Symptoms:

• Configuring an Edge port for Fast Start
• Blocked addresses not ageing out until aging time has passed.
• A temporary network loop is created in the network
• SuperStack 3 Switch 3226
• SuperStack 3 Switch 3250

Facts:

• SuperStack 3 Switch 4400
• SuperStack 3 Switch 4400 SE
• 3C17203
• 3C17204
• 3C17206
• Switch 4050
• 3C17708
• Switch 4060
• 3C17709
• SuperStack 3 Switch 4900
• 3C17700
• SuperStack 3 Switch 4200
• 3C17300
• 3C17302
• 3C17304
• Rapid Spanning Tree
• RSTP Spanning Tree
• STP
• Edge
• Edge Port
• FastStart
• Fast Start

Causes: Legacy spanning tree takes approx. 30 seconds to converge, whilst Rapid Spanning tree (RSTP) converges much quicker and takes approximately 5 second. RSTP has a function called FastStart that is even much quicker. FastStart is instantaneous and as soon as the link is up, if it's set to ?ON?, the port will go into a Spanning Tree Forwarding state, rather than going though the intermediate states first. This effectively minimises link initialisation times. With FastStart set to ON, if two Switches are connected together and there is more than one link between them, a temporary loop in the network will result, causing some devices being temporarily unreachable.

There is an underlying problem with the design of the Spanning Tree protocol itself. This problem is not specific to any product, so when connecting Switches together and Fast Start is not disabled on the interconnecting ports, any addresses learnt in that time will not be flushed. Furthermore, any previously learned addresses on links that become blocked will not correctly age out until the aging time has passed. If the Switch is power cycled then the device will learn the addresses correctly.

Fixes: When connecting switches together, it?s recommended that Fast Start is disabled on the interconnecting ports. This can be done using the Physical Interface > Ethernet > Setup command on the Web interface.

It is recommended to configure Fast Start as “OFF” on ports being used to interconnect Switches. It should only be enabled on ports that are directly connected to end stations.

Notes: Switch 3200 - Fast Start is enabled by default on all 10/100 ports and disabled by default on 10/100/1000 and SFP ports.

Switch 4400 ? Fast Start is enabled by default on all Switch 4400 front panel ports, and it is recommended that it is configured to ON for all Desktop Ports, unless configured as a trunk.

The Switch has an Edge port administration feature called ?FastStart? which is configurable on a port by port basis.

The Edge port administration feature ?FastStart? is configurable on a port by port basis.

• Product(s): SuperStack 3 Switches, Switch 4050, Switch 4060, Switch 4070
• Sub Product(s): SuperStack 3 Switch 4200, SuperStack 3 Switch 4400, SuperStack 3 Switch 4900, SuperStack 3 Switch 4924, SuperStack 3 Switch 4950, SuperStack 3 Switch 3824, SuperStack 3 Switch 3812, SuperStack 3 Switch 3226, SuperStack 3 Switch 3250

— David Gonzalez 2021/04/14 15:27