9-7
Cisco Systems IntelligentGigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
24R9746
Chapter9 Configuring STP
Understanding Spanning-Tree Features

Disabled State

A Layer 2 interface in the disabled state does not particip ate in frame forwarding or in the spanning tree.
An interface in the disabled state is nonoperational.
A disabled interface performs as follows:
•Discards frames received on the port
•Discards frames switched from another interface for forwar ding
•Does not learn addresses
•Does not receive BPDUs
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 9-2, 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 topol ogy with the
ideal switch as the root.
Figure9-2 Spanning-Tree Topology
When the spanning-tree topology is calculated based on default p arameters, 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 100 Mbps 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 interface to a higher
priority (lower numerical value) than the root port, the Gigabit Ethernet interface becomes the new root
port.
86475
DP
DP
RP
DP
RP
DP
RP = Root Port
DP = Designated Port
DP
RP
DA
CB