BiDirectional SPAN on Cisco Catalyst 3750 question

Posted on 2006-06-06
Last Modified: 2010-05-18
I want to make sure that I am doing this right. I want to setup my Cisco Catalyst 3750 to span my default VLAN1. I have a server with two NIC's and believe this is the way I should set it up, let me know if I am wrong:

1) On one NIC attach to a port that is on a separate VLAN from the port I want to monitor. This NIC will only be used to listen to traffic from the switch. It will be attached to port Gi1/0/1

2) On my other NIC I will leave it attached to the network. It will be attached to port Fa2/0/1

3) I will enter the following command into my Cisco switch in config mode:
monitor session 1 source vlan 1 both
monitor session 1 destination interface fa2/0/1

I am going to use Ethereal to view network traffic. The link to the resource I am using is

Thanks in advance.
Question by:Natldiag
    1 Comment
    LVL 10

    Accepted Solution

    That looks good.  A couple of quick comments.  On the second interface, within the server, disable microsoft client and  disable TCP/IP (the sniffing interface) as you do not want it to get an address, especially if you are doing this on a server.  Your destination is a fast ethernet port, from the commands it looks like you are monitoring a stack of 3750 switches, I would seriously consider moving this to a gig interface.  Also be prepared to see some false duplicate packets when sniffing the bidirectionally on the whole vlan.  Good luck

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