Users Guide

By default, this feature is enabled for RSTP, Rapid-PVST and MSTP. This feature is useful in a scalable topology with MSTP &
rapid-PVST (multi-instance), where multiple MAC flush calls are invoked.
RSTP
RSTP allows per port-based flush until the number of calls sent is equal to the MAC flush threshold value that you have
configured.
When the number of calls that are sent reaches the configured threshold, RSTP ignores further per-port based flush and starts
the MAC flush timer. When the timer expires, RSTP invokes an entire table flush, where it requests one flush for all the ports.
RSTP is a single instance and hence, MAC flush optimization is not required. However, to enable this feature, configure the
MAC flush timer to a non-zero value. This configuration is applied globally and applies for RSTP, MSTP, and rapid-PVST. This
configuration is retained when you change the STP mode.
For RSTP, the threshold is set to a higher value (65,535) because RSTP does not require this optimization. Even when this
feature is enabled, the global flush is invoked only after the flush count reaches 65,535.
MSTP
MSTP allows (VLAN-list, port) based flush until the number of calls sent is equal to the MAC flush threshold value that you have
configured.
When the number of calls exceeds the configured threshold, MSTP ignores further (VLAN-list, port) based flush and starts the
MAC flush timer. When the timer starts, the system blocks all further flush indications. When the timer expires for that specific
instance, the system triggers instance-based flushing.
The default MAC flush threshold value for MSTP is 5.
Rapid-PVST
Rapid-PVST allows (VLAN, port) based flush until the number of calls sent is equal to the MAC flush threshold value that is
configured.
When the number of calls sent exceeds the configured threshold, rapid-PVST ignores further (VLAN, port) based flush and
starts the MAC flush timer. When the timer starts, the system blocks further flush. When the timer expires for that specific
instance, the system triggers VLAN-based flushing.
By default, the MAC flush threshold value is set to 5. However, Dell EMC recommends that you configure this value based on
the number of ports that participate in the STP topology.
Spanning-tree link type for rapid state transitions
As specified in IEEE 802.1w, OS10 assumes a port that runs in full-duplex mode is a point-to-point link. A point-to-point link
transitions to forwarding state faster. By default, OS10 derives the link type of a port from the duplex mode. You can override
the duplex mode using the spanning-tree link-type command.
OS10 assumes a port that runs in half-duplex mode is a shared link, to which the fast transition feature is not applicable. Also, If
you explicitly designate a port as a shared link, you cannot use the fast transition feature, regardless of the duplex setting.
To hasten the spanning-tree state transitions, you can set the link type to point-to-point. To set the link type to point-to-point:
● Use the following command in INTERFACE mode.
spanning-tree link-type point-to-point
Dynamic path cost calculation
Path cost of an interface (physical or port-channel) is calculated based on the speed of the port or port-channel. When the
speed of the port or port-channel changes, the past cost recalculation is triggered based on the user defined configuration.
You can enable/disable dynamic recalculation of path cost using the spanning-tree path-cost command.
This cmd allows the protocol to do dynamic cost calculation whenever the channel-members are added or deleted. By default,
this dynamic path cost calculation is enabled.
When dynamic path cost is disabled , protocol calculate the path cost when the port channel is coming up for the first time
after creation or whenever dynamic path cost calculation is enabled and then disabled by management or when the user adds/
removes member port to/from the port channel.
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