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HAPTER
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| Configuring the Switch
Configuring the Spanning Tree Algorithm
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CONFIGURING THE SPANNING TREE ALGORITHM
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.
This switch supports Rapid Spanning Tree Protocol (RSTP), but is backward
compatible with Spanning Tree Protocol (STP).
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 (Multiple Spanning Tree Protocol).
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.
x
Designated
Root
Designated
Port
Designated
Bridge
x x
x
Root
Port
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