Auto-Negotiation

Auto-Negotiation
Auto-negotiation
is a process that enables clients and switches to agree on a link capability. This
is used to determine the link speed as well as the duplex being used. The auto-negotiation process
uses priorities to set the link configuration. Obviously, if both a client and a switch port can
use 100Mbps, full-duplex connectivity, that would be the highest priority ranking, whereas
half-duplex, 10Mbps Ethernet would be the lowest ranking.
Auto-negotiation uses Fast Link Pulse (FLP), which is an extension to the Normal Link Pulse
(NLP) standard used to verify link integrity. NLP is part of the original 10BaseT standard. Commonly,
these auto-negotiation protocols do not work that well, and you would be better off to
configure the switch and NICs to run in a dedicated mode instead of letting the clients and
switches auto-negotiate. Later in this chapter, we’ll show you how to configure your switches
with both the speed and duplex options.
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Chapter 13 
Connecting the Switch Block
Auto-negotiation is one of the most common causes of frame check sequence (FCS) and alignment
errors. If two devices are connected and one is set to full-duplex and the other to half-duplex,
one is sending and receiving on the same two wires while the other is using two wires to send and
two wires to receive. Statically configuring the duplex on the ports eliminates this problem.
Intermittent connectivity issues can often be traced to auto-negotiation problems.
If a single user occasionally has long connectivity outages, statically setting
speed and duplex on both ends often helps.