don't use cheap switches under some scenarios

don't use cheap switches under some scenarios

[Xinwen Fu]

Hi, all,
	I asked before why my linksys workgroup 5-port switch worked like
a hub
under heavy traffic (10mb/s) into one socket of the switch. The conclusion
is that the switch has some problem.

	In fact, for the first 5 minutes, it
works like a switch and then it works like a hub. The switching table is
messed up by the intense traffic, we believe. Other cheaper switches
(netgear fast esthernet switch FS108 ) have the same problem. We use a
CentreCom FS708, and then the problem is solved. Of course other expensive
and professional switches should be ok too, we think.

	Just some information.


Xinwen Fu


---------- Forwarded message ----------
Date: Thu, 8 May 2003 22:49:01 -0500 (CDT)
From: Xinwen Fu <xinwenfu@cs.tamu.edu>
To: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: linksys workgroup switch works like a hub

It is really weird!

Host_10.1--->10.254_router_1.254--->Linksys 5-port workgroup switch-->Host1.1
                                       |               |
                                       |               |
                                       v               v
                                    Host_1.10        Host_1.5

Host_10.1 sends 1000 packets/s (pps) to Host_1.10. Host_1.10 receives the
packets. But Host_1.5 receives the packets too, while Host_1.1 does not
(by tcpdump -i any).

	What is the possible reason?

	Thanks!
Xinwen Fu

Re: don't use cheap switches under some scenarios

[Chuck Ebbert]

Xinwen Fu wrote:

> The switching table is
> messed up by the intense traffic, we believe. Other cheaper switches
> (netgear fast esthernet switch FS108 ) have the same problem.

  What do you mean by "intense traffic?"  Many switches will get
confused if you fill their tables with a large number of different
addresses.  Is someone maybe spoofing MAC addresses on your net?
Or is the switch plugged into a larger switched network where it will
see many different MAC addresses on the uplink port?

  Also, almost every switch will flood all ports with traffic for
machines that have disappeared recently (disconnected or changed
their MAC addresses.)  High-end switches have settings to control
this problem but it's impossible to avoid some leakage.

  Never count on an ethernet switch to provide privacy for traffic 
on a single VLAN.

Re: don't use cheap switches under some scenarios

[H. Peter Anvin]

Followup to:  <Pine.GSO.4.44.0305101411001.11628-100000@unix.cs.tamu.edu>
By author:    Xinwen Fu <xinwenfu@cs.tamu.edu>
In newsgroup: linux.dev.kernel
>
> Hi, all,
> 	I asked before why my linksys workgroup 5-port switch worked like
> a hub
> under heavy traffic (10mb/s) into one socket of the switch. The conclusion
> is that the switch has some problem.
> 
> 	In fact, for the first 5 minutes, it
> works like a switch and then it works like a hub. The switching table is
> messed up by the intense traffic, we believe. Other cheaper switches
> (netgear fast esthernet switch FS108 ) have the same problem. We use a
> CentreCom FS708, and then the problem is solved. Of course other expensive
> and professional switches should be ok too, we think.
> 

Do you have lots of machines on your network?  All switches has a
limit on the number of MACs they can remember.

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
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