Справочник Пользователя для Cabletron Systems EMM-E6 Ethernet
BRIDGES
EMM-E6 User’s Guide
1-19
1.6.2
Spanning Tree Algorithm
The Spanning Tree Algorithm (STA) is used by bridges to detect data
loops (duplicate data paths). The bridges will then automatically break the
loop and use the now open path as a backup in case the primary path fails.
loops (duplicate data paths). The bridges will then automatically break the
loop and use the now open path as a backup in case the primary path fails.
When a bridge is powered up, it goes through a series of self tests to
check its internal operation. During this time the bridge is in a standby, or
blocking, condition and does not forward traffic. Also during this standby
period, the bridge sends out special bridge management packets called
configuration Bridge Protocol Data Units (BPDU). Bridges use the
BPDUs as a way of communicating with each other. The purpose of the
configuration BDPU is to notify other bridges on all of the connected
networks of the current topology.
check its internal operation. During this time the bridge is in a standby, or
blocking, condition and does not forward traffic. Also during this standby
period, the bridge sends out special bridge management packets called
configuration Bridge Protocol Data Units (BPDU). Bridges use the
BPDUs as a way of communicating with each other. The purpose of the
configuration BDPU is to notify other bridges on all of the connected
networks of the current topology.
After the bridge has informed the network of its presence, the bridge
enters a second standby state, called listening. During listening, the
bridge monitors the network for the BPDUs of other bridges. Having
received packets from the networks, the bridge enters the learning state,
continuing to block traffic as it examines the information it receives.
enters a second standby state, called listening. During listening, the
bridge monitors the network for the BPDUs of other bridges. Having
received packets from the networks, the bridge enters the learning state,
continuing to block traffic as it examines the information it receives.
Based on bridge priorities and MAC addresses, the interconnected bridges
will set bridge ports to either forwarding or standby conditions, allowing
a single access path to all parts of the network. The bridge or bridges
involved in this primary data path will then remain in the forwarding
state, and the bridges with lower priority involved in the backup path(s)
will remain in a standby condition. Any redundant paths (those placed in
standby) will be automatically used as need is detected by the operation
of the Spanning Tree Algorithm.
will set bridge ports to either forwarding or standby conditions, allowing
a single access path to all parts of the network. The bridge or bridges
involved in this primary data path will then remain in the forwarding
state, and the bridges with lower priority involved in the backup path(s)
will remain in a standby condition. Any redundant paths (those placed in
standby) will be automatically used as need is detected by the operation
of the Spanning Tree Algorithm.
The other type of BPDU is the topology change BPDU. This BPDU is
made up of four bytes and notifies the other bridges that a change has
taken place. Upon receipt of the topology change BPDU, the bridges
re-arbitrate, or re-span, to form a legal topology.
made up of four bytes and notifies the other bridges that a change has
taken place. Upon receipt of the topology change BPDU, the bridges
re-arbitrate, or re-span, to form a legal topology.
CH1Book Page 19 Wednesday, March 20, 1996 7:48 AM