Information Technology Reference
In-Depth Information
VLAN and someone accidentally deleted the VLAN to which the ports belong. When you add the VLAN
back into the VLAN table, the ports will become active again because a port remembers its assigned
VLAN.
If you have a link and the ports show that they are connected, but you cannot communicate with another
device, this can be particularly perplexing. It usually indicates a problem above the physical layer: Layer
2 or Layer 3. Try the actions suggested in the next paragraphs.
Check the trunking mode on each side of the link. Make sure that both sides are in the same mode. If
you turn the trunking mode to on (as opposed to auto or desirable) for one port, and the other port has
the trunking mode set to off, the ports will not be capable of communicating. Trunking changes the
formatting of the packet; the ports must be in agreement as to what format they are using on the link, or
they will not understand each other.
Make sure that all devices are in the same VLAN. If they are not in the same VLAN, then a router must
be configured to allow the devices to communicate.
Make sure that your Layer 3 addressing is correctly configured.
Traffic Issues
In this section, we describe some of the things you can learn by looking at a port's traffic information.
Most switches have some way to track the packets going in and out of a port. Commands that generate
this type of output on the Catalyst 4000/5000/6000 switches are
show port
and
show mac
. Output from
these commands on the 4000/5000/6000 switches is described in the switch command references.
Some of these port traffic fields show how much data is being transmitted and received on the port. Other
fields show how many error frames are being encountered on the port. If you have a large amount of
alignment errors, FCS errors, or late collisions, this may indicate a duplex mismatch on the wire. Other
causes for these types of errors may be bad network interface cards or cable problems. If you have a large
number of deferred frames, it is a sign that your segment has too much traffic; the switch is not capable
of sending enough traffic on the wire to empty its buffers. Consider removing some devices to another
segment.
Switch Hardware Failure
If you have tried everything you can think of and the port will not work, there might be faulty hardware.
Sometimes ports are damaged by electrostatic discharge (ESD). You may or may not see any indication
of this.
Look at the power-on self-test (POST) results from the switch to see whether any failures are indicated
for any part of the switch.
If you see behavior that can be considered “strange,” this could indicate hardware problems, but it could
also indicate software problems. It is usually easier to reload the software than it is to get new hardware.
Try working with the switch software first.
The operating system might have a bug. Loading a newer operating system could fix this. You can
research known bugs by reading the release notes for the version of code that you are using or by using
Cisco's Bug Navigator tool (www.cisco.com/support/bugtools).
The operating system could have somehow become corrupted. Reloading the same version of the
operating system could fix the problem.
