Configuring Spanning Tree Parameters Spanning Tree Overview
OmniSwitch AOS Release 8 Network Configuration Guide December 2017 page 6-9
For Flat + MSTP MSTI instance
2014 May 19 15:26:44 U28E_7_12_7 swlogd: stpCmm _STPt warn(4) TCN Storm detected
on port 0/10 for Msti 1001
For Flat + RSTP 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 Per VLAN + RSTP 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 VLAN 1001
For more information about the switch logging utility, see Chapter 36, “Using Switch Logging.”
Loop-guard on OmniSwitch
STP relies on continuous reception or transmission of BPDUs based on the port role. The designated port
or primary port transmits BPDUs, and the non-designated ports receive BPDUs. When one of the non-
designated ports in a spanning tree network stop receiving BPDUs, then the STP conceives that the
network is loop free. However, when a non-designated (Alternate, Root, or Backup) port becomes desig-
nated and moves to a forwarding state, a loop is created in the network.
With Loop Guard, if a switch stops receiving BPDUs on a non-designated port, the switch places the port
into the STP loop-inconsistent blocking state thus preventing the occurrence of loop in the network.
Loop-guard can be configured on individual ports on per-port or per-VLAN basis. A port can have both
roles:
• Designated
• Non-designated for mutually exclusive set of VLANs or MSTP-instances (in MSTP mode)
If loop-guard is enabled on the port, it does not affect the forward or blocking state for a designated port.
In case of BPDU timeout, if a loop-guard enabled port fails to receive three consecutive BPDUs, STP
converts the port explicitly to a blocked port.
When loop-guard is enabled, if a switch stops receiving BPDUs on a non-designated port, the switch
places the port into the STP loop-inconsistent blocking state.
By default, loop-guard is disabled on all the switch ports. User can configure loop-guard on any port
irrespective of its STP state or role. However, the feature functions only on non-designated (Alternate,
Root, or Backup) STP ports.
Notes:
• In the flat mode, as there is a single STP instance on all the VLANs, the loop-guard state of the ports is
same across all the VLANs on the switch.
• In MSTP mode, there is a single STP instance for each MSTI instance. In this case, loop-guard state of
port is same across all the VLANs of a given MSTP instance. Hence if a loop-guard error occurs on
any single port, it affects all the other ports related to the MSTI.
• In 1X1 mode, there is a single STP instance assigned for each VLAN. Hence if a loop-guard error
occurs on any single VLAN, it does not affect the other VLANs.
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