Topology Examples - Alcatel OmniSwitch 6624 Network Configuration Manual

Spanning Tree Overview
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.

Topology Examples

The following diagram shows an example of a physical network topology that incorporates data path
redundancy to ensure fault tolerance. These redundant paths, however, create loops in the network config-
uration. If a device connected to Switch A sends broadcast packets, Switch A will flood the packets out all
of its active ports. The switches connected to Switch A will in turn flood the broadcast packets out their
active ports, and Switch A will eventually receive the same packets back and the cycle will start over
again. This causes severe congestion on the network, often referred to as a broadcast storm.
The Spanning Tree Algorithm prevents network loops by ensuring that there is always only one active link
between any two switches. This is done by transitioning one of the redundant links into a blocking state,
leaving only one link actively forwarding traffic. If the active link goes down, then Spanning Tree will
transition one of the blocked links to the forwarding state to take over for the downed link. If a new switch
is added to the network, the Spanning Tree topology is automatically recalculated to include the monitor-
ing of links to the new switch.
page 5-6
Switch D
OmniSwitch 7700
TM
OmniSwitch 7700
TM
Switch A
Physical Topology Example
OmniSwitch 6624/6648 Network Configuration Guide
Configuring Spanning Tree Parameters
Switch C
OmniSwitch 6648
OmniSwitch 6648
OmniSwitch 6648
OmniSwitch 6648
Switch B
April 2004