Spanning-Tree Address Management - Cisco Catalyst 2960 series Configuration Manual

Information About Spanning Tree Protocol
are the same, the port priority and port ID are added together, and spanning tree disables the link with the
highest value.
Figure 11: Spanning Tree and Redundant Connectivity
You can also create redundant links between switches by using EtherChannel groups.

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, each switch in the stack receives but does not forward packets destined
for addresses between 0x0180C2000000 and 0x0180C200000F.
If spanning tree is enabled, the CPU on the switch or on each switch in the stack receives packets destined
for 0x0180C2000000 and 0x0180C2000010. If spanning tree is disabled, the switch or each switch in the
stack 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.
Related Topics
Configuring the Root Switch , on page 227
Restrictions for STP, on page 211
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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