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224 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Configuring STP related features
8
• Fast Port Span eliminates unnecessary MAC cache aging that can be caused by topology
change notifications. Bridging devices age out the learned MAC addresses in their MAC caches
if the addresses are unrefreshed for a given period of time, sometimes called the MAC aging
interval. When STP sends a topology change notification, devices that receive the notification
use the value of the STP forward delay to quickly age out their MAC caches. For example, if a
device normal MAC aging interval is 5 minutes, the aging interval changes temporarily to the
value of the forward delay (for example, 15 seconds) in response to an STP topology change.
In normal STP, the accelerated cache aging occurs even when a single host goes up or down.
Because Fast Port Span does not send a topology change notification when a host on a Fast
Port Span port goes up or down, the unnecessary cache aging that can occur in these
circumstances under normal STP is eliminated.
Fast Port Span is a system-wide parameter and is enabled by default. Thus, when you boot a
device, all the ports that are attached only to end stations run Fast Port Span. For ports that are not
eligible for Fast Port Span, such as ports connected to other networking devices, the device
automatically uses the normal STP settings. If a port matches any of the following criteria, the port
is ineligible for Fast Port Span and uses normal STP instead:
• The port is 802.1Q tagged
• The port is a member of a trunk group
• The port has learned more than one active MAC address
• An STP Configuration BPDU has been received on the port, thus indicating the presence of
another bridge on the port.
You also can explicitly exclude individual ports from Fast Port Span if needed. For example, if the
only uplink ports for a wiring closet switch are Gbps ports, you can exclude the ports from Fast Port
Span.
Disabling and re-enabling fast port span
Fast Port Span is a system-wide parameter and is enabled by default. Thus all ports that are
eligible for Fast Port Span use it.
To disable or re-enable Fast Port Span, enter the following commands.
PowerConnect(config)#no fast port-span
PowerConnect(config)#write memory
Syntax: [no] fast port-span
NOTE
The fast port-span command has additional parameters that let you exclude specific ports. These
parameters are shown in the following section.
To re-enable Fast Port Span, enter the following commands.
PowerConnect(config)#fast port-span
PowerConnect(config)#write memory
Excluding specific ports from fast port span
To exclude a port from Fast Port Span while leaving Fast Port Span enabled globally, enter
commands such as the following.
PowerConnect(config)#fast port-span exclude ethernet 1
PowerConnect(config)#write memory