Troubleshooting. Duplex mismatch for Full-Duplex

Problema planteado:

En la parte de Switching and Bridging uno de los temas más comunes y que generalmente no le colocamos la importancia debida es el tema Duplex y velocidad. Hubo un caso que me ha pasado en el entorno es que un enlace no puede negociar a Full Duplex, a pesar que ambos tengan la capacidad de soportarlo. Los equipos involucrados son como sigue:

Router 1841 —Straight cable— Switch 2960

Parece simple no?. Veamos la configuración de ambos

Router 1841 Switch 2960
interface FastEthernet0/0 interface FastEthernet0/24
no ip address switchport mode trunk
no ip redirects speed 100
no ip unreachables duplex full
no ip proxy-arp
duplex auto
speed auto
no mop enabled

Este es el estado de las interfaces:

RO-Router1841#show int fa 0/0 PAN-Switch2960#show int fasteth 0/24
FastEthernet0/0 is up, line protocol is up FastEthernet0/24 is up, line protocol is up (connected)
Hardware is Gt96k FE, address is 001f.6ceb.f3de (bia 001f.6ceb.f3de) Hardware is Fast Ethernet, address is 0021.1c3c.f298 (bia 0021.1c3c.f298)
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255 reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set Encapsulation ARPA, loopback not set
Keepalive set (10 sec) Keepalive set (10 sec)
Half-duplex, 100Mb/s, 100BaseTX/FX Full-duplex, 100Mb/s, media type is 10/100BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00 ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:02, output 00:00:02, output hang never Last input 00:00:46, output 00:00:00, output hang never
Last clearing of "show interface" counters never Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo Queueing strategy: fifo
Output queue: 0/40 (size/max) Output queue: 0/40 (size/max)
5 minute input rate 7000 bits/sec, 5 packets/sec 5 minute input rate 5000 bits/sec, 4 packets/sec
5 minute output rate 12000 bits/sec, 4 packets/sec 5 minute output rate 6000 bits/sec, 7 packets/sec
132341 packets input, 21551237 bytes 140522 packets input, 128814977 bytes, 0 no buffer
Received 1853 broadcasts, 0 runts, 0 giants, 0 throttles Received 406 broadcasts (0 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 30 input errors, 11 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog 0 watchdog, 247 multicast, 0 pause input
0 input packets with dribble condition detected 0 input packets with dribble condition detected
152623 packets output, 128721643 bytes, 0 underruns 156155 packets output, 23528094 bytes, 0 underruns
29 output errors, 22 collisions, 35 interface resets 0 output errors, 0 collisions, 1 interface resets
0 babbles, 29 late collision, 0 deferred 0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier 0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out 0 output buffer failures, 0 output buffers swapped out
RO-Router1841# PAN-Switch2960#

Como podemos ver, no es posible negociar con el full-duplex del lado extremo. Bueno como caso curioso se configura el duplex full en el Router 1841, pero entonces la interfaz se cae.

¿Qué esta pasando? ¿Solo funciona en modo half-duplex?

Solución:

Al principio pensaba que era problema del cableado, pero esto conduce a que si por lo menos uno de los pares no funciona, entonces ninguno de los dos se puede comunicar simultaneo (full-duplex) o ninguno a la vez (half-duplex).

LA manera de solucionarlo era activar el speed 100, pero debe configurarse bajo este orden:
duplex full
speed 100

Otra alternativa (que no he probado por cierto) es realizar un shutdown y no shutdown bajo el escenario original.

Esta situación traté de simularlo una y otra vez, pero la auto negociación funcionaba. Curiosamente el libro de CCIE Routing and Switching Exam Certification Guide 4th Edition mostraba este escenario en las preguntas y respuestas. He aqui el extracto del libro:


Consider an Ethernet cross-over cable between two 10/100 ports on Cisco switches. One
switch has been configured for 100-Mbps full duplex. Which of the following is true about
the other switch?
a. It will use a speed of 10 Mbps.
b. It will use a speed of 100 Mbps.
c. It will use a duplex setting of half duplex.
d. Itwill use a duplex setting of full duplex.

Cisco switches disable auto-negotiation after the speed and duplex have been
configured. The other switch attempts auto-negotiation and fails. However, the
unconfigured switch can detect the speed even without auto-negotiation. By default,
10-Mbps and 100-Mbps ports use half duplex when they are unable to auto-negotiate
a duplex setting.


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