852-1328
Configuration in the WBM
74
Product manual | Version: 1.2.0
Industrial Managed Switch
If the topology changes in a LAN coupled via bridge, a new tree is spanned. Once a sta-
ble network topology has been established, all bridges listen for Hello BPDUs transmitted
from the Root Bridge. If a bridge does not get a Hello BPDU after a predefined interval
(Max Age), the bridge assumes that the link to the Root Bridge is down. This bridge then
initiates negotiations with other bridges to reconfigure the network to re-establish a valid
network topology.
RSTP Switch Port States:
•
Discarding
If a port causes a Switching Loop (looping connection between two ports), user data
can no longer be sent or received. However, the port can go into the Forwarding state
if the other active connections fail and the Spanning Tree algorithm determines that the
port may transition to that state. BPDU data
(B
ridge
P
rotocol
D
ata
U
nit, configuration
message) continues to be received and sent in the "Discarding" state.
•
Learning
Even before the port has forwarded any frames (packets), it can learn source ad-
dresses from frames received and add them to the filter database (Switching Data-
base).
•
Forwarding
The port is in normal operating mode; it receives and sends data. RSTP still monitors
incoming BPDUs, which would indicate that the port should return to the Blocking state
to prevent a loop.
RSTP Bridge Port Roles
•
Root
The Root Port is a forwarding port that can best transmit data from the Non-Root
Bridge to the Root Bridge.
•
Designated
This is a forwarding port for every LAN segment.
•
Alternate
This port represents an alternate path to the Root Bridge. However, this path differs
from the Root Port.
•
Backup
This port is used as a backup/redundant path to a segment to which another Bridge
Port is already connected.
•
Disabled
This is not actually part of RSTP because a network administrator can manually dis-
able a port.
Other important terms:
Table 41: Other important terms
Term
Description
Forward Time
The Forward Time or Forward Delay is the maximum time (in seconds) that the switch
waits before it changes states. This delay is required because every switch must first re-
ceive information on topology changes before it forwards frames. In addition, each port
needs time to receive information on conflicts that would make it return to the blocking
state. Otherwise, temporary data loops might result. The valid range is 4 to 30 seconds.
Max Age
The Max Age is the maximum time (in seconds) that the switch can wait without receiv-
ing a BPDU (Bridge Protocol Data Unit, configuration message) before attempting to re-
configure. All switch ports (except for Designated Ports) receive BPDUs at regular inter-
