Why sniffer can capture packet on switch

epmahc1 used Ask the Experts™
I use cisco 2900 seriesXL . After i ran Sniffer Pro
I can see packet from A to B , but my computer is C
There're many kind of packet http icmp
but there's some notice that this event is one-way
means A send to B but B not reply.......
I set nothing on switch...

Is switch becomes hub ???
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Sr Manager Cloud Networking Ops
So you set up a monitor port on the 2900? It sounds as though you might only be seeing traffic actually destined for the port you are plugged into or broadcast traffic.

Also, you won't be able to see the transmit traffic for the Sniffer Pro machine.


A switch does not mirror all traffic to all ports like a hub.  Once the switch learns the layer 2 address for the devices hanging off of it, it will only forward unicast traffic to the port where the destination node is located.

If you want to capture the traffic on the entire switch, you need to configure a SPAN port.  Keep in mind that SPAN ports can only span ports within the same VLAN.  If you have more than one VLAN configured on your switch, you will need a seperate SPAN port.

Les MooreSr. Systems Engineer
Top Expert 2008

G'day, epmahc1
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