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Chapter 5
Applying Cisco's Diagnostic Tools
Internet address is 172.16.60.1/24
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/
255, load 46/255
Encapsulation ARPA, loopback not set, keepalive set (10
sec)
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters 00:00:50
Queueing strategy: fifo
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
5 minute input rate 1209000 bits/sec, 620 packets/sec
5 minute output rate 1819000 bits/sec, 477 packets/sec
21386 packets input, 3979009 bytes, 0 no buffer
Received 9 broadcasts, 0 runts, 0 giants
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,
0 abort
0 input packets with dribble condition detected
6000 packets output, 8237684 bytes, 0 underruns
0 output errors, 1020 collisions, 0 interface resets
0 babbles, 0 late collision, 1006 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped
out
So, what do you think? What are your observations? This exercise was
designed specifically to educate you about Ethernet capabilities. The principal
observation that you should have made was the increasing number of collisions
on the interface.
Collisions are a normal occurrence for CSMA/CD protocols. The fact that
a connection is not full duplex creates the opportunity for collisions.
Although collisions are normal, excessive collisions can be detrimental to a
network. Once collisions exceed 5 to 8 percent of the output packets, the
interface becomes very ineffective. The higher the collision rate, the more
packets that have to be retransmitted.
The output queue for the Ethernet interface doesn't stop filling up just
because of collisions on the line. Therefore, not only does the interface have to
transmit the normal queue of packets, it has to retransmit all of the frames that
were lost due to collisions. The number of packets that need to be transmitted
can grow exponentially. Let's calculate the collision percentage for the four
show interface
outputs:
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