Dell Force10 MXL Blade Configuration Manual page 592

In STP topology 2
application is started to connect to the network. Because the priority of the bridge in device D is lower than
the root bridge in Switch A, device D is elected as root, causing the link between Switches A and B to enter
a Blocking state. Network traffic then begins to flow in the directions indicated by the BPDU arrows in the
topology. If the links between Switches C and A or Switches C and B cannot handle the increased traffic
flow, frames may be dropped.
In STP topology 3
Switch C that connects to device D, and device D sends a superior BPDU that would trigger the election of
device D as the new root bridge, the BPDU is ignored and the port on Switch C transitions from a
forwarding to a Root-Inconsistent state (shown by the green X icon). As a result, Switch A becomes the
root bridge.
All incoming and outgoing traffic is blocked on an STP port in a Root-Inconsistent state. After the timeout
period, the Switch C port automatically transitions to a Forwarding state as soon as device D stops sending
BPDUs that advertise a lower priority.
If you enable a root guard on all STP ports on the links where the root bridge should not appear, you can
ensure a stable STP network topology and avoid bridging loops.
590
| Spanning Tree Protocol (STP)
(Figure 33-12 upper right), STP is enabled on device D on which a software bridge
(Figure 33-12 lower middle), if you enabled the root guard feature on the STP port on