Configuring Spanning Tree Parameters Spanning Tree Overview
OmniSwitch AOS Release 8 Network Configuration Guide December 2017 page 6-8
• When a bridge receives BPDU on its designated port that contains information that is less attractive
(lower priority values and/or higher path costs), it forwards its own information to other LANs to
which it is connected for consideration.
STP evaluates BPDU parameter values to select the best BPDU based on the following order of
precedence:
1 The lowest root bridge ID (lowest priority value, then lowest MAC address).
2 The best root path cost.
3 If root path costs are equal, the bridge ID of the bridge sending the BPDU.
4 If the previous three values tie, then the port ID (lowest priority value, then lowest port number).
Topology Change Notification
When a topology change occurs, such as when a link goes down or a switch is added to the network, the
affected bridge sends a Topology Change Notification (TCN) BPDU to the designated bridge for its LAN.
The designated bridge then forwards the TCN to the root bridge. The root then sends out a Configuration
BPDU and sets a Topology Change (TC) flag within the BPDU to notify other bridges that there is a
change in the configuration information. Once this change is propagated throughout the Spanning Tree
network, the root stops sending BPDU with the TC flag set and the Spanning Tree returns to an active,
stable topology.
Detecting the Source of Topology Changes
The following information and logging mechanisms are available on each switch to help identify the
source of topology changes within an active network:
• The port on which the last TCN was received on the local switch. The “Topology Change Port” field of
the show spantree vlan, show spantree cist, and show spantree msti commands displays the switch
port on which the last TCN was received. This information can be used to track down the switch that
triggered the topology change in an active RSTP or MSTP topology (not supported for STP
topologies).
• Switch logging entries to identify root port and root bridge changes for all Spanning Tree protocols
(STP, RSTP, and MSTP). For example:
2014 May 19 15:26:44 U28E_7_12_7 swlogd: stpCmm _TRPt info(5) TRAP:newRoot stp=0
2014 May 19 15:39:54 U28E_7_12_7 swlogd: stpCmm _TRPt info(5) TRAP:newRootPort
stp=0 port=101005
For more information about the switch logging utility, see Chapter 36, “Using Switch Logging.”
• Topology change storm detection to identify excessive topology changes for all Spanning Tree
protocols (STP, RSTP, and MSTP). The switch uses internal calculations based on the number of
topology changes within a specific period of time to determine if the number of topology changes
exceeds a specific threshold. When this threshold value is reached, switch logging entries are triggered
as a warning of potential instability within the network. For example:
For Flat + MSTP CIST instance:
2014 May 19 15:26:44 U28E_7_12_7 swlogd: stpCmm _STPt warn(4) TCN Storm detected
on port 1/1/1 for Cist
Note. You can restrict the propagation of TCNs on a port. See “Restricting TCN Propagation” on page 6-42
for more information.
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