2.3 Troubleshoot interface and cable issues (collisions, errors, duplex, speed)

This topic seems to be the hardest to explain because there is no specific back-up information that is indeed specifying the issue in Cisco troubleshooting.But first let me explain collision and I hope that we can go further to this topic.

Collision refers to the scenario wherein two stations send a frame at the same time.If that happens, this may lead that both frames will be dropped or to be resent.

There are two ways that resolve this issue:

  1. CSMA/CD ( Carrier Sense Multiple Access with Collision Detection)
  2. CSMA/CA ( Carrier Sense Multiple Access with Collision Avoidance)

csmacd

First, I would like to acknowledge Behrouz Forouzan for writing a book of Data Communications and Network Engineering, it is really a great help especially in studying CSMA. This protocol evolved from Aloha protocol to CSMA to CSMA/CD. To make CSMA/CD simple, just follow the flowchart. If it is hard to follow since it involves technical term, the explanation is like this: it follows the one mouth rule in a meeting, it means to say we need to “SENSE” whether someone is speaking and then that is the right time for anyone to speak,if someone is speaking, we need to wait up until everyone is silent. If we detect a collision or if we observed someone is speaking just like you are speaking as well, it will send a jamming signal that states that we need to wait for a period of time to speak again. The frame will be totally discarded or dropped if it reached 1 6 times to  be resent as you see in the figure. It is safe to say that the frame will be totally discarded after 15 times but as you can see in the figure, there is a question k>15? let us say that k=15? then 15>15, the answer would be no and the next step would go back from the start.

343434

CSMA/CA almost the one mouth rule in a meeting.But the goal of CSMA/CA is not just simply detect a collision but to avoid collision. As you observed in the flow chart, the process goes back into the first process with question ” Idle channel?” up until it reached the 15th time that a frame is sent.

The difference of both is that CSMA/CD represents Ethernet or IEEE802.3 standards while CSMA/CA represents IEEE 802.11 which is Wi-Fi.

LATE COLLISIONS

To allow collision to work properly, protocol states that we can only detect collision restricted up to the 512th bit. If collision is still detected after the 512th bit, it will be tagged as late collision and it may cause a duplex mismatch.

DEFERRED COUNTER

here is an example of verifying deferred counter

router#show interface ethernet 0
Ethernet0 is up, line protocol is up
Hardware is Lance, address is 0010.7b36.1be8 (bia 0010.7b36.1be8)
Internet address is 10.200.40.74/22
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:06, output hang never
Last clearing of “show interface” counters never
Input queue: 1/75/1/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: random early detection(RED)
Output queue :0/40 (size/max)
5 minute input rate 1000 bits/sec, 2 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
2058015 packets input, 233768993 bytes, 1 no buffer
Received 1880947 broadcasts, 0 runts, 0 giants, 1 throttles
3 input errors, 0 CRC, 0 frame, 0 overrun, 3 ignored
0 input packets with dribble condition detected
298036 packets output, 32280269 bytes, 0 underruns
0 output errors, 10 collisions, 0 interface resets
0 babbles, 0 late collision, 143 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out

Deferred counter refers to the number of times that an interface tried to send a frame, but the carrier is sensed as not idle as the first attempt.This deferred counter may not cause any issue but it is good for us to know and would help us to eliminate choices.

 

COLLISION COUNTER

Same example but highlighting collision counter:

router#show interface ethernet 0
Ethernet0 is up, line protocol is up
Hardware is Lance, address is 0010.7b36.1be8 (bia 0010.7b36.1be8)
Internet address is 10.200.40.74/22
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:06, output hang never
Last clearing of “show interface” counters never
Input queue: 1/75/1/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: random early detection(RED)
Output queue :0/40 (size/max)
5 minute input rate 1000 bits/sec, 2 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
2058015 packets input, 233768993 bytes, 1 no buffer
Received 1880947 broadcasts, 0 runts, 0 giants, 1 throttles
3 input errors, 0 CRC, 0 frame, 0 overrun, 3 ignored
0 input packets with dribble condition detected
298036 packets output, 32280269 bytes, 0 underruns
0 output errors, 10 collisions, 0 interface resets
0 babbles, 0 late collision, 143 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out

Collision counter refers to the number of frames for which one or more collisions occur when the frames are sent. This will not affect the network but may lead us to a conclusion that a network is heavily loaded.

EXCESSIVE COLLISIONS

Excessive collisions refers to the collision after the 16th attempt to resend the frame. The frame will not be transmitted and marked as excessive collisions.

DUPLEX MISMATCH

Duplex mismatch means that router1 has half duplex and router2 has full duplex. It means to say that they are running a different duplex mechanism. It is difficult to diagnose difficult mismatch because the symptoms may be intermittent. Second, the interface display (up/up) state though the duplex settings are mismatched.

The indication to determine it when we verify it using show interface command shows not normal number of input errors and CRC.

RUNTS

Last thing, runts refers to packets less than 64 bytes that may cause excessive collisions.

*******************************END OF BLOG*****************************

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