Per VLAN Spanning Tree +
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OmniAccess 5740 Unified Services Gateway CLI Configuration Guide
Alcatel-Lucent
PER VLAN SPANNING TREE (PVST+) OVERVIEW
Spanning-Tree Protocol (STP) is a link management protocol that provides path
redundancy while preventing undesirable loops in the network. For an Ethernet
network to function properly, only one active path can exist between two stations.
Multiple active paths between stations cause loops in the network. If a loop exists
in the network topology, the potential exists for duplication of messages. When
loops occur, some switches see stations appear on both sides of the switch. This
condition confuses the forwarding algorithm, and allows duplicate frames to be
forwarded.
To provide path redundancy, Spanning-Tree Protocol defines a tree that spans all
switches in an extended network. Spanning-Tree Protocol forces certain
redundant data paths into a standby (blocked) state. If one network segment in
the Spanning-Tree Protocol becomes unreachable, or if Spanning-Tree Protocol
costs change, the spanning-tree algorithm reconfigures the spanning-tree
topology and reestablishes the link by activating the standby path. Spanning-Tree
Protocol operation is transparent to end stations, which are unaware whether they
are connected to a single LAN segment or a switched LAN of multiple segments.
Per-VLAN Spanning Tree + (PVST+) maintains a spanning tree instance for each
VLAN configured in the network. Since PVST + treats each VLAN as a separate
network, it has the ability to load balance traffic (at layer-2) by forwarding some
VLANs on one trunk and other VLANs on another trunk without causing a
Spanning Tree loop.
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