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LAN Overview 143
Autonegotiation uses a priority scheme to define the more preferred options for Fast and
10-Mbps Ethernet. The lower the priority value, the more preferred the specification. The
names of these standards are listed in Table 4-3, along with the priority used by the
autonegotiation process.
Autonegotiation uses a series of Fast Link Pulses (FLPs) to communicate with the device on
the other end of the cable. An exchange takes place as to what each endpoint is capable of
supporting. The Autonegotiation Priority in Table 4-3 shows the choice that the process would
make if more than one option was supported--of the supported options, the one with the lowest
autonegotiation priority is the option chosen.
The autonegotiation process has been known to fail. Cisco recommends that for more important
devices, you should configure the LAN switch to the correct setting rather than depend on
autonegotiation.
LAN Standards
Cisco expects CCNAs to be very familiar with Ethernet specifications, as well as be familiar
with the basics of FDDI and Token Ring standards. The IEEE defines most of the standards
for Ethernet and Token Ring, with ANSI defining standards for FDDI. Table 4-4 lists the
specification that defines the Media Access Control (MAC) and Logical Link Control (LLC)
sublayers of the three LAN types for comparison.
Table 4-3
100Bx Standards and Autonegotiation Priority
Standard
Full or Half Duplex
Autonegotiation Priority
100BaseT2
Full
1
100BaseT2
Half
2
100BaseTX
Full
3
100BaseTX
Half
4
100BaseT4
Half
5
10BaseT
Full
6
10BaseT
Half
7
Table 4-4
MAC and LLC Details for Three Types of LANs
Name
MAC Sublayer
Spec
LLC Sublayer
Spec
Other Comments
Ethernet Version 2
(DIX Ethernet)
Ethernet
Not applicable
This spec is owned by Digital,
Intel, and Xerox.
IEEE Ethernet
IEEE 802.3
IEEE 802.2
This is also popularly called
802.3 Ethernet.
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ch04.fm Page 143 Monday, March 20, 2000 5:02 PM