Autonegotiation

Autonegotiation

Autonegotiation is a mechanism that allows two devices at either end to negotiate speed and duplex settings at physical layer. The benefits of autonegotiation include minimal configuration and operability between dissimilar Ethernet technologies.

In today's networks, 10BASE-T and 100BASE-T are ubiquitous. Newer Cisco modules such as the WS-X6548-GE-TX have ports capable of 10/100/1000BASE-T. Most existing network interface cards (NICs) operate at 10/100 speeds, with newer NICs offering 10/100/1000BASE-T operation. NICs capable of autonegotiating speed and duplex are beneficial because more and more users are becoming mobile. One day, a user might be connected to the office Catalyst switch at 100 Mbps, and the next day, a remote site that supports only 10 Mbps. The primary objective is to ensure that the user not only has easy access to the network but also has network reliability. If the user's laptop NIC is hard coded at 100BASE-T full duplex, the user connectivity might be impacted because the two switches might have different types of modules that operate at different speeds. For instance, the module in the office building is WS-X5225 (24 port 10/100BASE-TX), and the remote site has WS-X5013 (24 port 10BASE-T). In this case, because the switches are set by default to autonegotiate, a user with a NIC hard coded to 100BASE-T full duplex will not get any connectivity. Setting up autonegotiation on both the switch and laptop gets rid of this problem. The user no longer has to worry about the laptop NIC settings because the NIC automatically negotiates the proper physical layer configuration with the end device to which it connects.

The actual mechanics behind autonegotiation are straightforward, as depicted in Figure 1-6. Autonegotiation attempts to match speed and duplex mode at the highest priority with its link partner. Since the introduction of 1000BASE-T, the priorities have been readjusted. Table 1-4 describes each priority level.

The 10BASE-T specification does not include autonegotiation between devices. Autonegotiation was first introduced in IEEE 802.3u Fast Ethernet specification as an optional parameter. In a 10BASE-T environment, a single pulse, called the Normal Link Pulse (NLP), is sent every 16 ms (±8 ms) on an idle link. The NLP performs a link integrity test for 10BASE-T. When no traffic is on the link, the 10BASE-T device generates a NLP on the wire to keep the link from going down. The 10BASE-T device stops generating pulses when it receives data packets. A link failure occurs under conditions when the 10BASE-T device does not receive NLPs or a single data packet within a specified time slot.

As mentioned earlier, the IEEE 802.3u specification has an optional programmable field for autonegotiation. Within autonegotiation, there are various other optional operations, such as Remote Fault Indication and Next Page Function. Remote Fault Indication detects and informs the link partner of physical layer errors. The Next Page Function provides more verbose information about the negotiation process. One of the more appealing features of autonegotiation is compatibility with dissimilar Ethernet technologies. For example, Fast Ethernet is backward-compatible with 10BASE-T through a Parallel Detection mechanism. Essentially, the Fast Ethernet switches to NLP to communicate with a 10BASE-T device. Parallel Detection is when only one of the two link partners is capable of autonegotiation.

Fast Ethernet uses the same pulse structure as 10BASE-T. In 10BASE-T, there is only a single pulse every 16 ms, whereas in Fast Ethernet, there are bursts of pulses in intervals of 16 (±8) ms. In these pulses, or groups of pulses, the capability of the device is encoded in a 16-bit word called a Link Code Word (LCW), also known as Fast Link Pulse (FLP). The length of the burst is approximately 2 ms.

Gigabit Ethernet implementation requires that all IEEE 802.3z compliant devices have autonegotiation capability. Autonegotiation can, however, be disabled through a software feature. From the actual hardware perspective, the 802.3z specification requires autonegotiation capabilities on the device. On Cisco Catalyst switches, autonegotiation can be disabled with the following command. Note that this command must be configured on both link partners:

set port negotiation <mod/port> enable | disable

The parameters that 802.3z devices negotiate are

• Duplex setting

• Flow control

• Remote fault information

Although duplex setting can be negotiated, Cisco switches operate Gigabit Ethernet in full-duplex mode only. With the introduction of the newer 1000/100/10 blades, a port can operate at various speeds and duplex settings. However, it is unlikely that Cisco will support Gigabit half duplex in any point-to-point configurations with even the aforementioned blades. Use the show port capabilities command that is available in Catalyst OS to view the features supported by the line module, as shown in Example 1-4.