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11-8
Catalyst 2940 Switch Software Configuration Guide
78-15507-02
Chapter 11 Configuring STP
Understanding Spanning-Tree Features
Figure 11-3 Spanning Tree and Redundant Connectivity
You can also create redundant links between switches by using EtherChannel groups. For more
information, see Chapter 25, “Configuring EtherChannels.”
Spanning-Tree Address Management
IEEE 802.1D specifies 17 multicast addresses, ranging from 0x00180C2000000 to 0x0180C2000010, to
be used by different bridge protocols. These addresses are static addresses that cannot be removed.
Regardless of the spanning-tree state, the switch receives but does not forward packets destined for
addresses between 0x0180C2000000 and 0x0180C200000F.
If spanning tree is enabled, the switch CPU receives packets destined for 0x0180C2000000 and
0x0180C2000010. If spanning-tree is disabled, the switch forwards those packets as unknown multicast
addresses.
Accelerated Aging to Retain Connectivity
The default for aging dynamic addresses is 5 minutes, the default setting of the mac-address-table
aging-time global configuration command. However, a spanning-tree reconfiguration can cause many
station locations to change. Because these stations could be unreachable for 5 minutes or more during a
reconfiguration, the address-aging time is accelerated so that station addresses can be dropped from the
address table and then relearned. The accelerated aging is the same as the forward-delay parameter value
(spanning-tree vlan vlan-id forward-time seconds global configuration command) when the spanning
tree reconfigures.
Because each VLAN is a separate spanning-tree instance, the switch accelerates aging on a per-VLAN
basis. A spanning-tree reconfiguration on one VLAN can cause the dynamic addresses learned on that
VLAN to be subject to accelerated aging. Dynamic addresses on other VLANs can be unaffected and
remain subject to the aging interval entered for the switch.
Catalyst 2940
switch
Workstations
Catalyst 2940
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Catalyst 2940
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Switch A
Switch B
Active link
Blocked link
Switch C
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