Even with the right IPs, a network can crawl or drop traffic because of L1 problems. This chapter teaches you to read 'show interfaces' counters, spot duplex/speed mismatches, and diagnose collisions versus runts versus CRC errors.
Counters accumulate from boot. Reset them with 'clear counters' before re-testing. The fields that matter most: input/output errors, CRC, frame, overrun, ignored, runts, giants, collisions, late collisions.
R1# show interfaces Gi0/0
GigabitEthernet0/0 is up, line protocol is up
Hardware is iGbE, address is 0050.56be.1a01
Internet address is 10.0.0.1/24
MTU 1500 bytes, BW 1000000 Kbit/sec
Full-duplex, 1000Mb/s, link type is auto, media type is RJ45
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
5 minute input rate 1000 bits/sec, 2 packets/sec
12345 packets input, 1234567 bytes
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
12345 packets output, 1234567 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resetsIf one side auto-negotiates and the other is hard-coded, the auto-side typically falls to half-duplex while the hard-coded side stays full. The link comes up, but you see late collisions on the half-duplex side and CRC errors on the full-duplex side. The fix: either set both ends statically the same OR let both ends auto-negotiate.
1 more section in this chapter, plus a 2-question quiz.
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