Auto-negotiation

I figured i'd put together some notes to concisely describe auto-negotiation, based off a particular client's requirements.

What is auto-negotiation?  It is an Ethernet procedure by which two connected devices choose common transmission parameters, such as speed, duplex mode, and flow control.  Basically, it is a mechanism for two devices to negotiate the BEST method of communicating across a common link.  The most common parameters negotiated are speed and duplex.

But why is this needed?  Well, in a word where end devices support half-duplex, full-duplex, 10 Mbps, 100 Mbps, 1 Gbps, etc...it would be a monumental task to maintain static duplex/speed configurations on a large enterprise network.  In addition to duplex and speed, auto-negotiation can also be used to detect physical layer issues.  

I like to compare half-duplex and full-duplex to a walkie-talkie and a telephone, respectively.  In half-duplex, only one end of the communication can send or receive at any given period of time.  With full-duplex, both nodes can send and receive simultaneously.  In an ideal scenario, full-duplex is the preferred means of communication..obviously.

Regardless of what is "best," both ends of the link go through the following list to determine the best possible mode of operation that is supported by both devices:

1. 10GBASE-T full duplex
2. 1000BASE-T full duplex
3. 1000BASE-T half duplex
4. 100BASE-T2 full duplex
5. 100BASE-TX full duplex
6. 100BASE-T2 half duplex
7. 100BASE-T4 half duplex
8. 100BASE-TX half duplex
9. 10BASE-T full duplex
10. 10BASE-T half duplex

In other words, among the modes that are supported by both devices, each device chooses the one that is the topmost in this list.

The real complications arise when either a switch port or a end device is hard-coded.  Why might something get hardcoded?  Well, there used to be a time when the auto-negotiation standards were interpreted wildly by every given vendor.  As a result of previous inconsistencies, many people today still fear auto-magically letting things negotiate.  The problem with hard-coding, is that there are often cases where doing so will "break" the link:

¹ A duplex mismatch can result in performance issues, intermittent connectivity, and loss of communication. When you troubleshoot NIC issues, verify that the NIC and switch use a valid configuration.

² Some third-party NIC cards can fall back to half-duplex operation mode, even though both the switch port 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 switch port and the NIC. Symptoms include poor port performance and frame check sequence (FCS) errors that increment on the switch port. In order to troubleshoot this issue, try to manually configure the switch port to 100 Mbps and 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.