Information Technology Reference
In-Depth Information
Troubleshooting a Failure
Unfortunately, there is no systematic procedure to troubleshoot an STP issue. This section instead looks
like a checklist, recapitulating some of the actions available to you. Most of the indications given here
apply to bridging loop troubleshooting. Other failures of the STP leading to a loss of connectivity can
be identified using a more conventional way, by exploring the path taken by traffic experiencing a
problem.
Note that most of these troubleshooting steps assume connectivity to the different devices of the bridge
network. This means having console access. During a bridging loop, for example, you will probably not
be able to Telnet.
Use the Diagram of the Network
You need to know some basic things about your network before troubleshooting a bridging loop.
You need to know at least the following:
•
The topology of the bridged network
•
Where the root bridge is located
•
Where the blocked ports (and the redundant links) are located
This knowledge is essential at least for two reasons:
How could you know what to fix in the network if you don't know how it should look when it is
working?
•
Most of the troubleshooting steps are simply using
show
commands to try to identify error
conditions. Knowledge of the network helps you focus on the critical ports on the key devices.
•
Identify a Bridging Loop
It used to be that a broadcast storm could have the same effect on the network. Nowadays, with
high-speed links and devices providing switching at hardware level, it is nearly impossible that, for
instance, a single server brings down a network by broadcasting. The real way of identifying a bridging
loop for sure is to capture the traffic on a saturated link and to check that you see similar packets multiple
times.
But practically, if all users in a certain bridging domain have connectivity issues at the same time, you
can already suspect a bridging loop.
Check the port utilization on your devices and look for abnormal values. See the “Check Port
Utilization” upcoming section for additional information.
On the Catalyst switches running a CatOS, you can easily check the overall backplane usage using the
show system
command. This command is very useful because it not only gives you the current usage of
the switch backplane, but it also specifies the peak usage (and its date). An unusual peak utilization
shows you whether there has ever been a bridging loop on this device.
Restore Connectivity Quickly and Be Ready for Another Time
Bridging loops have extremely severe consequences on a bridged network. Administrators generally
don't have time to look for the reason of the loop and prefer to restore connectivity as soon as possible.
If you do this, you will not find the real cause of the issue and need to be ready for the next time that it
occurs.
