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Spanning Tree Algorithm Configuration
113
NXA-ENET24 - Software Management Guide
Spanning Tree Algorithm Configuration
Overview
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.
Then 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. All ports connected to designated bridging devices are
assigned as designated ports. 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.
FIG. 118 Spanning Tree Algorithm Configuration
x
Designated
Root
Designated
Port
Designated
Bridge
x x
x
Root
Port
x