Troubleshooting Guide

and NIC configuration are manually configured for 100 Mbps, full-duplex. This is because NIC
autonegotiation link detection still operates when the NIC is manually configured. This causes duplex
inconsistency between the switchport and the NIC. Symptoms include poor port performance and frame
check sequence (FCS) errors that increment on the switchport. In order to troubleshoot this issue, try to
manually configure the switchport to 100 Mbps, half-duplex. If this action resolves the connectivity
problems, this NIC issue is the possible cause. Try to update to the latest drivers for your NIC, or contact
your NIC card vendor for additional support.
Why Is It That the Speed and Duplex Cannot Be Hardcoded on Only One Link Partner?
A
s indicated in Table 1, a manual setup of the speed and duplex for full-duplex on one link partner results
in a duplex mismatch. This happens when you disable autonegotiation on one link partner while the other
link partner defaults to a half-duplex configuration. A duplex mismatch results in slow performance,
intermittent connectivity, data link errors, and other issues. If the intent is not to use autonegotiation, both
link partners must be manually configured for speed and duplex for full-duplex settings.
Recommended Port Configuration (Autonegotiation or Manual Configuration)
There are many opinions on the subject of autonegotiation. Previously, many engineers advised
customers not to use autonegotiation with any switch-connected device. However, improvements in the
interoperation of autonegotiation and the maturity of the technology has recently changed the view of
autonegotiation and its use. In addition, performance issues due to duplex mismatches, caused by the
manual setting of speed and duplex on only one link partner, are more common. Because of these recent
issues, the use of autonegotiation is regarded as a valid practice.
EtherChannel and Trunking Between Catalyst Switches and NICs
EtherChannel can be configured dynamically with Port Aggregation Protocol (PAgP), and trunking can
also be configured dynamically with Dynamic Trunking Protocol (DTP). Both PAgP and DTP are Cisco
proprietary protocols and supported only on Catalyst switches. If you want to configure EtherChannel or
trunking between Catalyst switches and NICs, it is recommended that you configure these features
statically, as other vendor NICs can potentially not support PAgP and DTP. On Catalyst switches,
configure the EtherChannel mode to on and trunking mode to nonegotiate, which disables the PAgP
and DTP protocols. If you configure the switch port with auto or desirable mode, it is possible you can
not be able to form the EtherChannel or trunk with NICs.
Verifying Physical Connection and Link
When you troubleshoot NIC issues, the first step is to verify physical connectivity. Visual inspection of the
switch must show a LINK light indicator when connected to a link partner. In addition, the NIC can also
have a LINK light indicator. The Command Line Interface (CLI) of the switch must be checked in ordre to
verify physical connectivity. The port in question must show connected for Catalyst OS software and
line protocol up for Cisco IOS® Software on the switch.
Example for CatOS - Catalyst 2948G, 2980G, 4000, 5000, and 6000 that Run CatOS Software
 show port mod⁄port
Switch> (enable) show port 3/1
Port Name Status VLAN Level Duplex Speed Type
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3/1 notconnect 1 normal half 100 100BaseFX MM
Example for Cisco IOS Software on the Switch - Catalyst 2900XL, 3500XL, 2948G-L3, and 6000
that Run Cisco IOS Software
 show interfaces type
Switch# show interfaces fastethernet 0/1
FastEthernet0/1 is down, line protocol is down