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, congure the MAC ush timer value to 0.
When you congure 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 congured.
When the number of calls that are sent reaches the congured 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, congure the MAC ush timer
to a non-zero value. This conguration is applied globally and applies for RSTP, MSTP, and RPVST. This conguration 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
congured.
When the number of calls exceed the congured 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 specic 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 congured.
When the number of calls sent exceeds the congured 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 specic instance, the system triggers
VLAN-based ushing.
By default, the MAC ush threshold value is set to 5. However, Dell EMC recommends that you congure this value based on the number
of ports that participate in the STP topology.
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