Cisco SPAN question

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    Hi – I am having trouble setting up a SPAN port monitor on my Cisco 871 router.

    I have a DSL modem connected to the WAN port, which is FastEthernet4.

    Of course to set up authentication for my DSL modem, a “Dialer0” interface was also created. I am assigned a dynamic IP from AT&T, and I use “IP negotiated”.

    What I want to do is monitor all traffic coming into my router – I guess it would be into FastEthernet4 or Dialer0.

    But when I enter “monitor session 1 source interface FastEthernet4”, the cli won’t accept it. I also tried to use “Dialer0” instead of Fa4, but the same thing happens.

    I tried creating a destination port first, but it didnt make a difference.

    Everything worked fine, however, when I tried using “Vlan1” or FastEthernet3 for the source.

    So does anybody know what I am doing wrong? I can’t imagine that SPAN wont work for the WAN interface. I can use SPAN to monitor my WAN interface on my ASA5505. Somehow I seem to recall having this issue several years ago – and IIRC I think I found a solution, but I cant remember now if or how I was able to solve it. Somehow I think it involved the syntax of how I was typing in the command….but I may be wrong.

    I have searched high and low on the net and at but cannot find an answer to my question.

    I know that I can accomplish the same thing with a hub or a passive ethernet tap….but I would prefer to use SPAN if possible.

    Thank you in advance for any help that you can provide.

    Take care,

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