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ONFIGURING
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The Spanning Tree Protocols supported by the switch include the
following standards:
•STP – Spanning Tree Protocol (IEEE 802.1D).
•RSTP – Rapid Spanning Tree Protocol (IEEE 802.1w).
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 is designed as a general replacement for the slower, legacy
STP. RSTP achieves much faster reconfiguration (around one tenth
of that 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.
b_mgmt.book Page 52 Tuesday, July 8, 2003 5:24 PM