Enhanced Easy VPN tunnel establishes and then drops from DSL.

I have an Enhanced Easy VPN server running on a 2801 router at my corporate office.  I am using 851W routers in the field to connect with Easy VPN remote.  This is working great from three locations with cable modems, but when i place an 851W at any of my Bellsouth DSL sites the VPN tunnel establishes connection and stays up for 2-4 minutes and then I lose connectivity.  The VPN light is on on the router at the branch office and it shows as up from the SDM.  And then every hour on the hour for 1 minute the tunnel comes up.  If i ping -t the ip of the remote router from the corporate office with it is up for that 1 minute it will stay up as long as i keep pinging it.  It was up for 12 hours last night and when i ended the ping it was down in just a few minutes.   I am running 12.4 on the 851s.   The funny thing is that i can take this same router with the same config and put it at a cable modem site and the tunnel never fails.
MBAMikeAsked:
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MBAMikeAuthor Commented:
I think i may have found a solution to the problem.   I added the lines below to both routers and reset the tunnel. ( i did this initially with SDM which uses dymamic and not periodic by default)

crypto isakmp keepalive 10 periodic

I also found out why the connection would be active every hour.  The "crypto ipsec security-association lifetime" is defaulted to one hour.  I set mine to 2400 which is 20 minutes.  

crypto ipsec security-association lifetime seconds 2400

When i did this the tunnel would become active for a minute or two every 20 minutes.

I also found several posts that suggest this should be set to 24 hours.
crypto ipsec security-association lifetime seconds 86400

I configured the server and remote routers with with SDM and not the command line and i believe this is why the keepalive statements were missing.  

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Press2EscSystems IntegratorCommented:
Interesting....  thanks for the heads up MBA...  P2E
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