IPSec does not disturb the original IP header and can be routed as normal IP traffic. Routers and switches in the data path between the communicating hosts simply forward the packets to their destination. However, when there is a firewall or gateway in the data path, IP forwarding must be enabled at the firewall for the following IP protocols and UDP ports:

IP Protocol ID 50:
For both inbound and outbound filters. Should be set to allow Encapsulating Security Protocol (ESP) traffic to be forwarded.

IP Protocol ID 51:
For both inbound and outbound filters. Should be set to allow Authentication Header (AH) traffic to be forwarded.

UDP Port 500:
For both inbound and outbound filters. Should be set to allow ISAKMP traffic to be forwarded.

L2TP/IPSec traffic looks just like IPSec traffic on the wire. The firewall just has to allow IKE (UDP 500) and IPSec ESP formatted packets (IP protocol = 50).

Follow me!