Dell EMC SmartFabric OS10 User Guide Release 10.5.0

MAC flush optimization

OS10 offers a MAC address clearing technique that optimizes the number of MAC flush calls sent by the Spanning Tree Protocol (STP) module.

If the number of calls sent to the hardware is too high, traffic is dropped or flooded impacting system performance. To prevent traffic drops and flooding, you can use the MAC flush optimization feature.

This feature fine-tunes the MAC flush-related parameters, such as the MAC flush threshold and the MAC flush timer to reduce the number of calls sent to the hardware. The clear request sent to clear the MAC address table entry is called a flush indication. The number of calls that are sent is displayed as flush invocations in the show spanning-tree command.

You can enable the MAC flush optimization feature by setting the MAC flush timer to a non-zero value. This feature is enabled by default with a default timer value of 200 centi-seconds.

To disable MAC flush optimization, configure the MAC flush timer value to 0.

When you configure the MAC flush timer to a non-zero value and the threshold to zero, the system invokes instance-based flush once and starts the timer. When the timer expires, the system invokes an instance-based flush 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

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.