Specifications

12-9
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-07
Chapter 12 Configuring STP
Understanding Spanning-Tree Features
The external spanning-tree behavior on access ports and trunk ports is not affected by PVST+ or
PVRST+.
For more information on 802.1Q trunks, see Chapter 15, “Configuring VLANs.”
Spanning Tree and Redundant Connectivity
You can create a redundant backbone with spanning tree by connecting two switch interfaces to another
device or to two different devices. Spanning tree automatically disables one interface but enables it if
the other one fails, as shown in Figure 12-3. If one link is high-speed and the other is low-speed, the
low-speed link is always disabled. If the speeds are the same, the port priority and port ID are added
together, and spanning tree disables the link with the lowest value.
Figure 12-3 Spanning Tree and Redundant Connectivity
You can also create redundant links between switches by using EtherChannel groups. For more
information, see Chapter 28, “Configuring EtherChannels.”
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 2950 or 3550
switch
Workstations
Catalyst 2950 or 3550
switch
Catalyst 2950 or 3550
switch
Switch A
Switch B
Active link
Blocked link
Switch C
74620