Reference Guide

Interface
Name Role PortID Prio Cost Sts Cost Link-type Edge
---------- ------ -------- ---- ------- --- ----------------
Gi 0/6 Root 128.263 128 20000 FWD 20000 P2P No
Gi 0/7 ErrDis 128.264 128 20000 EDS 20000 P2P No
FTOS(conf-if-gi-0/7)#do show ip int br gi 0/7
Interface IP-Address OK Method Status Protocol
GigabitEthernet 0/7 unassigned YES Manual up up
Selecting STP Root
The STP determines the root bridge, but you can assign one bridge a lower priority to increase the likelihood that it
becomes the root bridge. You can also specify that a bridge is the root or the secondary root.
To change the bridge priority or specify that a bridge is the root or secondary root, use the following command.
• Assign a number as the bridge priority or designate it as the root or secondary root.
PROTOCOL SPANNING TREE mode
bridge-priority {priority-value | primary | secondary}
– priority-value: the range is from 0 to 65535. The lower the number assigned, the more likely this
bridge becomes the root bridge.
The primary option specifies a bridge priority of 8192.
The secondary option specifies a bridge priority of 16384.
The default is 32768.
To view only the root information, use the show spanning-tree root command from EXEC privilege mode.
Example of Viewing STP Root Information
FTOS#show spanning-tree 0 root
Root ID Priority 32768, Address 0001.e80d.2462
We are the root of the spanning tree
Root Bridge hello time 2, max age 20, forward delay 15
FTOS#
STP Root Guard
STP root guard is supported on the platform.
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
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