100
Full
Settings:
1000 Full
Settings:
10 Half
Result:
100
Half
F0/1
F0/2
F0/3
Result:
1000 Full
Result:
10 Half
10/100
Figure 7-2
IEEE Autonegotiation Results with Autonegotiation Disabled on One Side
Reviewing each link, left to right:
■
PC1: The switch receives no autonegotiation messages, so it senses the electrical signal
to learn that PC1 is sending data at 100 Mbps. The switch uses the IEEE default duplex
based on the 100 Mbps speed (half duplex).
■
PC2: The switch uses the same steps and logic as with the link to PC1, except that the
switch chooses to use full duplex because the speed is 1000 Mbps.
■
PC3: The user picks poorly, choosing the slower speed (10 Mbps) and the worse duplex
setting (half). However, the Cisco switch senses the speed without using IEEE autonego-
tiation and then uses the IEEE duplex default for 10-Mbps links (half duplex).
PC1 shows a classic and unfortunately common end result: a duplex mismatch. The two
nodes (PC1 and SW1’s port G0/1) both use 100 Mbps, so they can send data. However, PC1,
using full duplex, does not attempt to use carrier sense multiple access with collision detec-
tion (CSMA/CD) logic and sends frames at any time. Switch port F0/1, with half duplex,
does use CSMA/CD. As a result, switch port F0/1 will believe collisions occur on the link,
even if none physically occur. The switch port will stop transmitting, back off, resend
frames, and so on. As a result, the link is up, but it performs poorly. The upcoming section
titled “Interface Speed and Duplex Issues” will revisit this problem with a focus on how to
recognize the symptoms of a duplex mismatch.
Autonegotiation and LAN Hubs
LAN hubs also impact how autonegotiation works. Basically, hubs do not react to autonego-
tiation messages, and they do not forward the messages. As a result, devices connected to a
hub must use the IEEE rules for choosing default settings, which often results in the devices
using 10 Mbps and half duplex.
Figure 7-3 shows an example of a small Ethernet LAN that uses a 20-year-old 10BASE-T
hub. In this LAN, all devices and switch ports are 10/100/1000 ports. The hub supports only
10BASE-T.
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ptg29743230
162 CCNA 200-301 Official Cert Guide, Volume 1
1000 Full
Result:
10 Half
1000 Full
Result:
10 Half
Result:
10 Half
SW1
Hub 1
1
2
3
4
Figure 7-3
IEEE Autonegotiation with a LAN Hub
Note that the devices on the right need to use half duplex because the hub requires the use
of the CSMA/CD algorithm to avoid collisions.
NOTE
If you would like to learn more about collision domains and the impact of these
older LAN hubs, look to the companion website for Appendix K, “Analyzing Ethernet LAN
Designs,” to the section titled “Ethernet Collision Domains.”
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