Span port problem

[Cesar Sanabria Pineda <csanab () ife org mx>]

Hi, I want to sniff all traffic from my intranet -> internet, I've
already configure SPAN port on a Cisco 2950 series switch, and it's on
the main's network cable (segment),  when i sniff traffic, i just see
traffic from some segments not all, is there any special configuration
for my switch?, what i'm doing bad?


 **********        *************            **********
( INTRANET ) -----| main switch |----------( INTERNET )
 **********        *************            **********
                           ____|______
                         | cisco 2950 |
                          ------------
                                |
                             sniffer


Main switch and cisco 2950 are connected via trunked port (cascading)

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