Administrator Guide

We are the root of the spanning tree
Root Bridge hello time 2, max age 20, forward delay 15
Dell#
STP Root Guard
Use the STP root guard feature in a Layer 2 network to avoid bridging loops.
In STP, the switch in the network with the lowest priority (as determined by STP or set with the bridge-
priority command) is selected as the root bridge. If two switches have the same priority, the switch with
the lower MAC address is selected as the root. All other switches in the network use the root bridge as the
reference used to calculate the shortest forwarding path.
Because any switch in an STP network with a lower priority can become the root bridge, the forwarding
topology may not be stable. The location of the root bridge can change, resulting in unpredictable network
behavior. The STP root guard feature ensures that the position of the root bridge does not change.
Root Guard Scenario
For example, as shown in the following illustration (STP topology 1, upper left) Switch A is the root bridge in
the network core. Switch C functions as an access switch connected to an external device. The link between
Switch C and Switch B is in a Blocking state. The flow of STP BPDUs is shown in the illustration.
In STP topology 2 (shown in the upper right), STP is enabled on device D on which a software bridge
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 (shown in the lower middle), if you have enabled the root guard feature on the STP port on
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.
Spanning Tree Protocol (STP) 882