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Understanding Spanning-Tree Features
When the spanning-tree topology is calculated based on default parameters, the path between source and
destination end stations in a switched network might not be ideal. For instance, connecting higher-speed
links to an interface that has a higher number than the root port can cause a root-port change. The goal
is to make the fastest link the root port.
For example, assume that one port on Switch B is a Gigabit Ethernet link and that another port on
Switch B (a 10/100 link) is the root port. Network traffic might be more efficient over the Gigabit
Ethernet link. By changing the spanning-tree port priority on the Gigabit Ethernet port to a higher
priority (lower numerical value) than the root port, the Gigabit Ethernet port becomes the new root port.
Spanning Tree and Redundant Connectivity
You can create a redundant backbone with spanning tree by connecting two switch interfaces that are
participating in spanning tree to another device or to two different devices, as shown in
Spanning tree automatically disables one interface but enables it if the other one fails. 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 14-3
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, each switch receives but does not forward packets destined for
addresses between 0x0180C2000000 and 0x0180C200000F.
If spanning tree is enabled, the CPU on the switch receives packets destined for 0x0180C2000000 and
0x0180C2000010. If spanning tree is disabled, the switch forwards those packets as unknown multicast
addresses.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
14-8
Spanning Tree and Redundant Connectivity
Active link
Blocked link
Chapter 31, "Configuring EtherChannels."
Workstations
Chapter 14
Configuring STP
Figure
14-3.
78-17058-01
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