Alcatel-Lucent OmniSwitch 6800 Series Network Configuration Manual page 159

Configuring Spanning Tree Parameters
•
When a bridge receives BPDU on its root port that contains more attractive information (higher prior-
ity parameters and/or lower path costs), it forwards this information on to other LANs to which it is
connected for consideration.
•
When a bridge receives BPDU on its designated port that contains information that is less attractive
(lower priority values and/or higher path costs), it forwards its own information to other LANs to
which it is connected for consideration.
STP evaluates BPDU parameter values to select the best BPDU based on the following order of prece-
dence:
1
The lowest root bridge ID (lowest priority value, then lowest MAC address).
2
The best root path cost.
3
If root path costs are equal, the bridge ID of the bridge sending the BPDU.
4
If the previous three values tie, then the port ID (lowest priority value, then lowest port number).
When a topology change occurs, such as when a link goes down or a switch is added to the network, the
affected bridge sends Topology Change Notification (TCN) BPDU to the designated bridge for its LAN.
The designated bridge will then forward the TCN to the root bridge. The root then sends out a Configura-
tion BPDU and sets a Topology Change (TC) flag within the BPDU to notify other bridges that there is a
change in the configuration information. Once this change is propagated throughout the Spanning Tree
network, the root stops sending BPDU with the TC flag set and the Spanning Tree returns to an active,
stable topology.
Note. You can restrict the propagation of TCNs on a port. To restrict TCN propagation on a port, see
"Configuring STP Port Parameters" on page
OmniSwitch 6800/6850/9000 Network Configuration Guide
7-27.
March 2008
Spanning Tree Overview
page 7-9