Справочник Пользователя для SmartBridges sB3021

 

 

 

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2

 Nexus PRO™ User Guide

 

 

STP is a Layer 2 link management protocol that provides path redundancy while preventing loops in 
the network. For a Layer 2 Ethernet network to function properly, only one active path can exist 
between any two redundant links.  
 
To create a fault-tolerant network, there needs to be a loop-free path between all nodes in the 
network. The Spanning Tree Algorithm calculates the best loop-free path throughout a Layer 2 
network. Infrastructure devices such as wireless bridges and switches send and receive spanning tree 
frames, called Bridge Protocol Data Units (BPDUs), at regular intervals. The devices do not forward 
these frames but use them to construct the loop-free path.

 

 
Multiple active paths among end stations cause loops in the network. If a loop exists in the network, 
end stations might receive duplicate messages. Infrastructure devices might also learn end-station 
MAC addresses on multiple Layer 2 interfaces. Such conditions result in an unstable network. 
 
STP defines a tree with a root bridge and a loop-free path from the root to all infrastructure devices in 
the Layer 2 network. 
 
STP forces redundant data paths into a standby (blocked) state. If a network segment in the spanning 
tree fails and a redundant path exists, the Spanning Tree Algorithm recalculates the spanning tree 
topology and activates the standby path. 
 
When two interfaces on a device are part of a loop, the spanning tree port priority and path cost 
settings determine which interface is put in the forwarding state and which is put in the blocking state. 
The port priority value represents the location of an interface in the network topology and how well it is 
located to pass traffic. The path cost value represents the media speed. 
 

 

STP is disabled by default. The table below lists the default STP settings when STP is enabled.  

32768 

 0-65535 

A parameter used to identify the 
root bridge in a spanning tree 
(instance of STP). The bridge 
with the lowest value has the 
highest priority and is the root. 
 

20 

 6-40 

The interval a bridge will wait for 
a hello packet from the root 
bridge before initiating a topology 
change. 
 

2 

1-10 

The interval of time between 
each configuration BPDU sent by 
the root bridge.