IBM 12.1(22)EA6 User Manual
9-7
Cisco Systems Intelligent Gigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
24R9746
Chapter 9 Configuring STP
Understanding Spanning-Tree Features
Disabled State
A Layer 2 interface in the disabled state does not participate in frame forwarding or in the spanning tree.
An interface in the disabled state is nonoperational.
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 forwarding
•
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
MAC address becomes the root switch. In
, 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.
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 9-2
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.
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.
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 = Designated Port
DP
RP
D
A
C
B