To provide multiprotocol local networks over a single-protocol backbone.
To provide workarounds for networks containing protocols that have limited hop counts; for example, AppleTalk.
To connect discontinuous subnetworks.
To allow virtual private networks across WANs.
check this URL to get more information.
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/finter_c/icflogin.htm#wp1012601
rate if it helps ...
