While any one of you are trying to analyze a SPAN or RSPAN traffic you may notice that certain layer 2 frames are missing. Usually SPAN/RSPAN ignores layer 2 traffic like CDP, spanning-tree BPUDs, VTP, DTP and PagP frames. However, these traffic types can be forwarded along with the normal SPAN traffic if the “encapsulation replicate” Cisco IOS command is configured in a Cisco Catalyst Switch.
The below example shows how to enable this feature
CiscoSwitch(config)# monitor session 1 source interface g0/1
CiscoSwitch(config)# monitor session 1 destination interface g0/2 encapsulation replicate