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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 13 Configuring STP
Understanding Spanning-Tree Features
How a Switch or Port Becomes the Root Switch or Root Port
If all switches in a network are enabled with default spanning-tree settings, the switch with the lowest
MAC address becomes the root switch. In Figure 13-2, Switch A is elected as the root switch because
the switch priority of all the switches is set to the default (32768) and Switch A has the lowest MAC
address. However, because of traffic patterns, number of forwarding interfaces, or link types, Switch A
might not be the ideal root switch. By increasing the priority (lowering the numerical value) of the ideal
switch so that it becomes the root switch, you force a spanning-tree recalculation to form a new topology
with the ideal switch as the root.
Figure 13-2 Spanning-Tree Topology
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 Figure 13-3.
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.
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