Administrator Guide

Layer 2 Switching Commands 427
When the auto parameter is configured with a set of speeds, only those
speeds are advertised during auto-negotiation. Alternatively, if no speed
arguments are configured, then all the speeds which the port is capable of
supporting are advertised. Not all ports support all speeds, even if they are
available in the command. Entering an unsupported speed will produce the
following error message An invalid interface has been used
for this function.
Fiber ports (other than 1000Base-X) do not support auto-negotiation and
therefore require the operator to enter the speed command with the desired
operating rate. The link partner must be similarly configured. SFP+/QSFP
fiber ports using fiber media do not support auto-negotiation, although it
should be enabled when using Direct Attach Copper cables if the link partner
also supports auto-negotiation. 1G SFP fiber ports should have auto-
negotiation enabled. SFP transceivers in an SFP+ port should have auto-
negotiation enabled. The default behavior is to enable auto-negotiation when
an SFP transceiver is inserted into an SFP+ port, unless a fixed speed is
configured.
Likewise, SFP+ ports connected via copper Direct Attach Cables should have
auto-negotiation enabled if the link partner is also capable of performing
auto-negotiation. If the link partner cannot perform auto-negotiation, then a
fixed speed must be utilized. In all cases, the link partners need compatible
settings, e.g., both sides must be set to use auto-negotiation or a fixed speed.
In the case of a fixed speed link, both sides must be set to the same speed.
Failure to set both sides of a link to the same speed/duplex values (auto-
negotiation disabled) or compatible speed/duplex values (auto-negotiation
enabled) may give a false link-up indication when configured or when the link
is brought up (no shut). Setting one end of a link to auto-negotiate and the
link partner to a fixed speed is not supported. Disabling auto-negotiation on
interfaces that require it, for example, 1000BASE-X or 1G/10G copper
interfaces, is not supported.
Example
The following example configures Gigabit Ethernet port 1/0/5 to advertise
100-Mbps operation only via auto-negotiation.
console(config)#interface gigabitethernet 1/0/5
console(config-if)#speed auto 100