Autonegotiation

With the multiple combinations of network modes available, configuring devices gets confusing. You need to determine if the device needs to operate at 10 or 100 Mbps, whether it needs to run in half- or full-duplex mode, and what media type to use. The device configuration must match the hub configuration to which it attaches.

Autonegotiation attempts to simplify manual configuration requirements by enabling the device and hub to automatically agree upon the highest common operational level. The 802.3u committee defined Fast Link Pulse (FLP) to support the autonegotiation process. FLP, an enhanced version of 10BaseT's Link Integrity Test, sends a series of pulses on the link announcing its capabilities. The other end also transmits FLP announcements, and the two ends settle on whatever method has highest priority in common between them. Table 1-2 illustrates the priority scheme.

Table 1-2. Autonegotiation Prioritization

Priority

Method

1

100BaseT2 full duplex

2

100BaseT2 half duplex

3

100BaseTX full duplex

4

100BaseT4 (Only half duplex)

5

100BaseTX half duplex

6

10BaseT full duplex

7

10BaseT half duplex

According to Table 1-2, 100BaseT2 full-duplex mode has highest priority, whereas the slowest method, 10BaseT half-duplex, has lowest priority. Priority is determined by speed, cable types supported, and duplex mode. A system always prefers 100 Mbps over 10 Mbps, and always prefers full duplex over half duplex. Note that 100BaseT2 has higher priority than 100BaseTX. This is not a direct result of 100BaseT2 being a more recent medium. Rather, 100BaseT2 has higher priority because it supports more cable types than does 100BaseTX. 100BaseTX only supports Category 5 type cable, whereas 100BaseT2 supports Category 3, 4, and 5 cables.

Not all devices perform autonegotiation. We have observed at several customer locations failure of the autonegotiation process—either because of equipment not supporting the feature or poor implementations. We recommend that critical devices such as routers, switches, bridges, and servers be manually configured at both ends of the link to ensure that, upon reboot, the equipment operates in a common mode with its hub/switch port.

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