Hub cause Network Slow ?

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Jan 13th, 2009
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Hi NetPro,

any idea why Hub will cause the network slow ? based on the attachment. i found 1 hub (Cisco_Micro_Hub)is connected to my network but how could block this ?

this setup really drive me crazy .... From the Dist_1 i detected 1 micro_hub connected to the same port for Sw1 (Fa0/24). so, when i show cdp nei from Sw1 i saw the hub is connected to the same port (Fa4/31) with the Dist_1.

any idea ? thanks.



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Overall Rating: 3.8 (5 ratings)
mahmoodmkl Tue, 01/13/2009 - 21:10
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This is creating a STP loop in u r network,get rid of one of the connection and u will be fine.



Giuseppe Larosa Tue, 01/13/2009 - 22:50
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Hello Jack,

if the device in the middle is an hub it can only work at 100 Mbps half duplex not full as traffic increases you see limitations at around 30 Mbps or less.

What you see in CDP can be the result of the hub action but that is not a real problem: CDP messages are L2 multicast frames so if the device doesn't stop the frames you see two CDP neighbors on the same port but is not an issue.

The real issue is the half duplex nature of the hub this has an impact on performances.

I would suggest if possible to remove it but probably it is used for distance reasons between the two switches

In the diagram no risk of bridging loop appears until additional cables and ports are connected to the micro hub.

However verify that the ports on the switches are not configured for STP portfast.

Hope to help


Joseph W. Doherty Wed, 01/14/2009 - 04:51
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As Giuseppe notes, hubs won't support the same performance as switches since they don't support full duplex. Also, your diagram doesn't show it, but if there were other active links on the hub, performance can degrade even more since all the hub's links share (effectively) one link's bandwidth.

Ideally, you might just want to remove the hub and directly connect the two devices that are connected via the hub now. Or, you might try replacing the hub with an inexpensive small switch.


Another possible reason for the hub might have been to allow a tap point for a wire sniffer.

ney25 Wed, 01/14/2009 - 16:54
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Hi Bro,

yes, you were right, this hub only in the duplex half only. finally i found this hub and as understand due to the distance reason that's why Hub connect between uplink switch and access switch.

any other reason of the network slowness ? anyone can share about your experienced ?

your reply will be highly appreciated.




Joseph W. Doherty Wed, 01/14/2009 - 17:02
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A common cause of "network slowness" is dropped packets. You might want to check end-to-end interface stats for drops or other errors. An Ethernet duplex mismatch will pass traffic, but with a high lost rate. Something to check between host and network edge devices (or even between network devices).

ney25 Wed, 01/14/2009 - 17:07
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Hi Bro,

yes, i totally agree with you about the duplex mismatch could causing the network slowness. what if the vlan-mismatch ? as i understand that vlan-mismatch could suppress the switch CPU utilization. but does vlan-mismatch could cause the slowness ?




glen.grant Wed, 01/14/2009 - 17:19
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No the vlan mismatch would not be a problem in a non trunked setup. Check the switches on each end and make sure they are not hardcoded to say 100/full as this is a speed duplex mismatch if the hub port is half duplex and this will really slow down the transfers. The ports should be set as auto or hardcode them to 100/half.

ney25 Wed, 01/14/2009 - 18:27
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Thanks All.

i agreed with that. will schedule a downtime to fix the problem.




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