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Configuring Spanning Tree Protocol (STP) and IronSpan Features
December 2005
© Foundry Networks, Inc.
7 - 17
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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’s 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:
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The port is 802.1Q tagged
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The port is a member of a trunk group
•
The port has learned more than one active MAC address
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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 Gigabit 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:
FESX424 Router(config)# no fast port-span
FESX424 Router(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:
FESX424 Router(config)# fast port-span
FESX424 Router(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:
FESX424 Router(config)# fast port-span exclude ethernet 1
FESX424 Router(config)# write memory
To exclude a set of ports from Fast Port Span, enter commands such as the following:
FESX424 Router(config)# fast port-span exclude ethernet 1 ethernet 2 ethernet 3
FESX424 Router(config)# write memory
To exclude a contiguous (unbroken) range of ports from Fast Span, enter commands such as the following:
FESX424 Router(config)# fast port-span exclude ethernet 1 to 24
FESX424 Router(config)# write memory
Summary of Contents for FastIron Edge Switch X424
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