How many is too many collisions? |
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EXPERT RESPONSE FROM: Greg _Ferro
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QUESTION POSED ON: 01 May 2001
Is there any benchmark for too many collisions? We have an IP network
with Cisco Routers and Switches. Our routers are running 10meg1/2dux (as
well as the switch port hooked to them). We see about 5 to 8% collisions on
our Ethernet with 2meg of traffic throughput (that's all that's being
sent). We are running 11.2, BGP and OSPF.
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A well-behaved network should have less 50 to 100 broadcast packets per
second. It's important to measure the number of packets, not percentage of
bandwidth, because it is the number of packets that causes poor performance
on workstations. For each broadcast packet that is received the
workstations must analyze the packet at the processor. That is, the packet
must be opened and data analyzed to determine if the information is
destined for this workstation. If you have around 500 packets per second,
studies show that workstations are losing around 10% of CPU performance as
they read and analyze the broadcast packet contents.
There are two types of broadcasts, Layer 2 and Layer 3 broadcasts. Layer 2
broadcasts are used for applications like ARP and system discovery and are
the most common broadcast type. Problems due to Layer 2 are usually solved
with switches by adding more bandwidth. Inherently, there is not normally a
lot of Layer 2 broadcast because it isn't used much.
Layer 3 broadcasts are far more common. In IP environments, you can
broadcast the network searching for another machine. Say you have a
network, 192.168.1.0/24. If you want a packet to get to all IP enabled
machines, you send a packet to 192.168.1.255 and all workstations will hear
and process that packet. IPX has a similar mechanism. This is what
Microsoft NetBIOS will use if you do not have a WINS server in use and
commonly creates a large number of broadcast packets.
So to look at some solutions:
Incorrect speed/duplex sensing
First, 10MB Ethernet is always half duplex. That is what the standard
specifies. There are a number of pseudo-standardized 10MB full duplex (3Com
has one) solutions, but they only work under very specific conditions. (If
you have 10 MB full duplex, you usually have a problem unless you have a
specifically matched switch for the network interface card, again a 3Com
NIC with a 3Com switch).
The most common cause of this problem is a duplex mismatch between the
switch and the workstation or router. I would check the port that the
router is connected to and make sure that it is 10MB half duplex. If it
auto sensing, then set it to 10MB half duplex.
When you connect a device to an auto-sensing port, a system goes through a
process of "flash testing" each possible connection speed and duplex. This
process is less than perfect and sometimes can get it wrong, especially
when you have an auto-sensing card that is carrying out the same procedure.
Cabling problems
I would replace the cable on the router. Seventy percent of problems like this are
caused by a faulty cable. Also change the patch lead on your Linux boxes.
Packet analysis
To determine if you have a Layer 2 or Layer 3 broadcast problem, you should
get a packet sniffer and have a look at the broadcast packets. Look at the
source of the packets to determine what the application is. I use Network
Associates Sniffer as it has an intelligent engine that will analyze the
capture and report in clear English for most problems.
If you have a lot of IP and IPX network broadcasts and you use Microsoft
Windows, you should implement a Microsoft WINS server and set the NetBIOS
node type to hybrid mode. This stops Windows from broadcasting for name
resolution.
If you have IPX as well as IP on your workstations, you should disable
NetBIOS over IPX to stop name broadcasts.
If you have IPX only, you should implement IP to resolve this problem.
Otherwise, the Sniffer capture will have to be analysed by an experienced
person to determine what the source of broadcasts are. Good Luck.
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