CAIA Technical Report 031217A December 2003 Page 4 of 5
F. Investigating the number of packets tcpdump would record
when the Alloy switch is replaced with a hub.
This test used a 10Mbit/sec CentreCOM MR820TR
hub in place of the Alloy switch with the same three
tests (Group, Start All Cards and manual) run. In this
case, not only did the hub result in CRC errors but also
alignment errors and oversized packets rather than
undersized/fragmented packets when the Group function
was used. Once again, fewer errors occurred with the
“Start All Cards” option and no errors or packet loss
when the cards were started manually. The test showed
that the same trend occurred in the hub as in the switch
due to colliding packets.
F. Confirming tcpdump was not responsible for packet loss by
repeating the test using a high performance switch.
To dismiss the possibility that the packet loss was a
result of tcpdump failing to capture packets sent by the
NS-16J, a Cisco Catalyst 2900 Series XL switch was
tested while using the Group function in place of the
Alloy switch. Cisco Discovery Protocol was disabled
and all spanning-tree packets were filtered on all ports so
as to ensure that the switch did not send out any
broadcast packets not received by a SmartBits2000 card.
Graph 7 (note only the first 32,000 packets are
shown) shows the culumative packets versus cumulative
time when 40,000 packets were flooded by the four
SmartBits cards (10,000 packets per card). All 40,000
packets were accounted for by tcpdump with no errors,
proving that both the Catalyst 2900 and tcpdump were
capable of handling all packets.
Graph 8 shows a close up of the beginning of this
test. Each dot represents one packet. As we can see the
Catalyst 2900 was capable of handling bursts of four
packets every 0.2 seconds from the beginning of the test
to the end.
Figure 8: First 25 packets with the Cisco Catalyst 2900 Series switch
V. CONCLUSION
This investigation looked into the actual size of an
Alloy NS-16J CAM table. It also tested the switch using
three different packet burst start methods from four
sources. The investigation found that the CAM table
could hold 8,320 destination MAC address and port
combinations, well above the manufacturer’s quoted
number of 8,000. It also found that bursting four packets
onto the switch backplane at exactly the same time (or at
very tiny inter-packet intervals) caused some errors and
packets loss.
It was clear from the tcpdump test performed that the
Alloy switch is not capable of handling loads such as
those on high-speed networks and Internet backbones.
Switches such as the Cisco Catalyst 2900 Series XL
which would be more suited to this type of traffic
demand where packets may simultaneously converge on
the switch backplane. The Alloy NS-16J could, however,
be adequate for small-scale projects where the rate and
number of packets is significantly lower. This includes
the MAGIC project at the Center for Advanced Internet
Architectures.
It is important to note that the Cisco 2900 Series XL
is substantially more expensive than the Alloy NS-16J.
At the time of writing the Alloy NS-16J retails around
the mid $AU100 range. The Cisco 2900 Series XL is no
longer on the market. The next available Cisco switch is
the WS-C2950-24 model around the mid $AU800 range.
The Alloy switch is therefore more economical for
smaller-scale projects. This difference in price as well as
performace capabilities should be considered when
purchasing any switch.
Figure 7: Group test using a Cisco Catalyst 2900 Series switch
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