Spanning tree algorithm – Dell Broadcom NetXtreme Family of Adapters User Manual
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Broadcom Teaming Services: Broadcom NetXtreme II® Network Adapter User Guide
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represents a failover event in which the cable is unplugged on the Top Switch port 4. This is a successful failover with
all stations pinging each other without loss of connectivity.
Figure 5: Failover Event
Spanning Tree Algorithm
In Ethernet networks, only one active path may exist between any two bridges or switches. Multiple active paths between
switches can cause loops in the network. When loops occur, some switches recognize stations on both sides of the switch.
This situation causes the forwarding algorithm to malfunction allowing duplicate frames to be forwarded. Spanning tree
algorithms provide path redundancy by defining a tree that spans all of the switches in an extended network and then forces
certain redundant data paths into a standby (blocked) state. At regular intervals, the switches in the network send and receive
spanning tree packets that they use to identify the path. If one network segment becomes unreachable, or if spanning tree
costs change, the spanning tree algorithm reconfigures the spanning tree topology and re-establishes the link by activating
the standby path. Spanning tree operation is transparent to end stations, which do not detect whether they are connected to
a single LAN segment or a switched LAN of multiple segments.
Spanning Tree Protocol (STP) is a Layer 2 protocol designed to run on bridges and switches. The specification for STP is
defined in IEEE 802.1d. The main purpose of STP is to ensure that you do not run into a loop situation when you have
redundant paths in your network. STP detects/disables network loops and provides backup links between switches or bridges.
It allows the device to interact with other STP compliant devices in your network to ensure that only one path exists between
any two stations on the network.
After a stable network topology has been established, all bridges listen for hello BPDUs (Bridge Protocol Data Units)
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. The process to create a new topology can take up to 50 seconds. During
this time, end-to-end communications are interrupted.
The use of Spanning Tree is not recommended for ports that are connected to end stations, because by definition, an end
station does not create a loop within an Ethernet segment. Additionally, when a teamed adapter is connected to a port with
Spanning Tree enabled, users may experience unexpected connectivity problems. For example, consider a teamed adapter