The invention is directed towards routing a packet using both IPSec and common routing protocols within dynamic network topologies in a VPN. The routing of IPSec packets employs Open System Interconnection (OSI) layer three information. In one embodiment, a tree mechanism is used for looking up layer three information that may be associated with a protected subnetwork. When a packet is identified as being associated with a protected subnetwork, the packet may be encrypted and encapsulated, including the original destination and source IP address header information within another packet employing the IP Encapsulating Security Payload (ESP) protocol. New source and destination IP addresses are provided for the new packet using IP addresses associated with an entry gateway and an exit gateway to the VPN. The new packet may then be routed through the VPN using traditional routing protocols.