Specifications

HP Virtual Connect for Cisco Network Administrators (version 1.2x) page 36
Figure 16. VC Stacking Link Minimum Recommendations
Optimizing Virtual Connect Stacking Links
Virtual Connect stacking links provide the physical path between downlinks (server NIC ports) to VC
uplinks. As a result, the fewer VC uplinks a frame has to traverse, the less latency the frame incurs in
reaching the external network. Each VC module a frame must traverse adds approximately 3.8
microseconds of latency to the frame. Because of the additional latency that each hop can add,
an administrator may wish to add additional stacking links above the recommended minimum.
On a per-vNet or per-Shared Uplink Set basis, the VC-Enet module with the active uplink, or port
channel, is the module that all server frames will use to exit the VC domain towards the external
network. For example, if a server NIC port (assigned to vNet1) is connected to VC-Enet module 2
and the active uplink for vNet1 is connected to VC-Enet module 6, then module 2 will send the
frame towards module 8 via the shortest stacking link path. If the modules are stacked together
using stacking links as depicted in the figure above, module 2 will take the path to module eight via
the stacking link to module 7. Module 7 will then use it’s directly connected internal stacking link to
deliver the frame to module 8 and out the uplink towards the external network. Module 2 could use
the longer path via module 1 to module 3 to module 4 to module 8 if it loses its shorter path through
module 7.
An administrator could purposefully add additional stacking links to provide shorter stacking link
paths between modules. For example using the scenario above, the administrator could add an
additional stacking link between module 2 and module 8. If added, module 2 could send frames
towards the active uplink on module 8 using the directly connected stacking link to module 8.
Purposefully adding additional stacking links is not a common practice since each module’s latency
of 3.8 microseconds is so low.