Security with external cables

I have 2 Cat6 cables running between two buildings. They are underground (except where they go into the buildings). I need to make sure someone cannot merely cut the cable, attach a plug to it and plug it into a laptop thus having the same access as the computers inside the buildings. The cables inside the buildings are connected to a vanilla switch. All the inside computers are passworded etc., but this would still give someone a good start at hacking. Is there some kind of encryption devise that could go inside each building at the switch to protect the data going across the cable?
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Use vpn. Also monitor cable length
Michael ChisholmCommented:
Depending on your equipment you can use ACLs to only allow communication from a specific device across the port the cable is connected.  The switches would have to be capable of it.

A little more info about how the connection is connected at both ends would also help in determining the best solution.  if you are connected by a cable and it only goes into a switch there is nothing to VPN into.  Also are you sending DHCP across the connection. Is there a router on both ends of the wire.  Are you willing to purchase hardware or is there a server in both buildings?
Method 1:
Make sure link goes down on interface down event for good
Come back, measure cable length (i.e. if there is no bridge installed halfway), rise it up or panic

Method 2:
Common PC of today can do few gigabits of VPN per CPU core. Or you buy expensive VPN modules for routers, and worry not if anybody cuts your cables.
Mal OsborneAlpha GeekCommented:
You could reconfigure your network to use IPSEC. This will encrypt all LAN traffic. Supported by Windows server 2000 and up, Windows XP and up.

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Mal OsborneAlpha GeekCommented:
Seems the best answer to me.
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