Information Technology Reference
In-Depth Information
Break the Loop Disabling Ports
The easy way out of a bridging loop is to disable manually every single port that is providing redundancy
in the network. If you have been able to identify a part of the network that is more affected, start
disabling ports in this area. Even better, if possible, start by disabling ports that should be blocking. Each
time you disable a port, check if connectivity is restored in the network as if you are hit by a bridging
loop—its effect should stop immediately after you break it. Knowing which disabled port stopped the
loop, you can be sure that the failure was located on a redundant path where this port was located. If this
port should have been blocking, you have probably found the link on which the failure appeared.
Log STP Events on Devices Hosting Blocked Ports
If you couldn't precisely identify the source of the problem—or, for instance, if the problem is only
transient—enable the logging of the STP event on the bridges and switches of the network experiencing
the failure. If you want to limit the number of devices to configure, enable this logging at least on devices
hosting blocked ports because this is always the transition of a blocked port that creates a loop.
•
IOS
—Enter the exec command
debug spantree events
to enable STP debugging information being
generated. Use the
general config mode
command logging buffered to capture this debug
information in the device's buffers.
•
CatOS
—The command
set logging level spantree 7 default
increases the default level of
STP-related event to debugging. Be sure that you are logging a maximum amount of messages in
the switch's buffers using the
set logging buffer 500
command.
You can also try to send this output to a syslog device. Unfortunately, when a bridging loop occurs, you
seldom can keep connectivity to a syslog server.
Check Ports
As mentioned before, the critical ports to be investigated first are the blocking ports. The next section
gives a list of what you can look for on the different ports, with a quick description of the commands to
enter for both IOS-based machines and CatOS-based switches.
Check That Blocked Ports Receive BPDUs
Especially on blocked ports and root ports, check that you keep receiving BPDUs periodically. Several
issues can lead to a port not receiving packets/BPDUs:
•
If you are running an IOS release 12.0 or greater, the command
show spanning-tree bridge-group
#.
has a field named BPDU that will show you the number of BPDUs that you received for each
interface. Issuing the command once or twice more will quickly tell you if the device is receiving
BPDUs.
•
If you don't have the field BPDU on the output of the
show spanning-tree
command, then the
easiest way to checking whether you are receiving BPDUs is to simply enable STP debug with the
debug spantree tree
command.
For CatOS, the
show mac <module/port>
command will tell you the number of multicast packets
that a specific port receives. But the simplest is to use
show spantree statistic <modele#/port#>
<vlan#>
. This command displays the exact number of configuration BPDUs received for the
specified port on the specified VLAN (a port can belong to several VLANs, if trunking). See the
section “An Additional CatOS Command,” later in this chapter, for more information.
