17–QLogic Teaming Services
General Network Considerations
306
83840-546-00 D
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 that has a lost link on one of
its physical adapters. If the physical adapter were to be reconnected (also known
as fallback), the intermediate driver would detect that the link has been
reestablished and would begin to pass traffic through the port. Traffic would be
lost if the port was temporarily blocked by the Spanning Tree Protocol.
