From what I see of that, it is a firewall. Do you have another in place before that? From a MS whitepaper,
Firewalls and IPSec
If a firewall separates two hosts that use IPSec to secure the communication channel, the firewall must open the following ports:
TCP port 50 for IPSec Encapsulating Security Protocol (ESP) traffic
TCP port 51 for IPSec Authentication Header (AH) traffic
UDP port 500 for Internet Key Exchange (IKE) negotiation traffic
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