Question : Cisco PIX addition for extranet connection causes network outage

I have a Cisco netowrk for my Microsoft computing environment.  The core is a 6500 switch, its default gateway is a Cisco 3640 router who's default gateway is a Cisco PIX 515 to the Internet.  I run a supernet subnet with a 22 bit subnet mask internally.  I am trying to add a Cisco PIX 501 to the network for an extranet connection.  the WAN port connects to the client and the ethernet ports connect internally. When I try to connect the ethernet port on the 501 to a port on the 6500 I get network disruptions.  the PIX 501 begins to see a ton of ARP requests from all over the internal network and printers go off line.  What could be causing this?  there are no routing protocols running on either the 501 or the 6500.  

I am in need of a very quick response.

Answer : Cisco PIX addition for extranet connection causes network outage

Please try this and let me know if it helps.
set spantree portfast 9/7 ena
To cancel portfast use the following command
set spantree portfast 3/1 disable