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(Rapid) Spanning Tree
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5.2.3 Mixed operation of RSTP and STP
If a device with STP support is integrated into the network, only switch ports that receive
STP BPDUs are set to STP mode. All other ports that receive RSTP BPDUs remain in RSTP
mode.
5.2.4 Topology detection of a Rapid Spanning Tree network
(RSTP)
(Rapid) Spanning Tree switches continually exchange information about the network topol-
ogy using special messages (BPDUs - Bridge Protocol Data Units). In this way the switches
“learn” the current network topology and - based on this information - make the following de-
cisions:
–
Which switch is selected as the root switch
– Which data paths are disabled
If a switch is started using the (Rapid) Spanning Tree Protocol, it first expects to be the root
switch. However, no data communication is possible during the startup phase until the cur-
rent network topology has been learned and until the decisions described above have been
made. Therefore loops in the network startup phase which could occur because no data
path is interrupted, are prevented.
5.2.4.1 Topology modification
A topology modification can be triggered by the following:
–
Adding a data path
– Failure of a data path
– Adding a spanning tree switch, or
– Failure of a Spanning Tree switch
A topology modification is automatically detected and the network is reconfigured so that
another tree is created and all the devices in this tree can be accessed. During this process,
loops do not even occur temporarily.
If sending of traps was not deactivated, two traps are generated:
–
newRoot (OID: 1.3.6.1.2.1.17.0.1)
– topologyChange (OID 1.3.6.1.2.1.17.0.2)
5.2.4.2 Interrupted data paths and port states
The described data path interruption by the Spanning Tree Protocol is created by discon-
necting individual ports that no longer forward any data packets. A port can have the follow-
ing states:
–
Learning
– Forwarding
– Blocking/Discarding
– Disabled (link down or disconnected by the user)
The current port states are shown in the web interface.