You can start here check bellow links, IPsec is prety much an industry standard so as long both routers cisco and nortel do support Ipsec you should be ok, follow the Ike Phase-1 configuration as well as phase-2 , both sides have to coninside and match whichever policy both sides agreed on.

IPsec router-to-router

http://www.cisco.com/en/US/tech/tk583/tk372/technologies_configuration_example09186a008009448f.shtml

Check here several Ipsec config examples mid way down the page

http://www.cisco.com/en/US/products/hw/routers/ps274/prod_configuration_examples_list.html

Rgds

-Jorge