How to determine which switch is dropping network traffic?

Posted on 2009-12-29
Last Modified: 2012-05-08
I have 3 switches on my network:

3COM 2948-SFP (Managed)
Dell PowerConnect 3424 (managed)
3COM Baseline 2824 (Unmanaged)

The unmanaged switch has all my servers connected to it as well as the connection to the firewall.  Both the managed switches are connected to the unmanaged 3com via a network cable.

I had an AD issue with clients that I was able to determine is being caused by a network issue.  The problem goes away when clients are plugged into the unmanaged switch that is connected to the servers, it reappears when connected to the other two switches.

Some of the ports on the managed 3com switch are showing CRC errors, etc here is a copy/paste of one of the ports as a sample:

Received Bytes (Octets) 15153058829  
Received Packets 24932600  
Broadcast Packets Received 461867  
Multicast Packets Received 95578  
CRC & Align Errors 111  
Undersize Packets 0  
Oversize Packets 0  
Fragments 37  
Jabbers 0  
Collisions 2  
Frames of 64 Bytes 101220868  
Frames of 65 to 127 Bytes 61377083  
Frames of 128 to 255 Bytes 22986716  
Frames of 256 to 511 Bytes 5317032  
Frames of 512 to 1023 Bytes 3369123  
Frames of 1024 to 1518 Bytes 32121492  

I'm a bit confused as to which switch is the problem, or which switches.
Question by:jmtoman
    LVL 16

    Expert Comment

    I would remove autoneg for the port connected to the unmanaged switch on both managed switches. Try specifying your connection settings and see if you can come up with something that works.
    LVL 1

    Expert Comment

    Are the three switches connected in a daisy chain type of setup? ie a circle of switches so to speak?
    LVL 24

    Expert Comment

    LVL 21

    Expert Comment

    The number of errors on the port you showed aren't very many and probably not enough to cause any problems. Those errors are from physical layer issues like cables or adapters usually so if you are using the same PC and same cable they should react the same unless the switch port itself is to blame.
    Are the errors worse on the other ports and do they count up rapidly when the problem is going on? If they don't increment during the issue they can most likely be ignored. In that case you should consider doing a capture from the pc while connected to each both switches and compare them to see what is happening that is different.

    Are there any VLANs etc configured on the managed switches that might be an issue to the unmanaged switch?

    Accepted Solution

    I aplogize for the delay.  I was out sick for an extended period of time.  I replaced the switch and all is well.
    LVL 1

    Expert Comment

    We have a 3com switch that is blocking all traffic to another switch that is hp, we also have had switches block ports that have pcs on them, no traffic is allowed through the port until a reboot of the switch is done!

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