Information Technology Reference
In-Depth Information
Check for Duplex Mismatch
To look for a duplex mismatch, you obviously have to check each side of the point-to-point link.
IOS
—Simply use the
show interface
command to check the speed and duplex status of the specified
port.
•
CatOS
—The very first lines of the output of
show port <module#/port#>
will give you the speed
and duplex for which the port is configured.
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Check Port Utilization
We have seen that an interface overloaded can fail to transmit vital BPDUs. A very loaded link is also
an indication of a possible bridging loop.
•
IOS
—Use the command
show interface
to determine an interface utilization. Several fields will
help you here (Load, Packets Input/Output, and so on).
•
CatOS
—The command used to display statistics about packets received and sent on a port is
show
mac <modele#/port#>
. The command
show top
automatically evaluates the port utilization over a
30-second period of time and displays the result classified by percentage bandwidth utilization
(other options are available). Also, the
show system
command gives an indication on the backplane
utilization, even if it does not point to a specific port.
Check Packet Corruption
IOS
—Look for increasing figures in the input errors fields of the
show interface
command.
CatOS
—The command
show port <modele#/port#>
gives you some details with the Aling-Err,
FCS-Err, Xmit-Err, Rcv-Err, and Undersize fields. You will get even more detailed statistics using the
show counters <modele#/port#>
command.
An Additional CatOS Command
The Catalyst-specific software is richer than the IOS as far as STP troubleshooting is concerned. The
command
show spantree statistics <modele#/port#> <vlan#>
gives very accurate information on a
specific port. On suspected ports, run this command and pay special attention to the fields:
•
Forward trans count
—This counter remembers how many times a port transitions from learning
to forwarding. In a stable topology, this counter should always show 1. This counter is reset to 0 if
the corresponding port is going down and up. So, if the value is higher than 1, it means that the
transition that this port experienced is the result of a STP recalculation, not of a direct link failure.
•
Max age expiry count
—This counter tracks the number of times that the max age expired on this
link. Basically, a port expecting BPDUs will wait for the max age (default 20 seconds) before
considering its designated bridge as lost. Each time this event occurs, the counter is incremented.
When the value is not zero, you know that, for whatever reason, the designated bridge for this LAN
is unstable or has problems transmitting its BPDUs.
Look for Resource Errors
We have seen that a high CPU utilization can be dangerous for a system running the STA. Here is how
to check that the device is not running short of CPU resource:
IOS
—Use the
show processes cpu
command. Check that the CPU utilization is not getting too close
to 100 percent.
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