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Cisco IE 3000 Switch Hardware Installation Guide
Chapter 3 Troubleshooting
Diagnosing Problems
Spanning Tree Loops
Spanning Tree Protocol (STP) loops can cause serious performance issues that look like port or interface
problems. In this situation, the switch bandwidth is used over and over again by the same frames, leaving
little room for legitimate traffic.
Loops can be caused by a unidirectional link. A unidirectional link occurs whenever the traffic sent by
the switch is received by its neighbor, but the traffic from the neighbor is not received by the switch. A
broken fiber-optic cable, other cabling, or a port issue could cause this one-way communication.
You can enable UniDirectional Link Detection (UDLD) on the switch to help identify difficult-to-find
unidirectional link problems. UDLD supports two modes of operation: normal (the default) and
aggressive. In normal mode, UDLD detects unidirectional links due to misconnected interfaces on
fiber-optic connections. In aggressive mode, UDLD also detects unidirectional links due to one-way
traffic on fiber-optic and twisted-pair links and due to misconnected interfaces on fiber-optic links. For
information about enabling UDLD on the switch, see the “Understanding UDLD” section in the
“Configuring UDLD” chapter of the software configuration guide for this release.
Verify Switch Performance
Review this section when troubleshooting switch performance problems.
Speed, Duplex, and Autonegotiation
If the port statistics show a large amount of alignment errors, frame check sequence (FCS), or
late-collisions errors, this might indicate a speed or duplex mismatch.
A common issue with speed and duplex is when the duplex settings are mismatched between two
switches, between a switch and a router, or between the switch and a workstation or server. This can
happen when manually setting the speed and duplex or from autonegotiation issues between the two
devices. A mismatch occurs under these circumstances:
•
A manually set speed or duplex parameter is different from the manually set speed or duplex
parameter on the connected port.
•
A port is set to autonegotiate, and the connected port is set to full duplex with no autonegotiation.
To maximize switch performance and ensure a link, follow one of these guidelines when changing the
settings for duplex and speed:
•
Let both ports autonegotiate both speed and duplex.
•
Manually set the speed and duplex parameters for the ports on both ends of the connection.
•
If a remote device does not autonegotiate, configure the duplex settings on the two ports to match.
The speed parameter can adjust itself even if the connected port does not autonegotiate.
Autonegotiation and NIC
Problems sometimes occur between the switch and third-party network interface cards (NICs). By
default, the switch ports and interfaces are set to autonegotiate. It is common for devices like laptops or
other devices to be set to autonegotiate as well, yet sometimes autonegotation issues occur.
To troubleshoot autonegotiation problems, try manually setting both sides of the connection. If this does
not solve the problem, there could be a problem with the firmware or software on your NIC. You can
resolve this by upgrading the NIC driver to the latest version available from the manufacture.
