Connectivity Guide

You can enable the MAC ush optimization feature by setting the MAC ush timer to a non-zero value. This feature is enabled by default
with a default timer value of 200 centi-seconds.
To disable MAC ush optimization, congure the MAC ush timer value to 0.
When you congure the MAC ush timer to a non-zero value and the threshold to zero, the system invokes instance-based ush once and
starts the timer. When the timer expires, the system invokes an instance-based ush again.
The show spanning-tree {brief | details | active} command displays the following information:
Flush Interval 200 centi-sec, Flush Invocations 32
Flush Indication threshold 2
To clear MAC addresses:
• RSTP invokes a port-based MAC ush to clear the MAC address table entry for that port.
• MSTP invokes (VLAN-list associated to the instance, port) based ush to clear the MAC address table entry for that instance, port.
• RPVST invokes (VLAN, port) based ush to clear the MAC address table entry for that VLAN, port.
By default, this feature is enabled for RSTP, RPVST and MSTP. This feature is very useful in a scalable topology with MSTP & RPVST
(multi-instance), where multiple MAC ush calls are invoked.
RSTP
RSTP allows per port-based ush until the number of calls sent is equal to the MAC ush threshold value that you have congured.
When the number of calls that are sent reaches the congured threshold, RSTP ignores further per-port based ush and starts the MAC
ush timer. When the timer expires, RSTP invokes an entire table ush, where it requests one ush for all the ports.
RSTP is single instance and hence MAC ush optimization is not required. However, to enable this feature, congure the MAC ush timer
to a non-zero value. This conguration is applied globally and applies for RSTP, MSTP, and RPVST. This conguration 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 ush is invoked only after the ush count reaches 65,535.
MSTP
MSTP allows (VLAN-list, port) based ush until the number of calls sent is equal to the MAC ush threshold value that you have
congured.
When the number of calls exceed the congured threshold, MSTP ignores further (VLAN-list, port) based ush and starts the MAC ush
timer. When the timer starts, the system blocks all further ush indications. When the timer expires for that specic instance, the system
triggers instance-based ushing.
The default MAC ush threshold value for MSTP is 5.
RPVST
RPVST allows (VLAN, port) based ush until the number of calls sent is equal to the MAC ush threshold value that is congured.
When the number of calls sent exceeds the congured threshold, RPVST ignores further (VLAN, port) based ush and starts the MAC
ush timer. When the timer starts, the system blocks further ush. When the timer expires for that specic instance, the system triggers
VLAN-based ushing.
By default, the MAC ush threshold value is set to 5. However, Dell EMC recommends that you congure this value based on the number
of ports that participate in the STP topology.
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