Switch 7700 Configuration Guide

Implementing STP on the Switch 7700 163
receive the data forwarded from Switch A until spanning tree calculation is
triggered again by changes, for example, the link from Switch B to C is down.
Thus the spanning tree is stabilized. The tree with the root Switch A is
illustrated in
Figure 3.
Figure 3 The Final Stabilized Spanning Tree
The root ID and the designated switch ID in actual calculation should include both
switch priority and switch MAC address. The designated port ID should include
port priority and port MAC address. In the updating process of a configuration
BPDU, other configuration BPDUs besides the first four items make modifications
according to certain rules. The basic calculation process is described below.
Configuring the BPDU
Forwarding Mechanism
Upon the initiation of the network, all the switches regard themselves as the roots.
The designated ports send the configuration BPDUs of local ports at a regular
interval of HelloTime. If it is the root port that receives the configuration BPDU, the
switch will enable a timer to time the configuration BPDU as well as increase
MessageAge carried in the configuration BPDU by certain rules. If a path goes
wrong, the root port on this path will not receive configuration BPDUs any more
and the old configuration BPDUs will be discarded due to timeout. Hence,
recalculation of the spanning tree will be initiated to generate a new path to
replace the failed one and thus restore the network connectivity.
However, the new configuration BPDU as now recalculated will not be propagated
throughout the network right away, so the old root ports and designated ports
that have not detected the topology change will still forward the data through the
old path. If the new root port and designated port begin to forward data
immediately after they are elected, a occasional loop may still occur. In RSTP, a
transitional state mechanism is thus adopted to ensure the new configuration
BPDU has been propagated throughout the network before the root port and
designated port begin to send data again. That is, the root port and designated
port should undergo a transitional state for a period of Forward Delay before they
enter the forwarding state.
Implementing STP on
the Switch 7700
The Switch 7700 implements the Rapid Spanning Tree Protocol (RSTP), an
enhancement to STP. The Forward Delay for the root ports and designated ports to
enter forwarding state is greatly reduced in certain conditions, thereby shortening
the time period for stabilizing the network topology.
Switch A
with priority 0
Switch B
with priority 1
Switch C
with priority 2
4
5
E1/0/7
E1/0/1
E1/0/4
E1/0/1