When you span a port or a vlan you are copying traffic from that interface or vlan to a specific interface. When you span you copy all traffic, unicast, multicast or broadcast from that source. During that monitor session the destination interface is not part of any VLAN, so it is not receiving any traffic that is not from the monitor session source, so there is no need to create a seperate VLAN.

Edison Ortiz Wed, 03/28/2007 - 08:45
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The only time when you will need a separate VLAN for SPAN is when using RSPAN.

RSPAN enables you to mirror traffic from one switchport to another switchport located in another physical switch.


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