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We have a stack of 6, 48 port 3750 cisco switches and I would like to setup mirroring across the stack to send the data to a network analyser. I have done mirroring before on a single switch, but never on a stack of swtiches. My question is, what sort of things do I need to consider when trying to monitor a stack of c3750 switches with multiple vlans if I want to monitor all of the traffic on all VLANS with my network analyzer?

Since each switch is already stacked, do I only need to setup a single monitor port on one of the switches?

When I specify the port range that I want to monitor, should I just set the source port as being the various vlans I want to monitor, and then set the destination port as being a single port?

 switch 1 provision ws-c3750-48ts
 switch 2 provision ws-c3750-48ts
 switch 3 provision ws-c3750-48ts
 switch 4 provision ws-c3750-48ts
 switch 5 provision ws-c3750-48p
 switch 6 provision ws-c3750-48p

Sorry for asking multiple questions in a single post, I can break it up if needed - it just seems like all of the questions are a part of the overall question regarding configuring SPAN across a switch stack.

  • A switch stack is a single, logical device, with a single configuration, as if it were a chassis switch with multiple blades. – Ron Maupin Apr 18 '16 at 17:40
  • I figured so much, just wanted to check. Regarding the configuration of mirroring multiple vlans on the stack to a single port, would the configuration be as I described it above? – Richie086 Apr 18 '16 at 17:42
  • A single port on one of the switches will work, except for the fact that a single port probably can't all the traffic for that many ports, and you will probably end up dropping a lot of traffic destined for that single port. – Ron Maupin Apr 18 '16 at 17:45
  • Yeah that could be an issue, but the network analyser is not mission critical for us, we just want to get a better idea of the traffic on our LAN. A few dropped packets would not be the end of the world. – Richie086 Apr 18 '16 at 18:11
  • Ron, if you want to make your comment into an answer id be happy to accept it. BTW thank you for all you do for this site, I see you answer questions frequently and I always find your explanations to be of a very high quality. – Richie086 Apr 18 '16 at 18:12
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A switch stack basically works like a single switch with a single configuration, much like a chassis switch with multiple blades.

You would configure SPAN the same way you do on a single switch. You can set up all the traffic to go to a single monitor port on any of the switches, although you could easily overload the single port and drop a lot of traffic.

For understanding your traffic mix and quantities, NetFlow may be a better solution.

  • Thanks Ron! I will look into Netflow instead of mirroring – Richie086 Apr 18 '16 at 18:20
  • looks like netflow is not supported by the 3750s :( – Richie086 Apr 18 '16 at 23:28

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