System information

Spanning Tree Algorithm Configuration
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Spanning Tree Algorithm Configuration
The Spanning Tree Algorithm (STA) can be used to detect and disable network
loops, and to provide backup links between switches, bridges or routers. This allows
the switch to interact with other bridging devices (that is, an STA-compliant switch,
bridge or router) in your network to ensure that only one route exists between any
two stations on the network, and provide backup links which automatically take over
when a primary link goes down.
The spanning tree algorithms supported by this switch include these versions:
• STP – Spanning Tree Protocol (IEEE 802.1D)
• RSTP – Rapid Spanning Tree Protocol (IEEE 802.1w) STA uses a distributed
algorithm to select a bridging device (STA-compliant switch, bridge or router)
that serves as the root of the spanning tree network. It selects a root port on
each bridging device (except for the root device) which incurs the lowest path
cost when forwarding a packet from that device to the root device. It selects a
designated bridging device from each LAN which incurs the lowest path cost
when forwarding a packet from that LAN to the root device. It then selects a port
on the designated bridging device to communicate with each attached LAN or
host device as a designated port. After determining the lowest cost spanning
tree, it enables all root ports and designated ports, and disables all other ports.
Network packets are therefore only forwarded between root ports and
designated ports, eliminating any possible network loops.
Once 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 (Maximum Age),
the bridge assumes that the link to the Root Bridge is down. This bridge will then
initiate negotiations with other bridges to reconfigure the network to reestablish a
valid network topology.
RSTP is designed as a general replacement for the slower, legacy STP. RSTP
achieves must faster reconfiguration (i.e., around one tenth of the time required by
STP) by reducing the number of state changes before active ports start learning,
predefining an alternate route that can be used when a node or port fails, and
retaining the forwarding database for ports insensitive to changes in the tree
structure when reconfiguration occurs.
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Designated
Root
Designated
Port
Designated
Bridge
x x
x
Root
Port
x