25. Spanning Tree
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RuggedBackbone™ RX5000
The Spanning-Tree Statistics form clears all spanning tree statistics for ethernet ports. This form is
accessible from the clear-stp-stats menu action. The path to this menu action is switch/spanning-tree/
clear-stp-stats. To clear statistics, click the Perform button on the Clear Spanning-Tree Statistics form.
Figure 25.26. Clear Spanning-Tree Statistics form
25.7. Troubleshooting
Problem One
When I connect a new port the network locks up. The port status LEDs are flashing madly.
Occasionally, the network seems to experience a lot of flooding. All the ports seem to experience
significant traffic. The problem lasts a few seconds and then goes away.
One of my switches displays a strange behavior where the root port hops back and forth between two
switch ports and never settles down.
Is it possible that one of the switches in the network or one of the ports on a switch in the network has
STP disabled and accidentally connects to another switch? If this has occurred, then a traffic loop has
been formed.
If the problem appears to be transient in nature, it is possible that ports that are part of the spanning
tree have been configured as edge ports. After the link layers have come up on edge ports, STP will
directly transition them (perhaps improperly) to the forwarding state. If an RSTP configuration message
is then received, the port will be returned to blocking. A traffic loop may be formed for the length of time
the port was in forwarding.
If one of the switches appears to flip the root from one port to another, the problem may be one of traffic
prioritization (See problem five).
Another possible cause of intermittent operation is that of an auto-negotiation mismatch. If one end
of the link is fixed to full-duplex mode and the peer auto-negotiates, the auto-negotiating end will fall
back to half-duplex operation. At lower traffic, the volumes the link may display few if any errors. As
the traffic volume rises, the fixed negotiation side will begin to experience dropped packets while the
auto-negotiating side will experience collisions. Ultimately, as traffic loads approach 100%, the link will
become entirely unusable. At this point, RSTP will not be able to transmit configuration messages over
the link and the spanning tree topology will break down. If an alternate trunk exists, RSTP will activate
it in the place of the congested port. Since activation of the alternate port often relieves the congested
port of its traffic, the congested port will once again become reliable. RSTP will promptly enter it back
into service, beginning the cycle once again. The root port will flip back and forth between two ports
on the switch.
Problem Two
My PC/IED/Device is connected to your switch. After I reset the switch, it takes a long time before it
comes up.
Is it possible that the RSTP edge setting for this port is set to false? If Edge is set to false, the bridge will
make the port go through two forward delay times before the port can send or receive frames. If Edge
is set to true, the bridge will transition the port directly to forwarding upon link up.
