Switch throughput issue

Unanswered Question

I have a network with two switches at the core.  One is a SLM 2048 and the other is a 3524.

The 3524 is setup with a few Vlans and Voip and switches our Internet connection.  It is only connected to 7940 phones (about 10) and the SLM 2048.

The SLM 2048 is connected to PCs (about 25) and the 3524.

The issue I am having is when I do a major operation like copying a CD from a workstation to a server, which are both connected to the 2048, the network seems to be completly

saturated with traffic.  The VOIP audio gets metallic or worse, and any simple network transaction takes a very long time or just times out.  When

doing this the workstation shows a usage of about 5-8% of the network connection (50-80 mbps), all the PC's on the network are Gigabit machines. 

What should I check to try to see why this is happening?

I have this problem too.
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Steve Schubert Thu, 03/05/2009 - 18:54

Do you have port mirroring or in Cisco terminology, a span port spanning a VLAN? This could be left over from past troubleshooting and would allow traffic across ports that typically would not be propagated across a switch.

sschuber -

Thanks for the pointer.    I checked both switches and on the SLM2048 it is called Port mirroring, and no ports are setup for that.   On the 3524 it is called SPAN and no ports are configured for it.   I dug deeper into just to make sure and it looks like both devices cannot even do this for a port when VLAN's are enabled on that port.

Do you have any other ideas of what I should be checking for?


Steve Schubert Tue, 03/24/2009 - 08:18

The only reason that traffic from a device should be seen on all other ports is if it is broadcast or multicast traffic within the same broadcast domain (aka VLAN). Typically the switch directs traffic between ports based on the MAC addresses of the transmitting and recieving devices within the same VLAN. (sorry, if you already know this but I'm thinking out loud here). So once the PC knows the MAC address (via an ARP reply from the Server) of the Server, traffic should flow only between those two ports. (This is why I asked if Port Mirroring or Spanning was turned on). Have you taken a sniffer trace from one of the other PC or phone ports to see if the traffic between the PC and Server is showing up on that port? It would be good to at least confirm what we're suspecting is actually happening. That is, PC-to-Server traffic is being propagated across all ports.

Can you tell me some more about how the switches are configured? For example, which VLANs do you have configured on which switch and are you trunking between switches. How about the connection to the router? Is it a trunk connection to the 3524? What kind of router are you using?

I could probably guess on some of the answers but I think it would help if you tell me yourself.




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