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Port SPAN traffic

am capturing the traffic from my core switch through a Mirror PORT/SPAN Port.

i found there are unicast traffic in it, according to theory if you are hanging to a switch you should not see the unicast traffic.

is it right? or its normal to have unicast traffic
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Maen Abu-TabanjehNetwork Administrator, Network Consultant
Top Expert 2011


Great thanks, let me rephrase the question

1)if you are listening (wireshark capture) to traffic while connecting directly to a SPAN/Mirrored Port in a Switch, should we see UniCast traffic or not?

2) can (Cisco)network admin drop/stop unicat packets to go to the SPAN/Mirrored Port?
In general you would see only non-unicast packets on a mirror port that isn't mirroring another device yet.
However, when a switch receives a packet it doesn't have a corresponding MAC table entry for, it will flood it to all ports in that VLAN because it doesn't know where it lives.

One example would be if a PC A doesn't know that PC B is down and sends a packet to PC B anyway the switch will send it to all ports in the VLAN.

Another example is when a router/L3 switch hasn't aged out the ARP entry for a device no longer available it will continue to forward packets to switches in that VLAN and in turn they will send it to all of their ports.
Sorry I missed the second part of that question before sending.

I don't know of any way to filter out the unicast packets in this situation at the network device.

You could probably get what you want by filtering on the Wireshark, Sniffer or whatever tool you are using after capturing it.
1 You will see the traffic of the port that you monitoring. ie. if you are monitoring a web server port on a switch you will see the unicast http traffic that the server receives. So yes, you will unicast, multicast, broadcast traffic on pretty much any port that is connected to another device.
2 No, you will see all traffic, if you are using wireshark you can filter the traffic that is displayed during and after the capture. I was trying to find a way to filter unicast but only found the opposite...I'm not a wireshark expert though.
Generally, I would expect to see unicast traffic on an active switch port. What I wouldn't expect is to see unicast traffic for other devices not connected to that port - the reasons why it could happen given by Rick O Shay above.

If ports are being spanned and you are capturing at the destination, then you will see whatever traffic you have configured to monitor for those ports - this is what a port mirror is supposed to do.