IBM BladeCenter Management Module, Installation And User Manual

Page: 140 / 184

Bridge protocol data units
For STP to arrive at a stable network topology, the following information is used:
v The unique switch identifier
v
The path cost to the root associated with each switch port
v The port identifier
STP communicates between switches on the network using bridge protocol data
units (BPDUs). Each BPDU contains the following information:
v
The unique identifier of the switch that the transmitting switch currently
recognizes as the root switch
v The path cost to the root from the transmitting port
v The port identifier of the transmitting port
The switch sends BPDUs to communicate and construct the spanning-tree topology.
All switches connected to the LAN on which a packet is transmitted will receive the
BPDU. BPDUs are not directly forwarded by the switch, but the receiving switch
uses the information in the frame to calculate a BPDU, and, if the topology
changes, initiates a BPDU transmission.
The communication between switches through BPDUs causes the following results:
v
One switch is elected as the root switch.
v The shortest distance to the root switch is calculated for each switch.
v A designated switch is selected. This is the switch closest to the root switch
through which packets will be forwarded to the root.
v A port for each switch is selected. This is the port providing the best path from
the switch to the root switch.
v Ports included in the STP are selected.
Creating a stable topology
If all switches have STP enabled with default settings, the switch with the lowest
MAC address in the network will become the root switch. By increasing the priority
(lowering the priority number) of the best switch, STP can be forced to select the
best switch as the root switch.
When STP is enabled using the default parameters, the path between source and
destination stations in a switched network might not be ideal. For example,
connecting higher-speed links to a port that has a higher number than the current
root port can cause a root-port change. The goal is to make the fastest link the root
port.
STP port states
The BPDUs take some time to pass through a network. This propagation delay can
result in topology changes in which a port that changed directly from a blocking
state to a forwarding state could create temporary data loops. Ports must wait for
new network topology information to propagate throughout the network before
starting to forward packets. They must also wait for the packet lifetime to expire for
BPDU packets that were forwarded based on the old topology. The forward delay
timer is used to enable the network topology to stabilize after a topology change. In
addition, STP specifies a series of states a port must go through to further ensure
that a stable network topology is created after a topology change.
122
IBM BladeCenter 4-Port Gb Ethernet Switch Module: Installation and User’s Guide