cisco 3560G Span port

Hello,

We are inserting a IDS appliance into our system and need to setup a span port for it to use.  I am not really clear on how to do this.  The Cisco 3560 switch connects to our asa firewall which is in routed mode via a trunk port on port 12 of the switch.  I assume this is the best port to monitor?  I have an open port 15 that I was going to connect to the IDS system.  Would the correct command be
monitor session 1 source interface fastethernet 0/12  
&
monitor session 1 destination interface fastethernet 0/15

I have never done this before and am looking for any help or advice to make sure this looks ok and will not cause any issues on the switch/network doing it like this.  Any help is appreciated.

Thank you
LVL 1
Tim LewisNetwork ManagerAsked:
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Cyclops3590Commented:
close

monitor session 1 source interface fastethernet 0/12
monitor session 1 destination interface fastethernet 0/15 encapsulation dot1q

need that at the end so it doesn't strip the vlan tagging info.
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Tim LewisNetwork ManagerAuthor Commented:
Thank you. Does doing this affect the monitored port in anyway?
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Cyclops3590Commented:
not that i've ever seen.  there's clearly going to be resources used by the switch to do the operation but none that should affect the actual data traffic.
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Tim LewisNetwork ManagerAuthor Commented:
I found that the command to monitor should be monitor session 1 destination interface fastethernet 0/15 encapsulation replicate.    otherwise good to go.  Thanks for the help
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