, Cisco Systems provides an STP configuration feature called PortFast that can place specific switch ports into forwarding state as soon as a link is detected. Without PortFast enabled, a switch port has to transition through the listening and learning stages (30 seconds total) of Spanning Tree reconvergence before the switch port can actually pass valid traffic. For PortFast capable Cisco switches, enabling PortFast on all switch ports that OSA-Express QDIO is connected into allows network administrators to both preserve their original Spanning Tree configuration by not having to change it, while also providing a viable mechanism to avoid potential static or dynamic VIPA problems.
For more information on configuring STP and immediate port forwarding, see the bridge or switch operational manual.