Hello Leonides,
Here is a configuration example for a concentrator connecting to a PIX firewall.
I do not know about any config examples between a linksys PPPOE router and a VPN concentrator.
The concept is pretty much the same and the configuration on the concentrator is explained in this example.
http://cisco.com/en/US/partner/products/hw/vpndevc/ps2284/products_configuration_example09186a00801dd672.shtml
Hope this answers your question.
Rate this post, if it helps.
Cheers
Gilbert