Configuring the Switch

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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)
• MSTP – Multiple Spanning Tree Protocol (IEEE 802.1s)
STP – STP uses a distributed algorithm to select a bridging device (STP-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 – RSTP is designed as a general replacement for the slower, legacy STP.
RSTP is also incorporated into MSTP. RSTP achieves must faster reconfiguration
(i.e., around 1 to 3 seconds, compared to 30 seconds or more for 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.