Spanning Tree And Redundant Connectivity; Spanning-Tree Address Management; Accelerated Aging To Retain Connectivity - Cisco Catalyst 2950 Software Configuration Manual

Understanding Spanning-Tree Features

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
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 13-3
You can also create redundant links between switches by using EtherChannel groups. For more
information, see

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.
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
13-8
Figure
Spanning Tree and Redundant Connectivity
Active link
Blocked link
Chapter 30, "Configuring EtherChannels."
13-3. If one link is high-speed and the other is low-speed, the
Workstations
Chapter 13 Configuring STP
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